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219-463: For information about the composition of non-human animal nervous systems, see nervous system . For information about the typical structure of the Homo sapiens nervous system, see human brain or peripheral nervous system . This article discusses information pertinent to the study of neuroanatomy. The first known written record of a study of the anatomy of the human brain is an ancient Egyptian document,

438-537: A connectome including its synapses. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic ; the nervous systems of the two sexes, males and female hermaphrodites , have different numbers of neurons and groups of neurons that perform sex-specific functions. In C. elegans , males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons. Arthropods , such as insects and crustaceans , have

657-418: A model system for studying neurophysiology . The form of neural plasticity known as long-term potentiation (LTP) was initially discovered to occur in the hippocampus and has often been studied in this structure. LTP is widely believed to be one of the main neural mechanisms by which memories are stored in the brain. In rodents as model organisms , the hippocampus has been studied extensively as part of

876-457: A multiple-systems model , suggesting that some effects may not be simply mediated by one portion of the brain. Studies on freely moving rats and mice have shown many hippocampal neurons to act as place cells that cluster in place fields , and these fire bursts of action potentials when the animal passes through a particular location. This place-related neural activity in the hippocampus has also been reported in monkeys that were moved around

1095-452: A nerve net , a diffuse network of isolated cells. In bilaterian animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the Ediacaran period, over 550 million years ago. The nervous system contains two main categories or types of cells: neurons and glial cells . The nervous system is defined by the presence of

1314-399: A sensory input and ends with a motor output, passing through a sequence of neurons connected in series . This can be shown in the "withdrawal reflex" causing a hand to jerk back after a hot stove is touched. The circuit begins with sensory receptors in the skin that are activated by harmful levels of heat: a special type of molecular structure embedded in the membrane causes heat to change

1533-408: A Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C-shape , then straightens, thereby propelling itself rapidly forward. Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not

1752-414: A brain system responsible for spatial memory and navigation. Many neurons in the rat and mouse hippocampus respond as place cells : that is, they fire bursts of action potentials when the animal passes through a specific part of its environment. Hippocampal place cells interact extensively with head direction cells , whose activity acts as an inertial compass, and conjecturally with grid cells in

1971-418: A broader system that incorporates both the memory and the spatial perspectives in its role that involves the use of a wide scope of cognitive maps. This relates to the purposive behaviorism born of Tolman's original goal of identifying the complex cognitive mechanisms and purposes that guided behaviour. It has also been proposed that the spiking activity of hippocampal neurons is associated spatially, and it

2190-633: A candidate mechanism for long-term memory , LTP has since been studied intensively, and a great deal has been learned about it. However, the complexity and variety of the intracellular signalling cascades that can trigger LTP is acknowledged as preventing a more complete understanding. The hippocampus is a particularly favorable site for studying LTP because of its densely packed and sharply defined layers of neurons, but similar types of activity-dependent synaptic change have also been observed in many other brain areas. The best-studied form of LTP has been seen in CA1 of

2409-401: A capability for neurons to exchange signals with each other. Networks formed by interconnected groups of neurons are capable of a wide variety of functions, including feature detection, pattern generation and timing, and there are seen to be countless types of information processing possible. Warren McCulloch and Walter Pitts showed in 1943 that even artificial neural networks formed from

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2628-400: A cell can send signals to other cells. One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-to-point" signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells. Thus, neural signaling

2847-563: A central brain with three divisions and large optical lobes behind each eye for visual processing. The brain of a fruit fly contains several million synapses, compared to at least 100 billion in the human brain. Approximately two-thirds of the Drosophila brain is dedicated to visual processing . Thomas Hunt Morgan started to work with Drosophila in 1906, and this work earned him the 1933 Nobel Prize in Medicine for identifying chromosomes as

3066-564: A central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons. In some cases groups of intermediate neurons are clustered into discrete ganglia . The development of

3285-432: A chemically gated ion channel is activated, it forms a passage that allows specific types of ions to flow across the membrane. Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory. When a second messenger system is activated, it starts a cascade of molecular interactions inside the target cell, which may ultimately produce a wide variety of complex effects, such as increasing or decreasing

3504-402: A command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances. At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. There are multiple ways that

3723-484: A common wormlike ancestor that appear as fossils beginning in the Ediacaran period, 550–600 million years ago. The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an enlargement (a "ganglion") for each body segment, with an especially large ganglion at the front, called the "brain". Even mammals, including humans, show the segmented bilaterian body plan at

3942-418: A comparison with the shape of the folded back forelimbs and webbed feet of the mythological hippocampus , a sea monster with a horse's forequarters and a fish's tail. The hippocampus was then described as pes hippocampi major , with an adjacent bulge in the occipital horn , described as the pes hippocampi minor and later renamed as the calcar avis . The renaming of the hippocampus as hippocampus major, and

4161-478: A corresponding temporally structured stimulus, is called a central pattern generator . Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or longer. One of the most important types of temporal pattern is circadian rhythmicity —that is, rhythmicity with a period of approximately 24 hours. All animals that have been studied show circadian fluctuations in neural activity, which control circadian alternations in behavior such as

4380-529: A distinct pattern of neural population activity and waves of electrical activity as measured by an electroencephalogram (EEG). These modes are named after the EEG patterns associated with them: theta and large irregular activity (LIA). The main characteristics described below are for the rat, which is the animal most extensively studied. The theta mode appears during states of active, alert behavior (especially locomotion), and also during REM (dreaming) sleep. In

4599-417: A full-fledged theory of the role of the hippocampus in anxiety. The inhibition theory is currently the least popular of the three. The second major line of thought relates the hippocampus to memory. Although it had historical precursors, this idea derived its main impetus from a famous report by American neurosurgeon William Beecher Scoville and British-Canadian neuropsychologist Brenda Milner describing

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4818-556: A greatly simplified mathematical abstraction of a neuron are capable of universal computation . Historically, for many years the predominant view of the function of the nervous system was as a stimulus-response associator. In this conception, neural processing begins with stimuli that activate sensory neurons, producing signals that propagate through chains of connections in the spinal cord and brain, giving rise eventually to activation of motor neurons and thereby to muscle contraction, i.e., to overt responses. Descartes believed that all of

5037-487: A group of proteins that cluster together to form a structure resembling a postsynaptic density (the signal-receiving part of a synapse). However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction. Jellyfish , comb jellies , and related animals have diffuse nerve nets rather than

5256-449: A hidden goal. Other cells have been discovered since the finding of the place cells in the rodent brain that are either in the hippocampus or the entorhinal cortex. These have been assigned as head direction cells , grid cells and boundary cells . Speed cells are thought to provide input to the hippocampal grid cells. Approach-avoidance conflict happens when a situation is presented that can either be rewarding or punishing, and

5475-449: A hierarchy of processing stages. At each stage, important information is extracted from the signal ensemble and unimportant information is discarded. By the end of the process, input signals representing "points of light" have been transformed into a neural representation of objects in the surrounding world and their properties. The most sophisticated sensory processing occurs inside the brain, but complex feature extraction also takes place in

5694-497: A hundred known neurotransmitters, and many of them have multiple types of receptors. Many synapses use more than one neurotransmitter—a common arrangement is for a synapse to use one fast-acting small-molecule neurotransmitter such as glutamate or GABA , along with one or more peptide neurotransmitters that play slower-acting modulatory roles. Molecular neuroscientists generally divide receptors into two broad groups: chemically gated ion channels and second messenger systems . When

5913-422: A lateral structure may be said to lie medial to something else that lies even more laterally). Commonly used terms for planes of orientation or planes of section in neuroanatomy are "sagittal", "transverse" or "coronal", and "axial" or "horizontal". Again in this case, the situation is different for swimming, creeping or quadrupedal (prone) animals than for Man, or other erect species, due to the changed position of

6132-405: A life-long memory of the event even after a single training session. The memory of such an event appears to be first stored in the hippocampus, but this storage is transient. Much of the long-term storage of the memory seems to take place in the anterior cingulate cortex . When such an intense learning event was experimentally applied, more than 5,000 differently methylated DNA regions appeared in

6351-474: A line of work that eventually led to their very influential 1978 book The Hippocampus as a Cognitive Map . There is now almost universal agreement that hippocampal function plays an important role in spatial coding, but the details are widely debated. Later research has focused on trying to bridge the disconnect between the two main views of hippocampal function as being split between memory and spatial cognition. In some studies, these areas have been expanded to

6570-482: A microscope. The author Michael Nikoletseas wrote: "It is difficult to believe that until approximately year 1900 it was not known that neurons are the basic units of the brain ( Santiago Ramón y Cajal ). Equally surprising is the fact that the concept of chemical transmission in the brain was not known until around 1930 ( Henry Hallett Dale and Otto Loewi ). We began to understand the basic electrical phenomenon that neurons use in order to communicate among themselves,

6789-414: A nervous system made up of a series of ganglia , connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly . Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia. The head segment contains the brain, also known as the supraesophageal ganglion . In the insect nervous system ,

