Misplaced Pages

#859140

100-406: The neuraxis or sometimes neuroaxis is the axis of the central nervous system . It denotes the direction in which the central nervous system lies. During embryological development, the neuraxis is bent by various flexures , contributing to the mature structure of the brain and spinal cord . Embryonic development can help in understanding the structure of the adult brain because it establishes

200-469: A myelinated axon , which are found periodically interspersed between segments of the myelin sheath. Therefore, at the point of the node of Ranvier, the axon is reduced in diameter. These nodes are areas where action potentials can be generated. In saltatory conduction , electrical currents produced at each node of Ranvier are conducted with little attenuation to the next node in line, where they remain strong enough to generate another action potential. Thus in

300-459: A barrier to chemicals dissolved in the blood, protecting the brain from most neurotoxins commonly found in food. Within the meninges the brain and spinal cord are bathed in cerebral spinal fluid which replaces the body fluid found outside the cells of all bilateral animals . In vertebrates, the CNS is contained within the dorsal body cavity , while the brain is housed in the cranial cavity within

400-407: A framework on which more complex structures can be built. First, the neural tube establishes the anterior–posterior dimension of the nervous system, which is called the neuraxis. The embryonic nervous system in mammals can be said to have a standard arrangement. Humans (and other primates, to some degree) make this complicated by standing up and walking on two legs. The anterior–posterior dimension of

500-482: A myelinated axon, action potentials effectively "jump" from node to node, bypassing the myelinated stretches in between, resulting in a propagation speed much faster than even the fastest unmyelinated axon can sustain. An axon can divide into many branches called telodendria (Greek for 'end of tree'). At the end of each telodendron is an axon terminal (also called a terminal bouton or synaptic bouton, or end-foot ). Axon terminals contain synaptic vesicles that store

600-425: A nerve in the peripheral nervous system can be described as neurapraxia , axonotmesis , or neurotmesis . Concussion is considered a mild form of diffuse axonal injury . Axonal injury can also cause central chromatolysis . The dysfunction of axons in the nervous system is one of the major causes of many inherited and acquired neurological disorders that affect both peripheral and central neurons. When an axon

700-514: A neurite, causing it to elongate, will make it become an axon. Nonetheless, axonal development is achieved through a complex interplay between extracellular signaling, intracellular signaling and cytoskeletal dynamics. The extracellular signals that propagate through the extracellular matrix surrounding neurons play a prominent role in axonal development. These signaling molecules include proteins, neurotrophic factors , and extracellular matrix and adhesion molecules. Netrin (also known as UNC-6)

800-505: A neurite, converting it into an axon. As such, the overexpression of phosphatases that dephosphorylate PtdIns leads into the failure of polarization. The neurite with the lowest actin filament content will become the axon. PGMS concentration and f-actin content are inversely correlated; when PGMS becomes enriched at the tip of a neurite, its f-actin content is substantially decreased. In addition, exposure to actin-depolimerizing drugs and toxin B (which inactivates Rho-signaling ) causes

900-422: A number of primitive emotions or feelings such as hunger , thirst and maternal bonding . This is regulated partly through control of secretion of hormones from the pituitary gland . Additionally the hypothalamus plays a role in motivation and many other behaviors of the individual. The cerebrum of cerebral hemispheres make up the largest visual portion of the human brain. Various structures combine to form

1000-403: A presynaptic terminal, it activates the synaptic transmission process. The first step is rapid opening of calcium ion channels in the membrane of the axon, allowing calcium ions to flow inward across the membrane. The resulting increase in intracellular calcium concentration causes synaptic vesicles (tiny containers enclosed by a lipid membrane) filled with a neurotransmitter chemical to fuse with

1100-412: A region called the ventricular zone . The neural stem cells, principally radial glial cells , multiply and generate neurons through the process of neurogenesis , forming the rudiment of the CNS. The neural tube gives rise to both brain and spinal cord . The anterior (or 'rostral') portion of the neural tube initially differentiates into three brain vesicles (pockets): the prosencephalon at

SECTION 10

#1732877275860

1200-554: A secreted protein, functions in axon formation. When the UNC-5 netrin receptor is mutated, several neurites are irregularly projected out of neurons and finally a single axon is extended anteriorly. The neurotrophic factors – nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NTF3) are also involved in axon development and bind to Trk receptors . The ganglioside -converting enzyme plasma membrane ganglioside sialidase (PMGS), which

1300-561: A specific clinical question and not as routine screening. Axon An axon (from Greek ἄξων áxōn , axis) or nerve fiber (or nerve fibre : see spelling differences ) is a long, slender projection of a nerve cell, or neuron , in vertebrates , that typically conducts electrical impulses known as action potentials away from the nerve cell body . The function of the axon is to transmit information to different neurons, muscles, and glands. In certain sensory neurons ( pseudounipolar neurons ), such as those for touch and warmth,

