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100-397: The posterior pituitary consists mainly of neuronal projections ( axons ) of magnocellular neurosecretory cells extending from the supraoptic and paraventricular nuclei of the hypothalamus . These axons store and release neurohypophysial hormones oxytocin and vasopressin into the neurohypophyseal capillaries, from there they get into the systemic circulation (and partly back into

200-419: A stimulus is a change in a living thing's internal or external environment . This change can be detected by an organism or organ using sensitivity, and leads to a physiological reaction. Sensory receptors can receive stimuli from outside the body, as in touch receptors found in the skin or light receptors in the eye, as well as from inside the body, as in chemoreceptors and mechanoreceptors . When

300-457: A basophilic ("base-loving") dye. These structures consist of rough endoplasmic reticulum and associated ribosomal RNA . Named after German psychiatrist and neuropathologist Franz Nissl (1860–1919), they are involved in protein synthesis and their prominence can be explained by the fact that nerve cells are very metabolically active. Basophilic dyes such as aniline or (weakly) hematoxylin highlight negatively charged components, and so bind to

400-426: A bit less than 1/10 of a volt at baseline. This voltage has two functions: first, it provides a power source for an assortment of voltage-dependent protein machinery that is embedded in the membrane; second, it provides a basis for electrical signal transmission between different parts of the membrane. Numerous microscopic clumps called Nissl bodies (or Nissl substance) are seen when nerve cell bodies are stained with

500-400: A cell in terms of movement, secretion, enzyme production, or gene expression. Receptors on cell surfaces are sensing components that monitor stimuli and respond to changes in the environment by relaying the signal to a control center for further processing and response. Stimuli are always converted into electrical signals via transduction . This electrical signal, or receptor potential , takes

600-544: A decrease in firing rate), or modulatory (causing long-lasting effects not directly related to firing rate). The two most common (90%+) neurotransmitters in the brain, glutamate and GABA , have largely consistent actions. Glutamate acts on several types of receptors and has effects that are excitatory at ionotropic receptors and a modulatory effect at metabotropic receptors . Similarly, GABA acts on several types of receptors, but all of them have inhibitory effects (in adult animals, at least). Because of this consistency, it

700-406: A dendrite to another dendrite. The signaling process is partly electrical and partly chemical. Neurons are electrically excitable, due to the maintenance of voltage gradients across their membranes . If the voltage changes by a large enough amount over a short interval, the neuron generates an all-or-nothing electrochemical pulse called an action potential . This potential travels rapidly along

800-436: A fast enough frequency, the depolarization is able to spread across the cell body to the axon hillock . From the axon hillock, an action potential can be generated and propagated down the neuron's axon , causing sodium ion channels in the axon to open as the impulse travels. Once the signal begins to travel down the axon, the membrane potential has already passed threshold , which means that it cannot be stopped. This phenomenon

900-421: A hormone which causes the retention of water in the kidneys. This process also increases an individual's thirst. By fluid retention or by consuming fluids, if an individual's blood pressure returns to normal, vasopressin release slows and less fluid is retained by the kidneys. Hypovolemia , or low fluid levels in the body, can also act as a stimulus to cause this response. Epinephrine , also known as adrenaline,

1000-406: A large response and cause neurological changes in the body. Pain also causes a behavioral change in the body, which is proportional to the intensity of the pain. The feeling is recorded by sensory receptors on the skin and travels to the central nervous system , where it is integrated and a decision on how to respond is made; if it is decided that a response must be made, a signal is sent back down to

1100-616: A muscle, which behaves appropriately according to the stimulus. The postcentral gyrus is the location of the primary somatosensory area , the main sensory receptive area for the sense of touch . Pain receptors are known as nociceptors . Two main types of nociceptors exist, A-fiber nociceptors and C-fiber nociceptors. A-fiber receptors are myelinated and conduct currents rapidly. They are mainly used to conduct fast and sharp types of pain. Conversely, C-fiber receptors are unmyelinated and slowly transmit. These receptors conduct slow, burning, diffuse pain. The absolute threshold for touch

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1200-748: A neuron leading to electrical activity, including pressure , stretch, chemical transmitters, and changes in the electric potential across the cell membrane. Stimuli cause specific ion-channels within the cell membrane to open, leading to a flow of ions through the cell membrane, changing the membrane potential. Neurons must maintain the specific electrical properties that define their neuron type. Thin neurons and axons require less metabolic expense to produce and carry action potentials, but thicker axons convey impulses more rapidly. To minimize metabolic expense while maintaining rapid conduction, many neurons have insulating sheaths of myelin around their axons. The sheaths are formed by glial cells: oligodendrocytes in