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7008-551: A neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high-rate bursting and quiescence. When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including attractor dynamics, periodicity, and even chaos . A network of neurons that uses its internal structure to generate temporally structured output, without requiring

7227-415: A number of studies that show the involvement of the hippocampus in conflict tasks. The authors suggest that one challenge is to understand how conflict processing relates to the functions of spatial navigation and memory and how all of these functions need not be mutually exclusive. The hippocampus has received renewed attention for its role in social memory. Epileptic human subjects with depth electrodes in

7446-511: A proportion of the claimed CA1 cells in the study. The dorsal CA2 and ventral CA1 subregions of the hippocampus have been implicated in social memory processing. Genetic inactivation of CA2 pyramidal neurons leads to pronounced loss of social memory, while maintaining intact sociability in mice. Similarly, ventral CA1 pyramidal neurons have also been demonstrated as critical for social memory under optogenetic control in mice. The hippocampus shows two major "modes" of activity, each associated with

7665-565: A protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts; axons typically extend throughout the body in bundles called nerves. Even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions. These include sensory neurons that transmute physical stimuli such as light and sound into neural signals, and motor neurons that transmute neural signals into activation of muscles or glands; however in many species

7884-411: A rat's location to be reconstructed with high confidence. The size of place fields varies in a gradient along the length of the hippocampus, with cells at the dorsal end showing the smallest fields, cells near the center showing larger fields, and cells at the ventral tip showing fields that cover the entire environment. In some cases, the firing rate of hippocampal cells depends not only on place but also

8103-531: A result of depression, but this can be stopped with anti-depressants even if they are not effective in relieving other symptoms. Chronic stress resulting in elevated levels of glucocorticoids , notably of cortisol , is seen to be a cause of neuronal atrophy in the hippocampus. This atrophy results in a smaller hippocampal volume which is also seen in Cushing's syndrome . The higher levels of cortisol in Cushing's syndrome

8322-579: A room whilst in a restraint chair. However, the place cells may have fired in relation to where the monkey was looking rather than to its actual location in the room. Over many years, many studies have been carried out on place-responses in rodents, which have given a large amount of information. Place cell responses are shown by pyramidal cells in the hippocampus and by granule cells in the dentate gyrus . Other cells in smaller proportion are inhibitory interneurons , and these often show place-related variations in their firing rate that are much weaker. There

8541-477: A severe impact on many types of cognition including memory . Even normal aging is associated with a gradual decline in some types of memory, including episodic memory and working memory (or short-term memory ). Because the hippocampus is thought to play a central role in memory, there has been considerable interest in the possibility that age-related declines could be caused by hippocampal deterioration. Some early studies reported substantial loss of neurons in

8760-444: A sharp wave, there is a dramatic increase in firing rate in up to 10% of the hippocampal population These two hippocampal activity modes can be seen in primates as well as rats, with the exception that it has been difficult to see robust theta rhythmicity in the primate hippocampus. There are, however, qualitatively similar sharp waves and similar state-dependent changes in neural population activity. The underlying currents producing

8979-409: A slice of nervous tissue, thanks to the transparency consequent to the lack of staining in the majority of surrounding cells. Modernly, Golgi-impregnated material has been adapted for electron-microscopic visualization of the unstained elements surrounding the stained processes and cell bodies, thus adding further resolutive power. Histochemistry uses knowledge about biochemical reaction properties of

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9198-430: A special type of cell—the neuron (sometimes called "neurone" or "nerve cell"). Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via synapses , which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. Many types of neuron possess an axon ,

9417-501: A three-layered system of membranes, including a tough, leathery outer layer called the dura mater . The brain is also protected by the skull, and the spinal cord by the vertebrae . The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not lie within the CNS. The large majority of the axon bundles called nerves are considered to belong to the PNS, even when

9636-633: A tracer virus which replicates from the synapse to the soma is the pseudorabies virus . By using pseudorabies viruses with different fluorescent reporters, dual infection models can parse complex synaptic architecture. Axonal transport methods use a variety of dyes (horseradish peroxidase variants, fluorescent or radioactive markers, lectins, dextrans) that are more or less avidly absorbed by neurons or their processes. These molecules are selectively transported anterogradely (from soma to axon terminals) or retrogradely (from axon terminals to soma), thus providing evidence of primary and collateral connections in

9855-410: A variant form of LTP that is conditioned on an extra input coming from a reward-signalling pathway that uses dopamine as neurotransmitter. All these forms of synaptic modifiability, taken collectively, give rise to neural plasticity , that is, to a capability for the nervous system to adapt itself to variations in the environment. The basic neuronal function of sending signals to other cells includes

10074-610: A very irregular temporal pattern. Sharp waves are less frequent during inactive waking states and are usually smaller. Sharp waves have also been observed in humans and monkeys. In macaques, sharp waves are robust but do not occur as frequently as in rats. One of the most interesting aspects of sharp waves is that they appear to be associated with memory. Wilson and McNaughton 1994, and numerous later studies, reported that when hippocampal place cells have overlapping spatial firing fields (and therefore often fire in near-simultaneity), they tend to show correlated activity during sleep following

10293-451: A very long time. Those who argued for the brain often contributed to the understanding of neuroanatomy as well. Herophilus and Erasistratus of Alexandria were perhaps the most influential with their studies involving dissecting human brains, affirming the distinction between the cerebrum and the cerebellum , and identifying the ventricles and the dura mater . The Greek physician and philosopher Galen , likewise, argued strongly for

10512-550: Is diffusion tensor imaging , which relies on the restricted diffusion of water in tissue in order to produce axon images. In particular, water moves more quickly along the direction aligned with the axons, permitting the inference of their structure. Certain viruses can replicate in brain cells and cross synapses. So, viruses modified to express markers (such as fluorescent proteins) can be used to trace connectivity between brain regions across multiple synapses. Two tracer viruses which replicate and spread transneuronal/transsynaptic are

10731-437: Is a major component of the brain of humans and other vertebrates . Humans and other mammals have two hippocampi, one in each side of the brain . The hippocampus is part of the limbic system , and plays important roles in the consolidation of information from short-term memory to long-term memory , and in spatial memory that enables navigation. The hippocampus is located in the allocortex , with neural projections into

10950-473: Is a special case of histochemistry that uses selective antibodies against a variety of chemical epitopes of the nervous system to selectively stain particular cell types, axonal fascicles, neuropiles, glial processes or blood vessels, or specific intracytoplasmic or intranuclear proteins and other immunogenetic molecules, e.g., neurotransmitters. Immunoreacted transcription factor proteins reveal genomic readout in terms of translated protein. This immensely increases

11169-407: Is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior individually. Such neurons appear most commonly in the fast escape systems of various species—the squid giant axon and squid giant synapse , used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of

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11388-541: Is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system . Nerves that exit from the brain are called cranial nerves while those exiting from the spinal cord are called spinal nerves . The nervous system consists of nervous tissue which, at a cellular level, is defined by the presence of a special type of cell, called the neuron . Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in

11607-462: Is also substantial evidence that it makes a contribution to memory, which can be distinguished from the contribution of the hippocampus. It is apparent that complete amnesia occurs only when both the hippocampus and the parahippocampus are damaged. The major input to the hippocampus is through the entorhinal cortex (EC), whereas its major output is via CA1 to the subiculum. Information reaches CA1 via two main pathways, direct and indirect. Axons from

11826-445: Is an abuse of terminology—it is the receptors that are excitatory and inhibitory, not the neurons—but it is commonly seen even in scholarly publications. One very important subset of synapses are capable of forming memory traces by means of long-lasting activity-dependent changes in synaptic strength. The best-known form of neural memory is a process called long-term potentiation (abbreviated LTP), which operates at synapses that use

12045-570: Is an anatomical convention that a cluster of neurons in the brain or spinal cord is called a nucleus , whereas a cluster of neurons in the periphery is called a ganglion . There are, however, a few exceptions to this rule, notably including the part of the forebrain called the basal ganglia . Sponges have no cells connected to each other by synaptic junctions , that is, no neurons, and therefore no nervous system. They do, however, have homologs of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express

12264-514: Is called identified if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties. In vertebrate nervous systems very few neurons are "identified" in this sense—in humans, there are believed to be none—but in simpler nervous systems, some or all neurons may be thus unique. In

12483-431: Is called presynaptic, and the cell that receives signals is called postsynaptic. Both the presynaptic and postsynaptic areas are full of molecular machinery that carries out the signalling process. The presynaptic area contains large numbers of tiny spherical vessels called synaptic vesicles , packed with neurotransmitter chemicals. When the presynaptic terminal is electrically stimulated, an array of molecules embedded in

12702-409: Is capable of a much higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second. At a more integrative level, the primary function of the nervous system is to control the body. It does this by extracting information from the environment using sensory receptors, sending signals that encode this information into