1400-466: Is also subcortical gray matter making up a large number of different nuclei . From and to the spinal cord are projections of the peripheral nervous system in the form of spinal nerves (sometimes segmental nerves ). The nerves connect the spinal cord to skin, joints, muscles etc. and allow for the transmission of efferent motor as well as afferent sensory signals and stimuli. This allows for voluntary and involuntary motions of muscles, as well as

1500-469: Is blocked and neutralized, it is possible to induce long-distance axonal regeneration which leads to enhancement of functional recovery in rats and mouse spinal cord. This has yet to be done on humans. A recent study has also found that macrophages activated through a specific inflammatory pathway activated by the Dectin-1 receptor are capable of promoting axon recovery, also however causing neurotoxicity in

1600-488: Is close to 1 millimeter in diameter, the size of a small pencil lead. The numbers of axonal telodendria (the branching structures at the end of the axon) can also differ from one nerve fiber to the next. Axons in the central nervous system (CNS) typically show multiple telodendria, with many synaptic end points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate branching allows for

1700-430: Is crushed, an active process of axonal degeneration takes place at the part of the axon furthest from the cell body. This degeneration takes place quickly following the injury, with the part of the axon being sealed off at the membranes and broken down by macrophages. This is known as Wallerian degeneration . Dying back of an axon can also take place in many neurodegenerative diseases , particularly when axonal transport

1800-403: Is distinct from somatic action potentials in three ways: 1. The signal has a shorter peak-trough duration (~150μs) than of pyramidal cells (~500μs) or interneurons (~250μs). 2. The voltage change is triphasic. 3. Activity recorded on a tetrode is seen on only one of the four recording wires. In recordings from freely moving rats, axonal signals have been isolated in white matter tracts including

1900-604: Is impaired, this is known as Wallerian-like degeneration. Studies suggest that the degeneration happens as a result of the axonal protein NMNAT2 , being prevented from reaching all of the axon. Demyelination of axons causes the multitude of neurological symptoms found in the disease multiple sclerosis . Dysmyelination is the abnormal formation of the myelin sheath. This is implicated in several leukodystrophies , and also in schizophrenia . A severe traumatic brain injury can result in widespread lesions to nerve tracts damaging

2000-507: Is involved in the activation of TrkA at the tip of neutrites, is required for the elongation of axons. PMGS asymmetrically distributes to the tip of the neurite that is destined to become the future axon. During axonal development, the activity of PI3K is increased at the tip of destined axon. Disrupting the activity of PI3K inhibits axonal development. Activation of PI3K results in the production of phosphatidylinositol (3,4,5)-trisphosphate (PtdIns) which can cause significant elongation of

2100-404: Is significant in that it consists of CNS tissue expressed in direct contact to the environment, allowing for administration of certain pharmaceuticals and drugs. At the anterior end of the spinal cord lies the brain. The brain makes up the largest portion of the CNS. It is often the main structure referred to when speaking of the nervous system in general. The brain is the major functional unit of

SECTION 20

#1732877275860

2200-402: Is the corpus callosum that connects the two cerebral hemispheres , and this has around 20 million axons. The structure of a neuron is seen to consist of two separate functional regions, or compartments – the cell body together with the dendrites as one region, and the axonal region as the other. The axonal region or compartment, includes the axon hillock, the initial segment,

2300-411: Is the area formed from the cell body of the neuron as it extends to become the axon. It precedes the initial segment. The received action potentials that are summed in the neuron are transmitted to the axon hillock for the generation of an action potential from the initial segment. The axonal initial segment (AIS) is a structurally and functionally separate microdomain of the axon. One function of

2400-426: Is thought to carry a different cargo. The studies on transport in the axon led to the naming of kinesin. In the nervous system, axons may be myelinated , or unmyelinated. This is the provision of an insulating layer, called a myelin sheath. The myelin membrane is unique in its relatively high lipid to protein ratio. In the peripheral nervous system axons are myelinated by glial cells known as Schwann cells . In

2500-449: Is unmyelinated and contains a specialized complex of proteins. It is between approximately 20 and 60 μm in length and functions as the site of action potential initiation. Both the position on the axon and the length of the AIS can change showing a degree of plasticity that can fine-tune the neuronal output. A longer AIS is associated with a greater excitability. Plasticity is also seen in