1300-465: A neuron responds at all, then it must respond completely. Greater intensity of stimulation, like brighter image/louder sound, does not produce a stronger signal but can increase firing frequency. Receptors respond in different ways to stimuli. Slowly adapting or tonic receptors respond to a steady stimulus and produce a steady rate of firing. Tonic receptors most often respond to increased stimulus intensity by increasing their firing frequency, usually as

1400-460: A neurotransmitter that binds to chemical receptors . The effect on the postsynaptic neuron is determined by the type of receptor that is activated, not by the presynaptic neuron or by the neurotransmitter. A neurotransmitter can be thought of as a key, and a receptor as a lock: the same neurotransmitter can activate multiple types of receptors. Receptors can be classified broadly as excitatory (causing an increase in firing rate), inhibitory (causing

1500-465: A power function of stimulus plotted against impulses per second. This can be likened to an intrinsic property of light where greater intensity of a specific frequency (color) requires more photons, as the photons can not become "stronger" for a specific frequency. Other receptor types include quickly adapting or phasic receptors, where firing decreases or stops with a steady stimulus; examples include skin which, when touched causes neurons to fire, but if

1600-510: A small sample of the air being inhaled. Olfactory receptors extend past the epithelial surface providing a base for many cilia that lie in the surrounding mucus. Odorant-binding proteins interact with these cilia stimulating the receptors. Odorants are generally small organic molecules. Greater water and lipid solubility is related directly to stronger smelling odorants. Odorant binding to G protein coupled receptors activates adenylate cyclase , which converts ATP to camp. cAMP , in turn, promotes

1700-447: A specific pathway through the nervous system to initiate a systematic response. Each type of receptor is specialized to respond preferentially to only one kind of stimulus energy, called the adequate stimulus . Sensory receptors have a well-defined range of stimuli to which they respond, and each is tuned to the particular needs of the organism. Stimuli are relayed throughout the body by mechanotransduction or chemotransduction, depending on

1800-400: A stimulus is detected by a sensory receptor, it can elicit a reflex via stimulus transduction . An internal stimulus is often the first component of a homeostatic control system . External stimuli are capable of producing systemic responses throughout the body, as in the fight-or-flight response . In order for a stimulus to be detected with high probability, its level of strength must exceed

1900-441: A technique called "double impregnation" and is still in use. In 1888 Ramón y Cajal published a paper about the bird cerebellum. In this paper, he stated that he could not find evidence for anastomosis between axons and dendrites and called each nervous element "an autonomous canton." This became known as the neuron doctrine , one of the central tenets of modern neuroscience . Stimulus (physiology) In physiology ,

2000-456: A universal classification of neurons that will apply to all neurons in the brain as well as across species. This is done by considering the three essential qualities of all neurons: electrophysiology, morphology, and the individual transcriptome of the cells. Besides being universal this classification has the advantage of being able to classify astrocytes as well. A method called patch-sequencing in which all three qualities can be measured at once

2100-447: Is a neurological disorder that results from the demyelination of axons in the central nervous system. Some neurons do not generate action potentials but instead generate a graded electrical signal , which in turn causes graded neurotransmitter release. Such non-spiking neurons tend to be sensory neurons or interneurons, because they cannot carry signals long distances. Neural coding is concerned with how sensory and other information

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2200-496: Is a powerful electrical insulator , but in neurons, many of the protein structures embedded in the membrane are electrically active. These include ion channels that permit electrically charged ions to flow across the membrane and ion pumps that chemically transport ions from one side of the membrane to the other. Most ion channels are permeable only to specific types of ions. Some ion channels are voltage gated , meaning that they can be switched between open and closed states by altering

2300-625: Is a synapse in which a neuron's axon connects to its dendrites. The human brain has some 8.6 x 10 (eighty six billion) neurons. Each neuron has on average 7,000 synaptic connections to other neurons. It has been estimated that the brain of a three-year-old child has about 10 synapses (1 quadrillion). This number declines with age , stabilizing by adulthood. Estimates vary for an adult, ranging from 10 to 5 x 10 synapses (100 to 500 trillion). Beyond electrical and chemical signaling, studies suggest neurons in healthy human brains can also communicate through: They can also get modulated by input from