12921-413: Is concealed by the parahippocampal gyrus . The cortex thins from six layers to the three or four layers that make up the hippocampus. The term hippocampal formation is used to refer to the hippocampus proper and its related parts. However, there is no consensus as to what parts are included. Sometimes the hippocampus is said to include the dentate gyrus and the subiculum . Some references include

13140-421: Is considered the foundation of modern neuroanatomy. The subsequent three hundred and fifty some years has produced a great deal of documentation and study of the neural system. At the tissue level, the nervous system is composed of neurons , glial cells , and extracellular matrix . Both neurons and glial cells come in many types (see, for example, the nervous system section of the list of distinct cell types in

13359-399: Is described as having an anterior and posterior part (in primates ) or a ventral and dorsal part in other animals. Both parts are of similar composition but belong to different neural circuits . In the rat, the two hippocampi resemble a pair of bananas, joined at the stems by the commissure of fornix (also called the hippocampal commissure). In primates , the part of the hippocampus at

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13578-437: Is little, if any, spatial topography in the representation; in general, cells lying next to each other in the hippocampus have uncorrelated spatial firing patterns. Place cells are typically almost silent when a rat is moving around outside the place field but reach sustained rates as high as 40 Hz when the rat is near the center. Neural activity sampled from 30 to 40 randomly chosen place cells carries enough information to allow

13797-458: Is not yet clear. In rats (the animals that have been the most extensively studied), theta is seen mainly in two conditions: first, when an animal is walking or in some other way actively interacting with its surroundings; second, during REM sleep . The function of theta has not yet been convincingly explained although numerous theories have been proposed. The most popular hypothesis has been to relate it to learning and memory. An example would be

14016-516: Is occasionally interrupted by large surges called sharp waves . These events are associated with bursts of spike activity lasting 50 to 100 milliseconds in pyramidal cells of CA3 and CA1. They are also associated with short-lived high-frequency EEG oscillations called "ripples", with frequencies in the range 150 to 200 Hz in rats, and together they are known as sharp waves and ripples . Sharp waves are most frequent during sleep when they occur at an average rate of around 1 per second (in rats) but in

14235-469: Is only gray in preserved tissue, and is better described as pink or light brown in living tissue) contains a high proportion of cell bodies of neurons. White matter is composed mainly of myelinated axons, and takes its color from the myelin. White matter includes all of the nerves, and much of the interior of the brain and spinal cord. Gray matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces. There

14454-445: Is partly why returning to a location where an emotional event occurred may evoke that emotion. There is a deep emotional connection between episodic memories and places. Due to bilateral symmetry the brain has a hippocampus in each cerebral hemisphere . If damage to the hippocampus occurs in only one hemisphere, leaving the structure intact in the other hemisphere, the brain can retain near-normal memory functioning. Severe damage to

14673-399: Is processed by the brain. In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive—the cell bodies serve only to provide metabolic support and do not participate in signalling. A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, most parts of

14892-675: Is related to the anterior parts of the right and left hippocampus. The right head of the hippocampus is more involved in executive functions and regulation during verbal memory recall. The tail of the left hippocampus tends to be closely related to verbal memory capacity. Damage to the hippocampus does not affect some types of memory, such as the ability to learn new skills (playing a musical instrument or solving certain types of puzzles, for example). This fact suggests that such abilities depend on different types of memory ( procedural memory ) and different brain regions. Furthermore, amnesic patients frequently show "implicit" memory for experiences even in

15111-412: Is small and simple in some species, such as the nematode worm, where the body plan is quite simple: a tube with a hollow gut cavity running from the mouth to the anus, and a nerve cord with an enlargement (a ganglion ) for each body segment, with an especially large ganglion at the front, called the brain. The nematode Caenorhabditis elegans has been studied because of its importance in genetics. In

15330-401: Is suggested that the memory, spatial cognition, and conflict processing functions may be seen as working together and not mutually exclusive. Psychologists and neuroscientists generally agree that the hippocampus plays an important role in the formation of new memories about experienced events ( episodic or autobiographical memory ). Part of this function is hippocampal involvement in

15549-431: Is the ability to extract biologically relevant information from combinations of sensory signals. In the visual system , for example, sensory receptors in the retina of the eye are only individually capable of detecting "points of light" in the outside world. Second-level visual neurons receive input from groups of primary receptors, higher-level neurons receive input from groups of second-level neurons, and so on, forming

15768-447: Is usually the result of medications taken for other conditions. Neuronal loss also occurs as a result of impaired neurogenesis. Another factor that contributes to a smaller hippocampal volume is that of dendritic retraction where dendrites are shortened in length and reduced in number, in response to increased glucocorticoids. This dendritic retraction is reversible. After treatment with medication to reduce cortisol in Cushing's syndrome,

15987-515: The Edwin Smith Papyrus . In Ancient Greece , interest in the brain began with the work of Alcmaeon , who appeared to have dissected the eye and related the brain to vision. He also suggested that the brain, not the heart, was the organ that ruled the body (what Stoics would call the hegemonikon ) and that the senses were dependent on the brain. The debate regarding the hegemonikon persisted among ancient Greek philosophers and physicians for

16206-541: The Great Hippocampus Question . The term hippocampus minor fell from use in anatomy textbooks and was officially removed in the Nomina Anatomica of 1895. Today, the structure is just called the hippocampus, with the term cornu Ammonis (that is, 'Ammon's horn') surviving in the names of the hippocampal subfields CA1-CA4 . The term limbic system was introduced in 1952 by Paul MacLean to describe

16425-505: The Herpes simplex virus type1 (HSV) and the Rhabdoviruses . Herpes simplex virus was used to trace the connections between the brain and the stomach, in order to examine the brain areas involved in viscero-sensory processing. Another study injected herpes simplex virus into the eye, thus allowing the visualization of the optical pathway from the retina into the visual system . An example of

16644-431: The brain , retina , and spinal cord , while the peripheral nervous system (PNS) is made up of all the nerves and ganglia (packets of peripheral neurons) outside of the CNS that connect it to the rest of the body. The PNS is further subdivided into the somatic and autonomic nervous systems. The somatic nervous system is made up of "afferent" neurons, which bring sensory information from the somatic (body) sense organs to

16863-410: The central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord . The PNS consists mainly of nerves , which are enclosed bundles of the long fibers, or axons , that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor nerves (efferent), while those nerves that transmit information from

17082-423: The entorhinal cortex via the perforant path . The entorhinal cortex (EC) is strongly and reciprocally connected with many cortical and subcortical structures as well as with the brainstem. Different thalamic nuclei , (from the anterior and midline groups), the medial septal nucleus , the supramammillary nucleus of the hypothalamus, and the raphe nuclei and locus coeruleus of the brainstem all send axons to

17301-409: The human brain , it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas. Among the most important functions of glial cells are to support neurons and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy pathogens and remove dead neurons; and to provide guidance cues directing

17520-468: The human brain , there are many other animals whose brains and nervous systems have received extensive study as model systems , including mice, zebrafish , fruit fly , and a species of roundworm called C. elegans . Each of these has its own advantages and disadvantages as a model system. For example, the C. elegans nervous system is extremely stereotyped from one individual worm to the next. This has allowed researchers using electron microscopy to map

17739-475: The hypothalamic mammillary body , and the anterior nuclear complex in the thalamus , and is generally accepted to be part of the limbic system. The hippocampus can be seen as a ridge of gray matter tissue , elevating from the floor of each lateral ventricle in the region of the inferior or temporal horn. This ridge can also be seen as an inward fold of the archicortex into the medial temporal lobe . The hippocampus can only be seen in dissections as it

17958-438: The insect brain have passive cell bodies arranged around the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called neuropil , in the interior. The cephalic molluscs have two pairs of main nerve cords organized around a number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. Most pairs of corresponding ganglia on both sides of

18177-479: The neocortex , in humans as well as other primates. The hippocampus, as the medial pallium , is a structure found in all vertebrates . In humans, it contains two main interlocking parts: the hippocampus proper (also called Ammon's horn ), and the dentate gyrus . In Alzheimer's disease (and other forms of dementia ), the hippocampus is one of the first regions of the brain to suffer damage; short-term memory loss and disorientation are included among

18396-470: The nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates, it consists of two main parts,

18615-406: The primary somatosensory cortex and the inferior parietal cortex . The function of the mirror system is a subject of much speculation. Many researchers in cognitive neuroscience and cognitive psychology consider that this system provides the physiological mechanism for the perception/action coupling (see the common coding theory ). They argue that mirror neurons may be important for understanding

18834-407: The radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include anemones , hydras , corals and jellyfish ) consist of a diffuse nerve net . All other animal species, with the exception of a few types of worm , have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. The size of

19053-474: The thalamus to field CA1. A very important projection comes from the medial septal nucleus, which sends cholinergic , and gamma amino butyric acid (GABA) stimulating fibers (GABAergic fibers) to all parts of the hippocampus. The inputs from the medial septal nucleus play a key role in controlling the physiological state of the hippocampus; destruction of this nucleus abolishes the hippocampal theta rhythm and severely impairs certain types of memory. Areas of