2600-418: The capillaries of the brain. Upon CNS injury astrocytes will proliferate, causing gliosis , a form of neuronal scar tissue, lacking in functional neurons. The brain ( cerebrum as well as midbrain and hindbrain ) consists of a cortex , composed of neuron-bodies constituting gray matter, while internally there is more white matter that form tracts and commissures . Apart from cortical gray matter there

2700-456: The cerebellum and transmit information between the cerebellum and the cerebral cortex . In the dorsal posterior pons lie nuclei that are involved in the functions of breathing, sleep, and taste. The midbrain, or mesencephalon, is situated above and rostral to the pons. It includes nuclei linking distinct parts of the motor system, including the cerebellum, the basal ganglia and both cerebral hemispheres , among others. Additionally, parts of

2800-418: The guidance of neuronal axon growth. These cells that help axon guidance , are typically other neurons that are sometimes immature. When the axon has completed its growth at its connection to the target, the diameter of the axon can increase by up to five times, depending on the speed of conduction required. It has also been discovered through research that if the axons of a neuron were damaged, as long as

2900-462: The immunoglobulin superfamily. Another set of molecules called extracellular matrix - adhesion molecules also provide a sticky substrate for axons to grow along. Examples of these molecules include laminin , fibronectin , tenascin , and perlecan . Some of these are surface bound to cells and thus act as short range attractants or repellents. Others are difusible ligands and thus can have long range effects. Cells called guidepost cells assist in

3000-560: The mesencephalic duct (cerebral aqueduct). The metencephalon becomes, among other things, the pons and the cerebellum , the myelencephalon forms the medulla oblongata , and their cavities develop into the fourth ventricle . Rhinencephalon , amygdala , hippocampus , neocortex , basal ganglia , lateral ventricles Epithalamus , thalamus , hypothalamus , subthalamus , pituitary gland , pineal gland , third ventricle Tectum , cerebral peduncle , pretectum , mesencephalic duct Pons , cerebellum Planarians , members of

3100-404: The neurotransmitter for release at the synapse . This makes multiple synaptic connections with other neurons possible. Sometimes the axon of a neuron may synapse onto dendrites of the same neuron, when it is known as an autapse . Some synaptic junctions appear along the length of an axon as it extends; these are called en passant boutons ("in passing boutons") and can be in the hundreds or even

Neuraxis - Misplaced Pages Continue

3200-465: The skull . The spinal cord is housed in the spinal canal within the vertebrae . Within the CNS, the interneuronal space is filled with a large amount of supporting non-nervous cells called neuroglia or glia from the Greek for "glue". In vertebrates, the CNS also includes the retina and the optic nerve ( cranial nerve II), as well as the olfactory nerves and olfactory epithelium . As parts of

3300-404: The striatum , the hippocampus and the neocortex , and its cavity becomes the first and second ventricles (lateral ventricles). Diencephalon elaborations include the subthalamus , hypothalamus , thalamus and epithalamus , and its cavity forms the third ventricle . The tectum , pretectum , cerebral peduncle and other structures develop out of the mesencephalon, and its cavity grows into

3400-427: The tectum ). The neocortex of monotremes (the duck-billed platypus and several species of spiny anteaters ) and of marsupials (such as kangaroos , koalas , opossums , wombats , and Tasmanian devils ) lack the convolutions – gyri and sulci – found in the neocortex of most placental mammals ( eutherians ). Within placental mammals, the size and complexity of the neocortex increased over time. The area of

3500-401: The vestibular organ . The two structures of the diencephalon worth noting are the thalamus and the hypothalamus. The thalamus acts as a linkage between incoming pathways from the peripheral nervous system as well as the optical nerve (though it does not receive input from the olfactory nerve) to the cerebral hemispheres. Previously it was considered only a "relay station", but it is engaged in

3600-449: The 116 genes involved in the nervous system of planarians, which includes genes related to the CNS, also exist in humans. In arthropods , the ventral nerve cord , the subesophageal ganglia and the supraesophageal ganglia are usually seen as making up the CNS. Arthropoda, unlike vertebrates, have inhibitory motor neurons due to their small size. The CNS of chordates differs from that of other animals in being placed dorsally in

3700-922: The Aα, Aβ, and Aγ nerve fibers, respectively. Later findings by other researchers identified two groups of Aa fibers that were sensory fibers. These were then introduced into a system (Lloyd classification) that only included sensory fibers (though some of these were mixed nerves and were also motor fibers). This system refers to the sensory groups as Types and uses Roman numerals: Type Ia, Type Ib, Type II, Type III, and Type IV. Lower motor neurons have two kind of fibers: Different sensory receptors are innervated by different types of nerve fibers. Proprioceptors are innervated by type Ia, Ib and II sensory fibers, mechanoreceptors by type II and III sensory fibers and nociceptors and thermoreceptors by type III and IV sensory fibers. The autonomic nervous system has two kinds of peripheral fibers: In order of degree of severity, injury to