2400-563: Is also influenced by the cellular responses to those same applied or endogenously generated forces. Mechanosensitive ion channels are found in many cell types and it has been shown that the permeability of these channels to cations is affected by stretch receptors and mechanical stimuli. This permeability of ion channels is the basis for the conversion of the mechanical stimulus into an electrical signal. Chemical stimuli, such as odorants, are received by cellular receptors that are often coupled to ion channels responsible for chemotransduction. Such

2500-494: Is also used commonly to respond to both internal and external changes. One common cause of the release of this hormone is the Fight-or-flight response . When the body encounters an external stimulus that is potentially dangerous, epinephrine is released from the adrenal glands . Epinephrine causes physiological changes in the body, such as constriction of blood vessels, dilation of pupils, increased heart and respiratory rate, and

2600-419: Is common for neuroscientists to refer to cells that release glutamate as "excitatory neurons", and cells that release GABA as "inhibitory neurons". Some other types of neurons have consistent effects, for example, "excitatory" motor neurons in the spinal cord that release acetylcholine , and "inhibitory" spinal neurons that release glycine . The distinction between excitatory and inhibitory neurotransmitters

2700-479: Is inhibitory, inhibitory neurotransmitters, normally GABA will be released into the synapse. This neurotransmitter causes an inhibitory postsynaptic potential in the postsynaptic neuron. This response will cause the postsynaptic neuron to become permeable to chloride ions, making the membrane potential of the cell negative; a negative membrane potential makes it more difficult for the cell to fire an action potential and prevents any signal from being passed on through

2800-411: Is known as an all-or-nothing response. Groups of sodium channels opened by the change in membrane potential strengthen the signal as it travels away from the axon hillock, allowing it to move the length of the axon. As the depolarization reaches the end of the axon, or the axon terminal , the end of the neuron becomes permeable to calcium ions, which enters the cell via calcium ion channels. Calcium causes

2900-478: Is not absolute. Rather, it depends on the class of chemical receptors present on the postsynaptic neuron. In principle, a single neuron, releasing a single neurotransmitter, can have excitatory effects on some targets, inhibitory effects on others, and modulatory effects on others still. For example, photoreceptor cells in the retina constantly release the neurotransmitter glutamate in the absence of light. So-called OFF bipolar cells are, like most neurons, excited by

3000-404: Is often considered to be a single drop of quinine sulfate in 250 gallons of water. Changes in pressure caused by sound reaching the external ear resonate in the tympanic membrane , which articulates with the auditory ossicles, or the bones of the middle ear. These tiny bones multiply these pressure fluctuations as they pass the disturbance into the cochlea, a spiral-shaped bony structure within

3100-459: Is represented in the brain by neurons. The main goal of studying neural coding is to characterize the relationship between the stimulus and the individual or ensemble neuronal responses and the relationships among the electrical activities of the neurons within the ensemble. It is thought that neurons can encode both digital and analog information. The conduction of nerve impulses is an example of an all-or-none response. In other words, if

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3200-448: Is the case in olfactory cells . Depolarization in these cells result from opening of non-selective cation channels upon binding of the odorant to the specific receptor. G protein-coupled receptors in the plasma membrane of these cells can initiate second messenger pathways that cause cation channels to open. In response to stimuli, the sensory receptor initiates sensory transduction by creating graded potentials or action potentials in

3300-424: Is the minimum amount of sensation needed to elicit a response from touch receptors. This amount of sensation has a definable value and is often considered to be the force exerted by dropping the wing of a bee onto a person's cheek from a distance of one centimeter. This value will change based on the body part being touched. Vision provides opportunity for the brain to perceive and respond to changes occurring around

3400-432: Is transferred to the axon, which fires. If the pressure is steady, the stimulus ends; thus, these neurons typically respond with a transient depolarization during the initial deformation and again when the pressure is removed, which causes the corpuscle to change shape again. Other types of adaptation are important in extending the function of several other neurons. The German anatomist Heinrich Wilhelm Waldeyer introduced

3500-631: Is used extensively by the Allen Institute for Brain Science . In 2023, a comprehensive cell atlas of the adult, and developing human brain at the transcriptional, epigenetic, and functional levels was created through an international collaboration of researchers using the most cutting-edge molecular biology approaches. Neurons communicate with each other via synapses , where either the axon terminal of one cell contacts another neuron's dendrite, soma, or, less commonly, axon. Neurons such as Purkinje cells in