19272-415: The CNS is often wrongly assumed to be more or less straight, but it actually shows always two ventral flexures (cervical and cephalic flexures) and a dorsal flexure (pontine flexure), all due to differential growth during embryogenesis. The pairs of terms used most commonly in neuroanatomy are: Note that such descriptors (dorsal/ventral, rostral/caudal; medial/lateral) are relative rather than absolute (e.g.,

19491-569: The CNS, and "efferent" neurons, which carry motor instructions out to the voluntary muscles of the body. The autonomic nervous system can work with or without the control of the CNS (that's why it is called 'autonomous'), and also has two subdivisions, called sympathetic and parasympathetic , which are important for transmitting motor orders to the body's basic internal organs, thus controlling functions such as heartbeat, breathing, digestion, and salivation. Autonomic nerves, unlike somatic nerves, contain only efferent fibers. Sensory signals coming from

19710-507: The DH were shown to cause spatial memory impairment while VH lesions did not. Its projecting pathways include the medial septal nucleus and supramammillary nucleus . The dorsal hippocampus also has more place cells than both the ventral and intermediate hippocampal regions. The intermediate hippocampus has overlapping characteristics with both the ventral and dorsal hippocampus. Using anterograde tracing methods, Cenquizca and Swanson (2007) located

19929-510: The DNA base excision repair pathway (see Epigenetics in learning and memory ). The between-systems memory interference model describes the inhibition of non-hippocampal systems of memory during concurrent hippocampal activity. Specifically, Fraser Sparks, Hugo Lehmann, and Robert Sutherland found that when the hippocampus was inactive, non-hippocampal systems located elsewhere in the brain were found to consolidate memory in its place. However, when

20148-448: The EC that originate in layer III are the origin of the direct perforant pathway and form synapses on the very distal apical dendrites of CA1 neurons. Conversely, axons originating from layer II are the origin of the indirect pathway, and information reaches CA1 via the trisynaptic circuit . In the initial part of this pathway, the axons project through the perforant pathway to the granule cells of

20367-403: The EC, so that it serves as the interface between the neocortex and the other connections, and the hippocampus. The EC is located in the parahippocampal gyrus , a cortical region adjacent to the hippocampus. This gyrus conceals the hippocampus. The parahippocampal gyrus is adjacent to the perirhinal cortex , which plays an important role in the visual recognition of complex objects. There

20586-498: The LIA mode, the EEG is dominated by sharp waves that are randomly timed large deflections of the EEG signal lasting for 25–50 milliseconds. Sharp waves are frequently generated in sets, with sets containing up to 5 or more individual sharp waves and lasting up to 500 ms. The spiking activity of neurons within the hippocampus is highly correlated with sharp wave activity. Most neurons decrease their firing rate between sharp waves; however, during

20805-481: The absence of conscious knowledge. For example, patients asked to guess which of two faces they have seen most recently may give the correct answer most of the time in spite of stating that they have never seen either of the faces before. Some researchers distinguish between conscious recollection , which depends on the hippocampus, and familiarity , which depends on portions of the medial temporal lobe. When rats are exposed to an intense learning event, they may retain

21024-668: The action potential, in the 1950s ( Alan Lloyd Hodgkin , Andrew Huxley and John Eccles ). It was in the 1960s that we became aware of how basic neuronal networks code stimuli and thus basic concepts are possible ( David H. Hubel and Torsten Wiesel ). The molecular revolution swept across US universities in the 1980s. It was in the 1990s that molecular mechanisms of behavioral phenomena became widely known ( Eric Richard Kandel )." A microscopic examination shows that nerves consist primarily of axons, along with different membranes that wrap around them and segregate them into fascicles . The neurons that give rise to nerves do not lie entirely within

21243-485: The actions of other people, and for learning new skills by imitation. Some researchers also speculate that mirror systems may simulate observed actions, and thus contribute to theory of mind skills, while others relate mirror neurons to language abilities. However, to date, no widely accepted neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions such as imitation. There are neuroscientists who caution that

21462-414: The adult human body ). Neurons are the information-processing cells of the nervous system: they sense our environment, communicate with each other via electrical signals and chemicals called neurotransmitters which generally act across synapses (close contacts between two neurons, or between a neuron and a muscle cell; note also extrasynaptic effects are possible, as well as release of neurotransmitters into

21681-432: The ancient Egyptian god often portrayed as such takes the name c ornu A mmonis . Its abbreviation CA is used in naming the hippocampal subfields CA1, CA2, CA3, and CA4 . It can be distinguished as an area where the cortex narrows into a single layer of densely packed pyramidal neurons , which curl into a tight U shape. One edge of the "U," – CA4, is embedded into the backward-facing, flexed dentate gyrus. The hippocampus

21900-402: The axis. Due to the axial brain flexures, no section plane ever achieves a complete section series in a selected plane, because some sections inevitably result cut oblique or even perpendicular to it, as they pass through the flexures. Experience allows to discern the portions that result cut as desired. According to these considerations, the three directions of space are represented precisely by

22119-501: The axons of neurons to their targets. A very important type of glial cell ( oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system) generates layers of a fatty substance called myelin that wraps around axons and provides electrical insulation which allows them to transmit action potentials much more rapidly and efficiently. Recent findings indicate that glial cells, such as microglia and astrocytes, serve as important resident immune cells within

22338-415: The behavior of the other, as though the observer were itself acting. Such neurons have been directly observed in primate species. Birds have been shown to have imitative resonance behaviors and neurological evidence suggests the presence of some form of mirroring system. In humans, brain activity consistent with that of mirror neurons has been found in the premotor cortex , the supplementary motor area ,

22557-493: The behavioral session. This enhancement of correlation, commonly known as reactivation , has been found to occur mainly during sharp waves. It has been proposed that sharp waves are, in fact, reactivations of neural activity patterns that were memorized during behavior, driven by strengthening of synaptic connections within the hippocampus. This idea forms a key component of the "two-stage memory" theory, advocated by Buzsáki and others, which proposes that memories are stored within

22776-502: The behaviors of animals, and most of the behaviors of humans, could be explained in terms of stimulus-response circuits, although he also believed that higher cognitive functions such as language were not capable of being explained mechanistically. Charles Sherrington , in his influential 1906 book The Integrative Action of the Nervous System , developed the concept of stimulus-response mechanisms in much more detail, and behaviorism ,

22995-402: The best known identified neurons are the gigantic Mauthner cells of fish. Every fish has two Mauthner cells, in the bottom part of the brainstem, one on the left side and one on the right. Each Mauthner cell has an axon that crosses over, innervating neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. The synapses generated by

23214-428: The body are linked by commissures (relatively large bundles of nerves). The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). The pedal ganglia, which control the foot, are below the esophagus and their commissure and connectives to the cerebral and pleural ganglia surround the esophagus in a circumesophageal nerve ring or nerve collar . A neuron

23433-410: The body to the CNS are called sensory nerves (afferent). The PNS is divided into two separate subsystems, the somatic and autonomic , nervous systems. The autonomic nervous system is further subdivided into the sympathetic , parasympathetic and enteric nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system

23652-417: The body, while all vertebrates have spinal cords that run along the dorsal midline. Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, earthworms have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by transverse nerves like

23871-413: The body. Nerves are made primarily of the axons or dendrites of neurons (axons in case of efferent motor fibres, and dendrites in case of afferent sensory fibres of the nerves), along with a variety of membranes that wrap around and segregate them into nerve fascicles . The vertebrate nervous system is divided into the central and peripheral nervous systems. The central nervous system (CNS) consists of

24090-428: The bottom, near the base of the temporal lobe , is much broader than the part at the top. This means that in cross-section the hippocampus can show a number of different shapes, depending on the angle and location of the cut. In a cross-section of the hippocampus, including the dentate gyrus, several layers will be shown. The dentate gyrus has three layers of cells (or four if the hilus is included). The layers are from

24309-414: The brain as the organ responsible for sensation and voluntary motion , as evidenced by his research on the neuroanatomy of oxen , Barbary apes , and other animals. The cultural taboo on human dissection continued for several hundred years afterward, which brought no major progress in the understanding of the anatomy of the brain or of the nervous system. However, Pope Sixtus IV effectively revitalized

24528-476: The brain is anatomically divided into the protocerebrum , deutocerebrum , and tritocerebrum . Immediately behind the brain is the subesophageal ganglion , which is composed of three pairs of fused ganglia. It controls the mouthparts , the salivary glands and certain muscles . Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation. The sensory information from these organs

24747-407: The brain largely contain astrocytes. The extracellular matrix also provides support on the molecular level for the brain's cells, vehiculating substances to and from the blood vessels. At the organ level, the nervous system is composed of brain regions, such as the hippocampus in mammals or the mushroom bodies of the fruit fly . These regions are often modular and serve a particular role within

24966-417: The brain that are either in the hippocampus itself or are strongly connected to it, such as the speed cells present in the medial entorhinal cortex . Together these cells form a network that serves as spatial memory. The first of such cells discovered in the 1970s were the place cells, which led to the idea of the hippocampus acting to give a neural representation of the environment in a cognitive map . When