3800-405: The CNS and PNS, respectively. Both act to add myelin sheaths to the axons, which acts as a form of insulation allowing for better and faster proliferation of electrical signals along the nerves. Axons in the CNS are often very short, barely a few millimeters, and do not need the same degree of isolation as peripheral nerves. Some peripheral nerves can be over 1 meter in length, such as the nerves to

3900-423: The CNS to and from the face, as well as to certain muscles (such as the trapezius muscle , which is innervated by accessory nerves as well as certain cervical spinal nerves ). Two pairs of cranial nerves; the olfactory nerves and the optic nerves are often considered structures of the CNS. This is because they do not synapse first on peripheral ganglia, but directly on CNS neurons. The olfactory epithelium

4000-403: The CNS, they connect directly to brain neurons without intermediate ganglia . The olfactory epithelium is the only central nervous tissue outside the meninges in direct contact with the environment, which opens up a pathway for therapeutic agents which cannot otherwise cross the meninges barrier. The CNS consists of two major structures: the brain and spinal cord . The brain is encased in

4100-510: The CNS. Different forms of glial cells have different functions, some acting almost as scaffolding for neuroblasts to climb during neurogenesis such as bergmann glia , while others such as microglia are a specialized form of macrophage , involved in the immune system of the brain as well as the clearance of various metabolites from the brain tissue . Astrocytes may be involved with both clearance of metabolites as well as transport of fuel and various beneficial substances to neurons from

Neuraxis - Misplaced Pages Continue

4200-419: The CNS. While the spinal cord has certain processing ability such as that of spinal locomotion and can process reflexes , the brain is the major processing unit of the nervous system. The brainstem consists of the medulla , the pons and the midbrain . The medulla can be referred to as an extension of the spinal cord, which both have similar organization and functional properties. The tracts passing from

4300-461: The ability of the AIS to change its distribution and to maintain the activity of neural circuitry at a constant level. The AIS is highly specialized for the fast conduction of nerve impulses . This is achieved by a high concentration of voltage-gated sodium channels in the initial segment where the action potential is initiated. The ion channels are accompanied by a high number of cell adhesion molecules and scaffold proteins that anchor them to

4400-399: The alveus and the corpus callosum as well hippocampal gray matter. In fact, the generation of action potentials in vivo is sequential in nature, and these sequential spikes constitute the digital codes in the neurons. Although previous studies indicate an axonal origin of a single spike evoked by short-term pulses, physiological signals in vivo trigger the initiation of sequential spikes at

4500-503: The axon length on the molecular level. These studies suggest that motor proteins carry signaling molecules from the soma to the growth cone and vice versa whose concentration oscillates in time with a length-dependent frequency. The axons of neurons in the human peripheral nervous system can be classified based on their physical features and signal conduction properties. Axons were known to have different thicknesses (from 0.1 to 20 μm) and these differences were thought to relate to

4600-400: The axon sometimes consists of several regions that function more or less independently of each other. Axons are covered by a membrane known as an axolemma ; the cytoplasm of an axon is called axoplasm . Most axons branch, in some cases very profusely. The end branches of an axon are called telodendria . The swollen end of a telodendron is known as the axon terminal or end-foot which joins

4700-415: The axon to its target, is one of the six major stages in the overall development of the nervous system . Studies done on cultured hippocampal neurons suggest that neurons initially produce multiple neurites that are equivalent, yet only one of these neurites is destined to become the axon. It is unclear whether axon specification precedes axon elongation or vice versa, although recent evidence points to

4800-399: The axon's membrane and empty their contents into the extracellular space. The neurotransmitter is released from the presynaptic nerve through exocytosis . The neurotransmitter chemical then diffuses across to receptors located on the membrane of the target cell. The neurotransmitter binds to these receptors and activates them. Depending on the type of receptors that are activated, the effect on

4900-447: The axon. During early development of the vertebrate embryo, a longitudinal groove on the neural plate gradually deepens and the ridges on either side of the groove (the neural folds ) become elevated, and ultimately meet, transforming the groove into a closed tube called the neural tube . The formation of the neural tube is called neurulation . At this stage, the walls of the neural tube contain proliferating neural stem cells in

5000-651: The axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body and from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction can be the cause of many inherited and acquired neurological disorders that affect both the peripheral and central neurons . Nerve fibers are classed into three types – group A nerve fibers , group B nerve fibers , and group C nerve fibers . Groups A and B are myelinated , and group C are unmyelinated. These groups include both sensory fibers and motor fibers. Another classification groups only