3600-535: The absolute threshold ; if a signal does reach threshold, the information is transmitted to the central nervous system (CNS), where it is integrated and a decision on how to react is made. Although stimuli commonly cause the body to respond, it is the CNS that finally determines whether a signal causes a reaction or not. Homeostatic outbalances are the main driving force for changes of the body. These stimuli are monitored closely by receptors and sensors in different parts of

3700-409: The central nervous system down neurons until they reach the motor neuron, which releases the neurotransmitter acetylcholine (ACh) into the neuromuscular junction. ACh binds to nicotinic acetylcholine receptors on the surface of the muscle cell and opens ion channels, allowing sodium ions to flow into the cell and potassium ions to flow out; this ion movement causes a depolarization, which allows for

3800-462: The central nervous system . These impulses inhibit the constriction of blood vessels and lower the heart rate. If these nerves do not detect stretching, the body determines perceives low blood pressure as a dangerous stimulus and signals are not sent, preventing the inhibition CNS action; blood vessels constrict and the heart rate increases, causing an increase in blood pressure in the body. Sensory feelings, especially pain, are stimuli that can elicit

3900-443: The cochlea , can interpret and convey to the brain information about equilibrium by a similar method as the one used for hearing. Hair cells in these parts of the ear protrude kinocilia and stereocilia into a gelatinous material that lines the ducts of this canal. In parts of these semi circular canals, specifically the maculae, calcium carbonate crystals known as statoconia rest on the surface of this gelatinous material. When tilting

4000-472: The glial cells that give them structural and metabolic support. The nervous system is made up of the central nervous system , which includes the brain and spinal cord , and the peripheral nervous system , which includes the autonomic , enteric and somatic nervous systems . In vertebrates, the majority of neurons belong to the central nervous system , but some reside in peripheral ganglia , and many sensory neurons are situated in sensory organs such as

4100-400: The hypophyseal portal system ). In addition to axons, the posterior pituitary also contains pituicytes , specialized glial cells resembling astrocytes assisting in the storage and release of the hormones. Classification of the posterior pituitary varies, but most sources include the two regions below: Also called the neural lobe or posterior lobe, this region constitutes the majority of

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4200-405: The occipital lobe of the CNS, specifically in the primary visual cortex . The absolute threshold for vision is the minimum amount of sensation needed to elicit a response from photoreceptors in the eye. This amount of sensation has a definable value and is often considered to be the amount of light present from someone holding up a single candle 30 miles away, if one's eyes were adjusted to

4300-402: The retina and cochlea . Axons may bundle into nerve fascicles that make up the nerves in the peripheral nervous system (like strands of wire that make up a cable). In the central nervous system bundles of axons are called nerve tracts . Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed in different parts of

4400-686: The squid giant axon could be used to study neuronal electrical properties. It is larger than but similar to human neurons, making it easier to study. By inserting electrodes into the squid giant axons, accurate measurements were made of the membrane potential . The cell membrane of the axon and soma contain voltage-gated ion channels that allow the neuron to generate and propagate an electrical signal (an action potential). Some neurons also generate subthreshold membrane potential oscillations . These signals are generated and propagated by charge-carrying ions including sodium (Na ), potassium (K ), chloride (Cl ), and calcium (Ca ) . Several stimuli can activate

4500-444: The synapse to another cell. Neurons may lack dendrites or have no axons. The term neurite is used to describe either a dendrite or an axon, particularly when the cell is undifferentiated . Most neurons receive signals via the dendrites and soma and send out signals down the axon. At the majority of synapses, signals cross from the axon of one neuron to the dendrite of another. However, synapses can connect an axon to another axon or

4600-408: The temporal lobe of the CNS, specifically in the primary auditory cortex . The absolute threshold for sound is the minimum amount of sensation needed to elicit a response from receptors in the ears. This amount of sensation has a definable value and is often considered to be a watch ticking in an otherwise soundless environment 20 feet away. Semi circular ducts, which are connected directly to

4700-562: The tongue and through the mouth. Gustatory cells are located on the surface of the tongue and adjacent portions of the pharynx and larynx . Gustatory cells form on taste buds , specialized epithelial cells , and are generally turned over every ten days. From each cell, protrudes microvilli, sometimes called taste hairs, through also the taste pore and into the oral cavity. Dissolved chemicals interact with these receptor cells; different tastes bind to specific receptors. Salt and sour receptors are chemically gated ion channels, which depolarize

4800-409: The tubulin of microtubules . Class III β-tubulin is found almost exclusively in neurons. Actin is predominately found at the tips of axons and dendrites during neuronal development. There the actin dynamics can be modulated via an interplay with microtubule. There are different internal structural characteristics between axons and dendrites. Typical axons seldom contain ribosomes , except some in