25185-430: The brain to the spinal cord that are capable of enhancing or inhibiting the reflex. Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more complex responses rely on signal processing in the brain. For example, when an object in the periphery of the visual field moves, and a person looks toward it many stages of signal processing are initiated. The initial sensory response, in

25404-403: The brain. One target is a set of spinal interneurons that project to motor neurons controlling the arm muscles. The interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate action potentials, which travel down their axons to the point where they make excitatory synaptic contacts with muscle cells. The excitatory signals induce contraction of

25623-444: The brain. These 'physiologic' methods (because properties of living, unlesioned cells are used) can be combined with other procedures, and have essentially superseded the earlier procedures studying degeneration of lesioned neurons or axons. Detailed synaptic connections can be determined by correlative electron microscopy. Serial section electron microscopy has been extensively developed for use in studying nervous systems. For example,

25842-470: The brakes!") was very popular up to the 1960s. It derived much of its justification from two observations: first, that animals with hippocampal damage tend to be hyperactive ; second, that animals with hippocampal damage often have difficulty learning to inhibit responses that they have previously been taught, especially if the response requires remaining quiet as in a passive avoidance test. British psychologist Jeffrey Gray developed this line of thought into

26061-441: The calcar avis as hippocampus minor, has been attributed to Félix Vicq-d'Azyr systematizing nomenclature of parts of the brain in 1786. Mayer mistakenly used the term hippopotamus in 1779, and was followed by some other authors until Karl Friedrich Burdach resolved this error in 1829. In 1861 the hippocampus minor became the center of a dispute over human evolution between Thomas Henry Huxley and Richard Owen , satirized as

26280-463: The capacity of researchers to distinguish between different cell types (such as neurons and glia ) in various regions of the nervous system. In situ hybridization uses synthetic RNA probes that attach (hybridize) selectively to complementary mRNA transcripts of DNA exons in the cytoplasm, to visualize genomic readout, that is, distinguish active gene expression, in terms of mRNA rather than protein. This allows identification histologically (in situ) of

26499-420: The cell bodies of the neurons to which they belong reside within the brain or spinal cord. The PNS is divided into somatic and visceral parts. The somatic part consists of the nerves that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons lie in dorsal root ganglia of the spinal cord. The visceral part, also known as the autonomic nervous system, contains neurons that innervate

26718-589: The cells involved in the production of genetically-coded molecules, which often represent differentiation or functional traits, as well as the molecular boundaries separating distinct brain domains or cell populations. By expressing variable amounts of red, green, and blue fluorescent proteins in the brain, the so-called " brainbow " mutant mouse allows the combinatorial visualization of many different colors in neurons. This tags neurons with enough unique colors that they can often be distinguished from their neighbors with fluorescence microscopy , enabling researchers to map

26937-436: The central nervous system, processing the information to determine an appropriate response, and sending output signals to muscles or glands to activate the response. The evolution of a complex nervous system has made it possible for various animal species to have advanced perception abilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals. In humans,

27156-409: The central nervous system. The nervous system of vertebrates (including humans) is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is the major division, and consists of the brain and the spinal cord . The spinal canal contains the spinal cord, while the cranial cavity contains the brain. The CNS is enclosed and protected by the meninges ,

27375-399: The chemical constituents of the brain (including notably enzymes) to apply selective methods of reaction to visualize where they occur in the brain and any functional or pathological changes. This applies importantly to molecules related to neurotransmitter production and metabolism, but applies likewise in many other directions chemoarchitecture, or chemical neuroanatomy. Immunocytochemistry

27594-424: The claims being made for the role of mirror neurons are not supported by adequate research. In vertebrates, landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through

27813-471: The dentate gyrus (first synapse). From then, the information follows via the mossy fibres to CA3 (second synapse). From there, CA3 axons called Schaffer collaterals leave the deep part of the cell body and loop up to the apical dendrites and then extend to CA1 (third synapse). Axons from CA1 then project back to the entorhinal cortex, completing the circuit. Basket cells in CA3 receive excitatory input from

28032-404: The dentate gyrus and the subiculum in the hippocampal formation, and others also include the presubiculum, parasubiculum , and entorhinal cortex . The neural layout and pathways within the hippocampal formation are very similar in all mammals. The hippocampus, including the dentate gyrus, has the shape of a curved tube, which has been compared to a seahorse, and to a horn of a ram, which after

28251-412: The detection of new events, places and stimuli. Some researchers regard the hippocampus as part of a larger medial temporal lobe memory system responsible for general declarative memory (memories that can be explicitly verbalized – these would include, for example, memory for facts in addition to episodic memory). The hippocampus also encodes emotional context from the amygdala . This

28470-410: The direction a rat is moving, the destination toward which it is traveling, or other task-related variables. The firing of place cells is timed in relation to local theta waves , a process termed phase precession . In humans, cells with location-specific firing patterns have been reported during a study of patients with drug-resistant epilepsy . They were undergoing an invasive procedure to localize

28689-422: The earliest widely held hypothesis was that the hippocampus is involved in olfaction . This idea was cast into doubt by a series of anatomical studies that did not find any direct projections to the hippocampus from the olfactory bulb . However, later work did confirm that the olfactory bulb does project into the ventral part of the lateral entorhinal cortex, and field CA1 in the ventral hippocampus sends axons to

28908-566: The early 1970s, Sydney Brenner chose it as a model system for studying the way that genes control development, including neuronal development. One advantage of working with this worm is that the nervous system of the hermaphrodite contains exactly 302 neurons, always in the same places, making identical synaptic connections in every worm. Brenner's team sliced worms into thousands of ultrathin sections and photographed every section under an electron microscope, then visually matched fibers from section to section, to map out every neuron and synapse in

29127-412: The early symptoms. Damage to the hippocampus can also result from oxygen starvation ( hypoxia ), encephalitis , or medial temporal lobe epilepsy . People with extensive, bilateral hippocampal damage may experience anterograde amnesia : the inability to form and retain new memories . Since different neuronal cell types are neatly organized into layers in the hippocampus, it has frequently been used as

29346-480: The elderly than in the young. Furthermore, a randomized control trial published in 2011 found that aerobic exercise could increase the size of the hippocampus in adults aged 55 to 80 and also improve spatial memory. The hippocampus contains high levels of glucocorticoid receptors , which make it more vulnerable to long-term stress than most other brain areas . There is evidence that humans having experienced severe, long-lasting traumatic stress show atrophy of

29565-408: The electrical field across the membrane. If the change in electrical potential is large enough to pass the given threshold, it evokes an action potential, which is transmitted along the axon of the receptor cell, into the spinal cord. There the axon makes excitatory synaptic contacts with other cells, some of which project (send axonal output) to the same region of the spinal cord, others projecting into

29784-408: The embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory. All bilaterian animals at an early stage of development form a gastrula , which is polarized, with one end called the animal pole and the other the vegetal pole . The gastrula has

30003-482: The ensuing decision-making has been associated with anxiety . fMRI findings from studies in approach-avoidance decision-making found evidence for a functional role that is not explained by either long-term memory or spatial cognition. Overall findings showed that the anterior hippocampus is sensitive to conflict, and that it may be part of a larger cortical and subcortical network seen to be important in decision-making in uncertain conditions. A review makes reference to

30222-466: The entire body, to give a complete connectome of the nematode. Nothing approaching this level of detail is available for any other organism, and the information has been used to enable a multitude of studies that would not have been possible without it. Drosophila melanogaster is a popular experimental animal because it is easily cultured en masse from the wild, has a short generation time, and mutant animals are readily obtainable. Arthropods have

30441-407: The fastest routes between them in order to pass a strict test known as The Knowledge in order to gain a license to operate. A study showed that the posterior part of the hippocampus is larger in these drivers than in the general public, and that a positive correlation exists between the length of time served as a driver and the increase in the volume of this part. It was also found the total volume of

30660-548: The first application of serial block-face scanning electron microscopy was on rodent cortical tissue. Circuit reconstruction from data produced by this high-throughput method is challenging, and the Citizen science game EyeWire has been developed to aid research in that area. Is a field that utilizes various imaging modalities and computational techniques to model and quantify the spatiotemporal dynamics of neuroanatomical structures in both normal and clinical populations. Aside from

30879-514: The first to illustrate the structure, also wavered between "seahorse" and "silkworm". "Ram's horn" was proposed by the Danish anatomist Jacob Winsløw in 1732; and a decade later his fellow Parisian, the surgeon de Garengeot, used cornu Ammonis – horn of Amun , the ancient Egyptian god who was often represented as having a ram's head. Another reference appeared with the term pes hippocampi , which may date back to Diemerbroeck in 1672, introducing