5100-416: The axons in a condition known as diffuse axonal injury . This can lead to a persistent vegetative state . It has been shown in studies on the rat that axonal damage from a single mild traumatic brain injury, can leave a susceptibility to further damage, after repeated mild traumatic brain injuries. A nerve guidance conduit is an artificial means of guiding axon growth to enable neuroregeneration , and

SECTION 50

#1732877275860

5200-519: The big toe. To ensure signals move at sufficient speed, myelination is needed. The way in which the Schwann cells and oligodendrocytes myelinate nerves differ. A Schwann cell usually myelinates a single axon, completely surrounding it. Sometimes, they may myelinate many axons, especially when in areas of short axons. Oligodendrocytes usually myelinate several axons. They do this by sending out thin projections of their cell membrane , which envelop and enclose

5300-415: The body, above the gut and notochord / spine . The basic pattern of the CNS is highly conserved throughout the different species of vertebrates and during evolution. The major trend that can be observed is towards a progressive telencephalisation: the telencephalon of reptiles is only an appendix to the large olfactory bulb , while in mammals it makes up most of the volume of the CNS. In the human brain,

5400-399: The body, including the eyes and head, as well as the limbs. Further, it is involved in motion that has been learned and perfected through practice, and it will adapt to new learned movements. Despite its previous classification as a motor structure, the cerebellum also displays connections to areas of the cerebral cortex involved in language and cognition . These connections have been shown by

5500-408: The body. Such functions may engage the heart , blood vessels , and pupils , among others. The brainstem also holds the reticular formation , a group of nuclei involved in both arousal and alertness . The cerebellum lies behind the pons. The cerebellum is composed of several dividing fissures and lobes. Its function includes the control of posture and the coordination of movements of parts of

5600-551: The brain and generate thousands of synaptic terminals. A bundle of axons make a nerve tract in the central nervous system , and a fascicle in the peripheral nervous system . In placental mammals the largest white matter tract in the brain is the corpus callosum , formed of some 200 million axons in the human brain . Axons are the primary transmission lines of the nervous system , and as bundles they form nerves . Some axons can extend up to one meter or more while others extend as little as one millimeter. The longest axons in

5700-409: The brain. The myelin gives the white appearance to the tissue in contrast to the grey matter of the cerebral cortex which contains the neuronal cell bodies. A similar arrangement is seen in the cerebellum . Bundles of myelinated axons make up the nerve tracts in the CNS. Where these tracts cross the midline of the brain to connect opposite regions they are called commissures . The largest of these

5800-443: The cell bodies of the neurons. In addition to propagating action potentials to axonal terminals, the axon is able to amplify the action potentials, which makes sure a secure propagation of sequential action potentials toward the axonal terminal. In terms of molecular mechanisms, voltage-gated sodium channels in the axons possess lower threshold and shorter refractory period in response to short-term pulses. The development of

5900-483: The cell body along the axon, carries mitochondria and membrane proteins needed for growth to the axon terminal. Ingoing retrograde transport carries cell waste materials from the axon terminal to the cell body. Outgoing and ingoing tracks use different sets of motor proteins . Outgoing transport is provided by kinesin , and ingoing return traffic is provided by dynein . Dynein is minus-end directed. There are many forms of kinesin and dynein motor proteins, and each

6000-594: The cell body and terminating at points where the axon makes synaptic contact with target cells. The defining characteristic of an action potential is that it is "all-or-nothing" – every action potential that an axon generates has essentially the same size and shape. This all-or-nothing characteristic allows action potentials to be transmitted from one end of a long axon to the other without any reduction in size. There are, however, some types of neurons with short axons that carry graded electrochemical signals, of variable amplitude. When an action potential reaches

6100-429: The central nervous system can cause severe illness and, when malignant , can have very high mortality rates. Symptoms depend on the size, growth rate, location and malignancy of tumors and can include alterations in motor control, hearing loss, headaches and changes in cognitive ability and autonomic functioning. Specialty professional organizations recommend that neurological imaging of the brain be done only to answer

SECTION 60

#1732877275860

6200-504: The central nervous system the myelin sheath is provided by another type of glial cell, the oligodendrocyte . Schwann cells myelinate a single axon. An oligodendrocyte can myelinate up to 50 axons. The composition of myelin is different in the two types. In the CNS the major myelin protein is proteolipid protein , and in the PNS it is myelin basic protein . Nodes of Ranvier (also known as myelin sheath gaps ) are short unmyelinated segments of

6300-440: The cerebral hemispheres, among others: the cortex, basal ganglia, amygdala and hippocampus. The hemispheres together control a large portion of the functions of the human brain such as emotion, memory, perception and motor functions. Apart from this the cerebral hemispheres stand for the cognitive capabilities of the brain. Connecting each of the hemispheres is the corpus callosum as well as several additional commissures. One of