31098-503: The form of electrochemical impulses traveling along thin fibers called axons , which can be directly transmitted to neighboring cells through electrical synapses or cause chemicals called neurotransmitters to be released at chemical synapses . A cell that receives a synaptic signal from a neuron may be excited , inhibited , or otherwise modulated . The connections between neurons can form neural pathways , neural circuits , and larger networks that generate an organism's perception of

31317-422: The form of electrochemical waves called action potentials , which produce cell-to-cell signals at points where axon terminals make synaptic contact with other cells. Synapses may be electrical or chemical. Electrical synapses make direct electrical connections between neurons, but chemical synapses are much more common, and much more diverse in function. At a chemical synapse, the cell that sends signals

31536-415: The general systemic pathways of the nervous system. For example, the hippocampus is critical for forming memories in connection with many other cerebral regions. The peripheral nervous system also contains afferent or efferent nerves , which are bundles of fibers that originate from the brain and spinal cord, or from sensory or motor sorts of peripheral ganglia, and branch repeatedly to innervate every part of

31755-468: The great majority of neurons participate in the formation of centralized structures (the brain and ganglia) and they receive all of their input from other neurons and send their output to other neurons. Glial cells (named from the Greek for "glue") are non-neuronal cells that provide support and nutrition , maintain homeostasis , form myelin , and participate in signal transmission in the nervous system. In

31974-422: The hippocampi in both hemispheres results in profound difficulties in forming new memories ( anterograde amnesia ) and often also affects memories formed before the damage occurred ( retrograde amnesia ). Although the retrograde effect normally extends many years back before the brain damage, in some cases older memories remain. This retention of older memories leads to the idea that consolidation over time involves

32193-615: The hippocampus neuronal genome of the rats at one hour and at 24 hours after training. These alterations in methylation pattern occurred at many genes that were down-regulated , often due to the formation of new 5-methylcytosine sites in CpG rich regions of the genome. Furthermore, many other genes were upregulated , likely often due to the removal of methyl groups from previously existing 5-methylcytosines (5mCs) in DNA. Demethylation of 5mC can be carried out by several proteins acting in concert, including TET enzymes as well as enzymes of

32412-422: The hippocampus and occurs at synapses that terminate on dendritic spines and use the neurotransmitter glutamate . The synaptic changes depend on a special type of glutamate receptor , the N -methyl-D-aspartate (NMDA) receptor , a cell surface receptor which has the special property of allowing calcium to enter the postsynaptic spine only when presynaptic activation and postsynaptic depolarization occur at

32631-401: The hippocampus are shown to be functionally and anatomically distinct. The dorsal hippocampus (DH), ventral hippocampus (VH) and intermediate hippocampus serve different functions, project with differing pathways, and have varying degrees of place cells. The dorsal hippocampus serves for spatial memory, verbal memory, and learning of conceptual information. Using the radial arm maze , lesions in

32850-475: The hippocampus during behavior and then later transferred to the neocortex during sleep. Sharp waves in Hebbian theory are seen as persistently repeated stimulations by presynaptic cells, of postsynaptic cells that are suggested to drive synaptic changes in the cortical targets of hippocampal output pathways. Suppression of sharp waves and ripples in sleep or during immobility can interfere with memories expressed at

33069-404: The hippocampus is dysfunctional, orientation is affected; people may have difficulty in remembering how they arrived at a location and how to proceed further. Getting lost is a common symptom of amnesia. Studies with animals have shown that an intact hippocampus is required for initial learning and long-term retention of some spatial memory tasks, in particular ones that require finding the way to

33288-432: The hippocampus more than of other parts of the brain. These effects show up in post-traumatic stress disorder , and they may contribute to the hippocampal atrophy reported in schizophrenia and severe depression . Anterior hippocampal volume in children is positively correlated with parental family income and this correlation is thought to be mediated by income related stress. A recent study has also revealed atrophy as

33507-542: The hippocampus of elderly people , but later studies using more precise techniques found only minimal differences. Similarly, some MRI studies have reported shrinkage of the hippocampus in elderly people, but other studies have failed to reproduce this finding. There is, however, a reliable relationship between the size of the hippocampus and memory performance; so that where there is age-related shrinkage, memory performance will be impaired. There are also reports that memory tasks tend to produce less hippocampal activation in

33726-530: The hippocampus to space. The spatial theory was originally championed by O'Keefe and Nadel, who were influenced by American psychologist E.C. Tolman's theories about " cognitive maps " in humans and animals. O'Keefe and his student Dostrovsky in 1971 discovered neurons in the rat hippocampus that appeared to them to show activity related to the rat's location within its environment. Despite skepticism from other investigators, O'Keefe and his co-workers, especially Lynn Nadel, continued to investigate this question, in

33945-453: The hippocampus was reactivated, memory traces consolidated by non-hippocampal systems were not recalled, suggesting that the hippocampus interferes with long-term memory consolidation in other memory-related systems. One of the major implications that this model illustrates is the dominant effects of the hippocampus on non-hippocampal networks when information is incongruent. With this information in mind, future directions could lead towards

34164-485: The hippocampus was unchanged, as the increase seen in the posterior part was made at the expense of the anterior part, which showed a relative decrease in size. There have been no reported adverse effects from this disparity in hippocampal proportions. Another study showed opposite findings in blind individuals. The anterior part of the right hippocampus was larger and the posterior part was smaller, compared with sighted individuals. There are several navigational cells in

34383-464: The internal organs, blood vessels, and glands. The autonomic nervous system itself consists of two parts: the sympathetic nervous system and the parasympathetic nervous system . Some authors also include sensory neurons whose cell bodies lie in the periphery (for senses such as hearing) as part of the PNS; others, however, omit them. The vertebrate nervous system can also be divided into areas called gray matter and white matter . Gray matter (which

34602-433: The left posterior, left anterior or right anterior hippocampus demonstrate distinct, individual cell responses when presented with faces of presumably recognizable famous people. Associations among facial and vocal identity were similarly mapped to the hippocampus of rheseus monkeys. Single neurons in the CA1 and CA3 responded strongly to social stimulus recognition by MRI. The CA2 was not distinguished, and may likely comprise

34821-402: The level of the behavior, nonetheless, the newly formed CA1 place cell code can re-emerge even after a sleep with abolished sharp waves and ripples, in spatially non-demanding tasks. Since at least the time of Ramon y Cajal (1852–1934), psychologists have speculated that the brain stores memory by altering the strength of connections between neurons that are simultaneously active. This idea

35040-734: The level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature. On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands. The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain. Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups ( superphyla ) called protostomes and deuterostomes . Deuterostomes include vertebrates as well as echinoderms , hemichordates (mainly acorn worms), and Xenoturbellidans . Protostomes,

35259-524: The local connections or mutual arrangement (tiling) between neurons. Optogenetics uses transgenic constitutive and site-specific expression (normally in mice) of blocked markers that can be activated selectively by illumination with a light beam. This allows researchers to study axonal connectivity in the nervous system in a very discriminative way. Magnetic resonance imaging has been used extensively to investigate brain structure and function non-invasively in healthy human subjects. An important example

35478-554: The main olfactory bulb, the anterior olfactory nucleus, and to the primary olfactory cortex. There continues to be some interest in hippocampal olfactory responses, in particular, the role of the hippocampus in memory for odors, but few specialists today believe that olfaction is its primary function. Over the years, three main ideas of hippocampal function have dominated the literature: response inhibition , episodic memory , and spatial cognition. The behavioral inhibition theory (caricatured by John O'Keefe and Lynn Nadel as "slam on

35697-460: The medial septum is more than just the controller of theta; it is also the main source of cholinergic projections to the hippocampus. It has not been established that septal lesions exert their effects specifically by eliminating the theta rhythm. During sleep or during resting, when an animal is not engaged with its surroundings, the hippocampal EEG shows a pattern of irregular slow waves, somewhat larger in amplitude than theta waves. This pattern

35916-485: The membrane are activated, and cause the contents of the vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the synaptic cleft . The neurotransmitter then binds to receptors embedded in the postsynaptic membrane, causing them to enter an activated state. Depending on the type of receptor, the resulting effect on the postsynaptic cell may be excitatory, inhibitory, or modulatory in more complex ways. For example, release of

36135-490: The moderate projections to two primary olfactory cortical areas and prelimbic areas of the medial prefrontal cortex . This region has the smallest number of place cells. The ventral hippocampus functions in fear conditioning and affective processes. Anagnostaras et al. (2002) showed that alterations to the ventral hippocampus reduced the amount of information sent to the amygdala by the dorsal and ventral hippocampus, consequently altering fear conditioning in rats. Historically,

36354-414: The more diverse group, include arthropods , molluscs , and numerous phyla of "worms". There is a basic difference between the two groups in the placement of the nervous system within the body: protostomes possess a nerve cord on the ventral (usually bottom) side of the body, whereas in deuterostomes the nerve cord is on the dorsal (usually top) side. In fact, numerous aspects of the body are inverted between

36573-542: The most active of them. An active cell typically stays active for half a second to a few seconds. As the rat behaves, the active cells fall silent and new cells become active, but the overall percentage of active cells remains more or less constant. In many situations, cell activity is determined largely by the spatial location of the animal, but other behavioral variables also clearly influence it. The LIA mode appears during slow-wave (non-dreaming) sleep, and also during states of waking immobility such as resting or eating. In

36792-440: The most intensively studied subject in medical history. In the ensuing years, other patients with similar levels of hippocampal damage and amnesia (caused by accident or disease) have also been studied, and thousands of experiments have studied the physiology of activity-driven changes in synaptic connections in the hippocampus. There is now universal agreement that the hippocampi play some sort of important role in memory; however,

37011-410: The muscle cells, which causes the joint angles in the arm to change, pulling the arm away. In reality, this straightforward schema is subject to numerous complications. Although for the simplest reflexes there are short neural paths from sensory neuron to motor neuron, there are also other nearby neurons that participate in the circuit and modulate the response. Furthermore, there are projections from

37230-460: The neighboring entorhinal cortex . The earliest description of the ridge running along the floor of the temporal horn of the lateral ventricle comes from the Venetian anatomist Julius Caesar Aranzi (1587), who likened it first to a silkworm and then to a seahorse ( Latin hippocampus , from Greek ἱππόκαμπος, from ἵππος, 'horse' + κάμπος, 'sea monster'). The German anatomist Duvernoy (1729),

37449-426: The nerves themselves—their cell bodies reside within the brain, spinal cord , or peripheral ganglia . All animals more advanced than sponges have nervous systems. However, even sponges , unicellular animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are precursors to those of neurons. In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of

37668-511: The nervous system cytoarchitecture . The classic Golgi stain uses potassium dichromate and silver nitrate to fill selectively with a silver chromate precipitate a few neural cells (neurons or glia, but in principle, any cells can react similarly). This so-called silver chromate impregnation procedure stains entirely or partially the cell bodies and neurites of some neurons - dendrites , axon - in brown and black, allowing researchers to trace their paths up to their thinnest terminal branches in

37887-403: The nervous system and looks for interventions that can prevent or treat them. In the peripheral nervous system, the most common problem is the failure of nerve conduction, which can be due to different causes including diabetic neuropathy and demyelinating disorders such as multiple sclerosis and amyotrophic lateral sclerosis . Neuroscience is the field of science that focuses on the study of

38106-402: The nervous system as well as many peripheral organs, but in mammals, all of these "tissue clocks" are kept in synchrony by signals that emanate from a master timekeeper in a tiny part of the brain called the suprachiasmatic nucleus . A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. Thus, the neuron "mirrors"

38325-561: The nervous system as well. However, there are some techniques that have been developed especially for the study of neuroanatomy. In biological systems, staining is a technique used to enhance the contrast of particular features in microscopic images. Nissl staining uses aniline basic dyes to intensely stain the acidic polyribosomes in the rough endoplasmic reticulum , which is abundant in neurons. This allows researchers to distinguish between different cell types (such as neurons and glia ), and neuronal shapes and sizes, in various regions of

38544-496: The nervous system in radiata is relatively unstructured. Unlike bilaterians , radiata only have two primordial cell layers, endoderm and ectoderm . Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type. The vast majority of existing animals are bilaterians , meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from

38763-531: The nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants . The central nervous system functions to send signals from one cell to others, or from one part of the body to others and to receive feedback. Malfunction of the nervous system can occur as a result of genetic defects, physical damage due to trauma or toxicity, infection, or simply senescence . The medical specialty of neurology studies disorders of

38982-435: The nervous system. The nervous system derives its name from nerves, which are cylindrical bundles of fibers (the axons of neurons ), that emanate from the brain and spinal cord , and branch repeatedly to innervate every part of the body. Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using

39201-453: The neural extracellular space), and produce our memories, thoughts, and movements. Glial cells maintain homeostasis, produce myelin (oligodendrocytes, Schwann cells) , and provide support and protection for the brain's neurons. Some glial cells ( astrocytes ) can even propagate intercellular calcium waves over long distances in response to stimulation, and release gliotransmitters in response to changes in calcium concentration. Wound scars in

39420-457: The neurodegenerative disorders Parkinson's, Huntington's, spinocerebellar ataxia and Alzheimer's disease. In spite of the large evolutionary distance between insects and mammals, many basic aspects of Drosophila neurogenetics have turned out to be relevant to humans. For instance, the first biological clock genes were identified by examining Drosophila mutants that showed disrupted daily activity cycles. Nervous system In biology ,

39639-486: The neurotransmitter acetylcholine at a synaptic contact between a motor neuron and a muscle cell induces rapid contraction of the muscle cell. The entire synaptic transmission process takes only a fraction of a millisecond, although the effects on the postsynaptic cell may last much longer (even indefinitely, in cases where the synaptic signal leads to the formation of a memory trace ). There are literally hundreds of different types of synapses. In fact, there are over

39858-450: The neurotransmitter glutamate acting on a special type of receptor known as the NMDA receptor . The NMDA receptor has an "associative" property: if the two cells involved in the synapse are both activated at approximately the same time, a channel opens that permits calcium to flow into the target cell. The calcium entry initiates a second messenger cascade that ultimately leads to an increase in

40077-474: The number of glutamate receptors in the target cell, thereby increasing the effective strength of the synapse. This change in strength can last for weeks or longer. Since the discovery of LTP in 1973, many other types of synaptic memory traces have been found, involving increases or decreases in synaptic strength that are induced by varying conditions, and last for variable periods of time. The reward system , that reinforces desired behaviour for example, depends on

40296-438: The only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response individually, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response. Mauthner cells have been described as command neurons . A command neuron

40515-410: The outer in – the molecular layer , the inner molecular layer , the granular layer , and the hilus . The CA3 in the hippocampus proper has the following cell layers known as strata: lacunosum-moleculare, radiatum, lucidum, pyramidal, and oriens. CA2 and CA1 also have these layers except the lucidum stratum . The input to the hippocampus (from varying cortical and subcortical structures) comes from

40734-422: The output to the EC, additional output pathways go to other cortical areas including the prefrontal cortex . A major output goes via the fornix to the lateral septal area and to the mammillary body of the hypothalamus (which the fornix interconnects with the hippocampus). The hippocampus receives modulatory input from the serotonin , norepinephrine , and dopamine systems, and from the nucleus reuniens of

40953-402: The paths and connections of all of the 302 neurons in this species. The fruit fly is widely studied in part because its genetics is very well understood and easily manipulated. The mouse is used because, as a mammal, its brain is more similar in structure to our own (e.g., it has a six-layered cortex , yet its genes can be easily modified and its reproductive cycle is relatively fast). The brain

41172-444: The phase with which theta rhythms, at the time of stimulation of a neuron, shape the effect of that stimulation upon its synapses. What is meant here is that theta rhythms may affect those aspects of learning and memory that are dependent upon synaptic plasticity . It is well established that lesions of the medial septum  – the central node of the theta system – cause severe disruptions of memory. However,

41391-430: The point of near convergence. In an attempt to reconcile the two disparate views, it is suggested that a broader view of the hippocampal function is taken and seen to have a role that encompasses both the organisation of experience ( mental mapping , as per Tolman's original concept in 1948) and the directional behaviour seen as being involved in all areas of cognition, so that the function of the hippocampus can be viewed as

41610-428: The possibilities for generating intricate temporal patterns become far more extensive. A modern conception views the function of the nervous system partly in terms of stimulus-response chains, and partly in terms of intrinsically generated activity patterns—both types of activity interact with each other to generate the full repertoire of behavior. The simplest type of neural circuit is a reflex arc , which begins with

41829-517: The precise nature of this role remains widely debated. A recent theory proposed – without questioning its role in spatial cognition – that the hippocampus encodes new episodic memories by associating representations in the newborn granule cells of the dentate gyrus and arranging those representations sequentially in the CA3 by relying on the phase precession generated in the entorhinal cortex . The third important theory of hippocampal function relates

42048-408: The pyramidal cells and then give an inhibitory feedback to the pyramidal cells. This recurrent inhibition is a simple feedback circuit that can dampen excitatory responses in the hippocampus. The pyramidal cells give a recurrent excitation which is an important mechanism found in some memory processing microcircuits. Several other connections play important roles in hippocampal function. Beyond

42267-783: The receptors that it activates. Because different targets can (and frequently do) use different types of receptors, it is possible for a neuron to have excitatory effects on one set of target cells, inhibitory effects on others, and complex modulatory effects on others still. Nevertheless, it happens that the two most widely used neurotransmitters, glutamate and GABA , each have largely consistent effects. Glutamate has several widely occurring types of receptors, but all of them are excitatory or modulatory. Similarly, GABA has several widely occurring receptor types, but all of them are inhibitory. Because of this consistency, glutamatergic cells are frequently referred to as "excitatory neurons", and GABAergic cells as "inhibitory neurons". Strictly speaking, this