6400-418: The cytoskeleton. Interactions with ankyrin-G are important as it is the major organizer in the AIS. The axoplasm is the equivalent of cytoplasm in the cell. Microtubules form in the axoplasm at the axon hillock. They are arranged along the length of the axon, in overlapping sections, and all point in the same direction – towards the axon terminals. This is noted by the positive endings of

6500-412: The dendrite or cell body of another neuron forming a synaptic connection. Axons usually make contact with other neurons at junctions called synapses but can also make contact with muscle or gland cells. In some circumstances, the axon of one neuron may form a synapse with the dendrites of the same neuron, resulting in an autapse . At a synapse, the membrane of the axon closely adjoins the membrane of

6600-481: The entire process adheres to surfaces and explores the surrounding environment. Actin plays a major role in the mobility of this system. Environments with high levels of cell adhesion molecules (CAMs) create an ideal environment for axonal growth. This seems to provide a "sticky" surface for axons to grow along. Examples of CAMs specific to neural systems include N-CAM , TAG-1  – an axonal glycoprotein  – and MAG , all of which are part of

6700-483: The fibers into three main groups using the letters A, B, and C. These groups, group A , group B , and group C include both the sensory fibers ( afferents ) and the motor fibers ( efferents ). The first group A, was subdivided into alpha, beta, gamma, and delta fibers – Aα, Aβ, Aγ, and Aδ. The motor neurons of the different motor fibers, were the lower motor neurons  – alpha motor neuron , beta motor neuron , and gamma motor neuron having

6800-407: The flexure in the brain stem, and at the top of the neck, that animal would be looking straight up instead of straight in front. Central nervous system The rest of this article exclusively discusses the vertebrate central nervous system, which is radically distinct from all other animals. In vertebrates , the brain and spinal cord are both enclosed in the meninges . The meninges provide

6900-411: The formation of multiple axons. Consequently, the interruption of the actin network in a growth cone will promote its neurite to become the axon. Growing axons move through their environment via the growth cone , which is at the tip of the axon. The growth cone has a broad sheet-like extension called a lamellipodium which contain protrusions called filopodia . The filopodia are the mechanism by which

7000-520: The front, the mesencephalon , and, between the mesencephalon and the spinal cord, the rhombencephalon . (By six weeks in the human embryo) the prosencephalon then divides further into the telencephalon and diencephalon ; and the rhombencephalon divides into the metencephalon and myelencephalon . The spinal cord is derived from the posterior or 'caudal' portion of the neural tube. As a vertebrate grows, these vesicles differentiate further still. The telencephalon differentiates into, among other things,

7100-433: The hallmark of traumatic brain injuries . Axonal damage is usually to the axon cytoskeleton disrupting transport. As a consequence protein accumulations such as amyloid-beta precursor protein can build up in a swelling resulting in a number of varicosities along the axon. Most axons carry signals in the form of action potentials, which are discrete electrochemical impulses that travel rapidly along an axon, starting at

7200-408: The human body are those of the sciatic nerve , which run from the base of the spinal cord to the big toe of each foot. The diameter of axons is also variable. Most individual axons are microscopic in diameter (typically about one micrometer (μm) across). The largest mammalian axons can reach a diameter of up to 20 μm. The squid giant axon , which is specialized to conduct signals very rapidly,

7300-441: The information out. The spinal cord relays information up to the brain through spinal tracts through the final common pathway to the thalamus and ultimately to the cortex. Apart from the spinal cord, there are also peripheral nerves of the PNS that synapse through intermediaries or ganglia directly on the CNS. These 12 nerves exist in the head and neck region and are called cranial nerves . Cranial nerves bring information to

7400-403: The initial segment is to separate the main part of an axon from the rest of the neuron; another function is to help initiate action potentials. Both of these functions support neuron cell polarity , in which dendrites (and, in some cases the soma ) of a neuron receive input signals at the basal region, and at the apical region the neuron's axon provides output signals. The axon initial segment

7500-451: The integration of synaptic messages at the scale of the neuron. Extracellular recordings of action potential propagation in axons has been demonstrated in freely moving animals. While extracellular somatic action potentials have been used to study cellular activity in freely moving animals such as place cells , axonal activity in both white and gray matter can also be recorded. Extracellular recordings of axon action potential propagation

7600-433: The latter. If an axon that is not fully developed is cut, the polarity can change and other neurites can potentially become the axon. This alteration of polarity only occurs when the axon is cut at least 10 μm shorter than the other neurites. After the incision is made, the longest neurite will become the future axon and all the other neurites, including the original axon, will turn into dendrites. Imposing an external force on

7700-419: The microtubules. This overlapping arrangement provides the routes for the transport of different materials from the cell body. Studies on the axoplasm has shown the movement of numerous vesicles of all sizes to be seen along cytoskeletal filaments – the microtubules, and neurofilaments , in both directions between the axon and its terminals and the cell body. Outgoing anterograde transport from

7800-554: The most important parts of the cerebral hemispheres is the cortex , made up of gray matter covering the surface of the brain. Functionally, the cerebral cortex is involved in planning and carrying out of everyday tasks. The hippocampus is involved in storage of memories, the amygdala plays a role in perception and communication of emotion, while the basal ganglia play a major role in the coordination of voluntary movement. The PNS consists of neurons, axons, and Schwann cells . Oligodendrocytes and Schwann cells have similar functions in

7900-439: The myelin sheath of a myelinated axon. Oligodendrocytes form the insulating myelin in the CNS. Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly spaced intervals. The myelination enables an especially rapid mode of electrical impulse propagation called saltatory conduction . The myelinated axons from the cortical neurons form the bulk of the neural tissue called white matter in

8000-1079: The neocortex of mice is only about 1/100 that of monkeys, and that of monkeys is only about 1/10 that of humans. In addition, rats lack convolutions in their neocortex (possibly also because rats are small mammals), whereas cats have a moderate degree of convolutions, and humans have quite extensive convolutions. Extreme convolution of the neocortex is found in dolphins , possibly related to their complex echolocation . There are many CNS diseases and conditions, including infections such as encephalitis and poliomyelitis , early-onset neurological disorders including ADHD and autism , seizure disorders such as epilepsy , headache disorders such as migraine , late-onset neurodegenerative diseases such as Alzheimer's disease , Parkinson's disease , and essential tremor , autoimmune and inflammatory diseases such as multiple sclerosis and acute disseminated encephalomyelitis , genetic disorders such as Krabbe's disease and Huntington's disease , as well as amyotrophic lateral sclerosis and adrenoleukodystrophy . Lastly, cancers of

8100-446: The neuraxis overlays the superior–inferior dimension of the body. However, there is a major curve between the brain stem and forebrain , which is called the cephalic flexure . Because of this, the neuraxis starts in an inferior position—the end of the spinal cord—and ends in an anterior position, the front of the cerebrum . This may be confusing and can be illustrated when looking at a four-legged animal standing up on two legs. Without

8200-499: The neuron. Axons vary largely in length from a few micrometers up to meters in some animals. This emphasizes that there must be a cellular length regulation mechanism allowing the neurons both to sense the length of their axons and to control their growth accordingly. It was discovered that motor proteins play an important role in regulating the length of axons. Based on this observation, researchers developed an explicit model for axonal growth describing how motor proteins could affect

8300-454: The neurons and tissue of the CNS and the peripheral nervous system (PNS). The CNS is composed of white and gray matter . This can also be seen macroscopically on brain tissue. The white matter consists of axons and oligodendrocytes , while the gray matter consists of neurons and unmyelinated fibers. Both tissues include a number of glial cells (although the white matter contains more), which are often referred to as supporting cells of

8400-423: The only vertebrates to possess the evolutionarily recent, outermost part of the cerebral cortex (main part of the telencephalon excluding olfactory bulb) known as the neocortex . This part of the brain is, in mammals, involved in higher thinking and further processing of all senses in the sensory cortices (processing for smell was previously only done by its bulb while those for non-smell senses were only done by

8500-433: The perception of senses. All in all 31 spinal nerves project from the brain stem, some forming plexa as they branch out, such as the brachial plexa , sacral plexa etc. Each spinal nerve will carry both sensory and motor signals, but the nerves synapse at different regions of the spinal cord, either from the periphery to sensory relay neurons that relay the information to the CNS or from the CNS to motor neurons, which relay

8600-399: The phylum Platyhelminthes (flatworms), have the simplest, clearly defined delineation of a nervous system into a CNS and a PNS . Their primitive brains, consisting of two fused anterior ganglia, and longitudinal nerve cords form the CNS. Like vertebrates, have a distinct CNS and PNS. The nerves projecting laterally from the CNS form their PNS. A molecular study found that more than 95% of

8700-477: The rest of the axon, and the axon telodendria, and axon terminals. It also includes the myelin sheath. The Nissl bodies that produce the neuronal proteins are absent in the axonal region. Proteins needed for the growth of the axon, and the removal of waste materials, need a framework for transport. This axonal transport is provided for in the axoplasm by arrangements of microtubules and type IV intermediate filaments known as neurofilaments . The axon hillock