42486-607: The results of surgical destruction of the hippocampi when trying to relieve epileptic seizures in an American man Henry Molaison , known until his death in 2008 as "Patient H.M." The unexpected outcome of the surgery was severe anterograde and partial retrograde amnesia ; Molaison was unable to form new episodic memories after his surgery and could not remember any events that occurred just before his surgery, but he did retain memories of events that occurred many years earlier extending back into his childhood. This case attracted such widespread professional interest that Molaison became

42705-646: The retina of the eye, and the final motor response, in the oculomotor nuclei of the brainstem , are not all that different from those in a simple reflex, but the intermediate stages are completely different. Instead of a one or two step chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the thalamus , cerebral cortex , basal ganglia , superior colliculus , cerebellum , and several brainstem nuclei. These areas perform signal-processing functions that include feature detection , perceptual analysis, memory recall , decision-making , and motor planning . Feature detection

42924-510: The roundworm C. elegans , whose nervous system is the most thoroughly described of any animal's, every neuron in the body is uniquely identifiable, with the same location and the same connections in every individual worm. One notable consequence of this fact is that the form of the C. elegans nervous system is completely specified by the genome, with no experience-dependent plasticity. The brains of many molluscs and insects also contain substantial numbers of identified neurons. In vertebrates,

43143-399: The rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two ganglia at the head (the " nerve ring ") end function similar to a simple brain . Photoreceptors on the animal's eyespots provide sensory information on light and dark. The nervous system of one very small roundworm, the nematode Caenorhabditis elegans , has been completely mapped out in

43362-453: The sagittal, transverse and horizontal planes, whereas coronal sections can be transverse, oblique or horizontal, depending on how they relate to the brain axis and its incurvations. Modern developments in neuroanatomy are directly correlated to the technologies used to perform research . Therefore, it is necessary to discuss the various tools that are available. Many of the histological techniques used to study other tissues can be applied to

43581-465: The same time. Drugs that interfere with NMDA receptors block LTP and have major effects on some types of memory, especially spatial memory. Genetically modified mice that are modified to disable the LTP mechanism, also generally show severe memory deficits. Age-related conditions such as Alzheimer's disease and other forms of dementia (for which hippocampal disruption is one of the earliest signs ) have

43800-713: The school of thought that dominated psychology through the middle of the 20th century, attempted to explain every aspect of human behavior in stimulus-response terms. However, experimental studies of electrophysiology , beginning in the early 20th century and reaching high productivity by the 1940s, showed that the nervous system contains many mechanisms for maintaining cell excitability and generating patterns of activity intrinsically, without requiring an external stimulus. Neurons were found to be capable of producing regular sequences of action potentials, or sequences of bursts, even in complete isolation. When intrinsically active neurons are connected to each other in complex circuits,

44019-400: The sensitivity of the cell to stimuli, or even altering gene transcription . According to a rule called Dale's principle , which has only a few known exceptions, a neuron releases the same neurotransmitters at all of its synapses. This does not mean, though, that a neuron exerts the same effect on all of its targets, because the effect of a synapse depends not on the neurotransmitter, but on

44238-412: The set of structures that line the deep edge of the cortex (Latin limbus meaning border ): These include the hippocampus, cingulate cortex , olfactory cortex , and amygdala . Paul MacLean later suggested that the limbic structures comprise the neural basis of emotion. The hippocampus is anatomically connected to parts of the brain that are involved with emotional behavior – the septum ,

44457-434: The shape of a disk with three layers of cells, an inner layer called the endoderm , which gives rise to the lining of most internal organs, a middle layer called the mesoderm , which gives rise to the bones and muscles, and an outer layer called the ectoderm , which gives rise to the skin and nervous system. Hippocampus The hippocampus ( pl. : hippocampi ; via Latin from Greek ἱππόκαμπος , ' seahorse ')

44676-531: The sleep-wake cycle. Experimental studies dating from the 1990s have shown that circadian rhythms are generated by a "genetic clock" consisting of a special set of genes whose expression level rises and falls over the course of the day. Animals as diverse as insects and vertebrates share a similar genetic clock system. The circadian clock is influenced by light but continues to operate even when light levels are held constant and no other external time-of-day cues are available. The clock genes are expressed in many parts of

44895-498: The sophistication of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other features of human society that would not exist without the human brain. Most neurons send signals via their axons , although some types are capable of dendrite-to-dendrite communication. (In fact, the types of neurons called amacrine cells have no axons, and communicate only via their dendrites.) Neural signals propagate along an axon in

45114-424: The source of their seizures , with a view to surgical resection. The patients had diagnostic electrodes implanted in their hippocampus and then used a computer to move around in a virtual reality town. Similar brain imaging studies in navigation have shown the hippocampus to be active. A study was carried out on taxi drivers. London's black cab drivers need to learn the locations of a large number of places and

45333-476: The spiking of hippocampal neurons and synchronise across the hippocampus in a travelling wave pattern. The trisynaptic circuit is a relay of neurotransmission in the hippocampus that interacts with many brain regions. From rodent studies it has been proposed that the trisynaptic circuit generates the hippocampal theta rhythm. Theta rhythmicity is very obvious in rabbits and rodents and also clearly present in cats and dogs. Whether theta can be seen in primates

45552-399: The spinal cord and in peripheral sensory organs such as the retina. Although stimulus-response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity. Because of the variety of voltage-sensitive ion channels that can be embedded in the membrane of

45771-503: The study of neuroanatomy by altering the papal policy and allowing human dissection. This resulted in a flush of new activity by artists and scientists of the Renaissance, such as Mondino de Luzzi , Berengario da Carpi , and Jacques Dubois , and culminating in the work of Andreas Vesalius . In 1664, Thomas Willis , a physician and professor at Oxford University, coined the term neurology when he published his text Cerebri Anatome which

45990-445: The study of these non-hippocampal memory systems through hippocampal inactivation, further expanding the labile constructs of memory. Additionally, many theories of memory are holistically based around the hippocampus. This model could add beneficial information to hippocampal research and memory theories such as the multiple trace theory . Lastly, the between-system memory interference model allows researchers to evaluate their results on

46209-401: The theta mode, the EEG is dominated by large regular waves with a frequency range of 6 to 9 Hz , and the main groups of hippocampal neurons ( pyramidal cells and granule cells ) show sparse population activity, which means that in any short time interval, the great majority of cells are silent, while the small remaining fraction fire at relatively high rates, up to 50 spikes in one second for

46428-426: The theta wave are generated mainly by densely packed neural layers of the entorhinal cortex, CA3, and the dendrites of pyramidal cells. The theta wave is one of the largest signals seen on EEG, and is known as the hippocampal theta rhythm . In some situations the EEG is dominated by regular waves at 3 to 10 Hz, often continuing for many seconds. These reflect subthreshold membrane potentials and strongly modulate

46647-499: The transfer of memories out of the hippocampus to other parts of the brain. Experiments using intrahippocampal transplantation of hippocampal cells in primates with neurotoxic lesions of the hippocampus have shown that the hippocampus is required for the formation and recall, but not the storage, of memories. It has been shown that a decrease in the volume of various parts of the hippocampus in people leads to specific memory impairments. In particular, efficiency of verbal memory retention

46866-419: The two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients. Most anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed by Geoffroy Saint-Hilaire for insects in comparison to vertebrates. Thus insects, for example, have nerve cords that run along the ventral midline of

47085-450: The vector of inheritance for genes. Because of the large array of tools available for studying Drosophila genetics, they have been a natural subject for studying the role of genes in the nervous system. The genome has been sequenced and published in 2000. About 75% of known human disease genes have a recognizable match in the genome of fruit flies. Drosophila is being used as a genetic model for several human neurological diseases including

47304-415: The viscera course into the CNS through the somatic sensory nerves (e.g., visceral pain), or through some particular cranial nerves (e.g., chemosensitive or mechanic signals). In anatomy in general and neuroanatomy in particular, several sets of topographic terms are used to denote orientation and location, which are generally referred to the body or brain axis (see Anatomical terms of location ). The axis of

47523-674: The world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support. Many of the cells and vasculature channels within the nervous system make up the neurovascular unit , which regulates cerebral blood flow in order to rapidly satisfy the high energy demands of activated neurons. Nervous systems are found in most multicellular animals , but vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges , placozoans , and mesozoans , which have very simple body plans. The nervous systems of

47742-402: Was formalized by Donald Hebb in 1949, but for many years remained unexplained. In 1973, Tim Bliss and Terje Lømo described a phenomenon in the rabbit hippocampus that appeared to meet Hebb's specifications: a change in synaptic responsiveness induced by brief strong activation and lasting for hours or days or longer. This phenomenon was soon referred to as long-term potentiation (LTP). As

47961-461: Was suggested that the mechanisms of memory and planning both evolved from mechanisms of navigation and that their neuronal algorithms were basically the same. Many studies have made use of neuroimaging techniques such as functional magnetic resonance imaging (fMRI), and a functional role in approach-avoidance conflict has been noted. The anterior hippocampus is seen to be involved in decision-making under approach-avoidance conflict processing. It

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