8800-761: The sensory fibers as Type I, Type II, Type III, and Type IV. An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron; the other type is a dendrite . Axons are distinguished from dendrites by several features, including shape (dendrites often taper while axons usually maintain a constant radius), length (dendrites are restricted to a small region around the cell body while axons can be much longer), and function (dendrites receive signals whereas axons transmit them). Some types of neurons have no axon and transmit signals from their dendrites. In some species, axons can emanate from dendrites known as axon-carrying dendrites. No neuron ever has more than one axon; however in invertebrates such as insects or leeches

8900-404: The simultaneous transmission of messages to a large number of target neurons within a single region of the brain. There are two types of axons in the nervous system: myelinated and unmyelinated axons. Myelin is a layer of a fatty insulating substance, which is formed by two types of glial cells : Schwann cells and oligodendrocytes . In the peripheral nervous system Schwann cells form

9000-455: The skull, and protected by the cranium. The spinal cord is continuous with the brain and lies caudally to the brain. It is protected by the vertebrae . The spinal cord reaches from the base of the skull, and continues through or starting below the foramen magnum , and terminates roughly level with the first or second lumbar vertebra , occupying the upper sections of the vertebral canal . Microscopically, there are differences between

9100-489: The soma (the cell body of a neuron) is not damaged, the axons would regenerate and remake the synaptic connections with neurons with the help of guidepost cells . This is also referred to as neuroregeneration . Nogo-A is a type of neurite outgrowth inhibitory component that is present in the central nervous system myelin membranes (found in an axon). It has a crucial role in restricting axonal regeneration in adult mammalian central nervous system. In recent studies, if Nogo-A

9200-452: The sorting of information that will reach cerebral hemispheres ( neocortex ). Apart from its function of sorting information from the periphery, the thalamus also connects the cerebellum and basal ganglia with the cerebrum. In common with the aforementioned reticular system the thalamus is involved in wakefulness and consciousness, such as though the SCN . The hypothalamus engages in functions of

9300-475: The speed at which an action potential could travel along the axon – its conductance velocity . Erlanger and Gasser proved this hypothesis, and identified several types of nerve fiber, establishing a relationship between the diameter of an axon and its nerve conduction velocity. They published their findings in 1941 giving the first classification of axons. Axons are classified in two systems. The first one introduced by Erlanger and Gasser, grouped

9400-431: The spinal cord to the brain pass through here. Regulatory functions of the medulla nuclei include control of blood pressure and breathing . Other nuclei are involved in balance , taste , hearing , and control of muscles of the face and neck . The next structure rostral to the medulla is the pons, which lies on the ventral anterior side of the brainstem. Nuclei in the pons include pontine nuclei which work with

9500-411: The target cell can be to excite the target cell, inhibit it, or alter its metabolism in some way. This entire sequence of events often takes place in less than a thousandth of a second. Afterward, inside the presynaptic terminal, a new set of vesicles is moved into position next to the membrane, ready to be released when the next action potential arrives. The action potential is the final electrical step in

9600-473: The target cell, and special molecular structures serve to transmit electrical or electrochemical signals across the gap. Some synaptic junctions appear along the length of an axon as it extends; these are called en passant boutons ("in passing boutons") and can be in the hundreds or even the thousands along one axon. Other synapses appear as terminals at the ends of axonal branches. A single axon, with all its branches taken together, can target multiple parts of

9700-413: The telencephalon covers most of the diencephalon and the entire mesencephalon . Indeed, the allometric study of brain size among different species shows a striking continuity from rats to whales, and allows us to complete the knowledge about the evolution of the CNS obtained through cranial endocasts . Mammals – which appear in the fossil record after the first fishes, amphibians, and reptiles – are

9800-444: The thousands along one axon. In the normally developed brain, along the shaft of some axons are located pre-synaptic boutons also known as axonal varicosities and these have been found in regions of the hippocampus that function in the release of neurotransmitters. However, axonal varicosities are also present in neurodegenerative diseases where they interfere with the conduction of an action potential. Axonal varicosities are also

9900-453: The use of medical imaging techniques, such as functional MRI and Positron emission tomography . The body of the cerebellum holds more neurons than any other structure of the brain, including that of the larger cerebrum , but is also more extensively understood than other structures of the brain, as it includes fewer types of different neurons. It handles and processes sensory stimuli, motor information, as well as balance information from

10000-421: The visual and auditory systems are located in the midbrain, including control of automatic eye movements. The brainstem at large provides entry and exit to the brain for a number of pathways for motor and autonomic control of the face and neck through cranial nerves, Autonomic control of the organs is mediated by the tenth cranial nerve . A large portion of the brainstem is involved in such autonomic control of

#859140