4900-458: The ability to generate electric signals first appeared in evolution some 700 to 800 million years ago, during the Tonian period. Predecessors of neurons were the peptidergic secretory cells. They eventually gained new gene modules which enabled cells to create post-synaptic scaffolds and ion channels that generate fast electrical signals. The ability to generate electric signals was a key innovation in

5000-456: The adult brain may regenerate functional neurons throughout the life of an organism (see neurogenesis ). Astrocytes are star-shaped glial cells that have been observed to turn into neurons by virtue of their stem cell-like characteristic of pluripotency . Like all animal cells, the cell body of every neuron is enclosed by a plasma membrane , a bilayer of lipid molecules with many types of protein structures embedded in it. A lipid bilayer

5100-429: The axon and activates synaptic connections as it reaches them. Synaptic signals may be excitatory or inhibitory , increasing or reducing the net voltage that reaches the soma. In most cases, neurons are generated by neural stem cells during brain development and childhood. Neurogenesis largely ceases during adulthood in most areas of the brain. Neurons are the primary components of the nervous system , along with

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5200-584: The axon terminal, it opens voltage-gated calcium channels , allowing calcium ions to enter the terminal. Calcium causes synaptic vesicles filled with neurotransmitter molecules to fuse with the membrane, releasing their contents into the synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and activate receptors on the postsynaptic neuron. High cytosolic calcium in the axon terminal triggers mitochondrial calcium uptake, which, in turn, activates mitochondrial energy metabolism to produce ATP to support continuous neurotransmission. An autapse

5300-507: The blood, oxygen levels, and water levels. Deviations from the homeostatic ideal may generate a homeostatic emotion , such as pain, thirst or fatigue, that motivates behavior that will restore the body to stasis (such as withdrawal, drinking or resting). Blood pressure, heart rate, and cardiac output are measured by stretch receptors found in the carotid arteries . Nerves embed themselves within these receptors and when they detect stretching, they are stimulated and fire action potentials to

5400-438: The body. Information, or stimuli, in the form of light enters the retina , where it excites a special type of neuron called a photoreceptor cell . A local graded potential begins in the photoreceptor, where it excites the cell enough for the impulse to be passed along through a track of neurons to the central nervous system . As the signal travels from photoreceptors to larger neurons, action potentials must be created for

5500-478: The body. These sensors are mechanoreceptors , chemoreceptors and thermoreceptors that, respectively, respond to pressure or stretching, chemical changes, or temperature changes. Examples of mechanoreceptors include baroreceptors which detect changes in blood pressure, Merkel's discs which can detect sustained touch and pressure, and hair cells which detect sound stimuli. Homeostatic imbalances that can serve as internal stimuli include nutrient and ion levels in

5600-506: The brain and spinal cord and help to receive and conduct impulses . Neurons communicate with other cells via synapses , which are specialized connections that commonly use minute amounts of chemical neurotransmitters to pass the electric signal from the presynaptic neuron to the target cell through the synaptic gap. Neurons are the main components of nervous tissue in all animals except sponges and placozoans . Plants and fungi do not have nerve cells. Molecular evidence suggests that

5700-416: The capacity to concentrate urine. Affected individuals excrete as much as 20 liters of dilute urine per day. Oversecretion of vasopressin causes the syndrome of inappropriate antidiuretic hormone (SIADH). Neuron A neuron , neurone , or nerve cell is an excitable cell that fires electric signals called action potentials across a neural network in the nervous system .They are located in

5800-406: The cell. Sweet, bitter, and umami receptors are called gustducins , specialized G protein coupled receptors . Both divisions of receptor cells release neurotransmitters to afferent fibers causing action potential firing. The absolute threshold for taste is the minimum amount of sensation needed to elicit a response from receptors in the mouth. This amount of sensation has a definable value and

5900-438: The central nervous system and Schwann cells in the peripheral nervous system. The sheath enables action potentials to travel faster than in unmyelinated axons of the same diameter, whilst using less energy. The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier , which contain a high density of voltage-gated ion channels. Multiple sclerosis

6000-420: The central nervous system are typically only about one micrometer thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon of a human motor neuron can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons can have axons that run from

6100-548: The cerebellum can have over 1000 dendritic branches, making connections with tens of thousands of other cells; other neurons, such as the magnocellular neurons of the supraoptic nucleus , have only one or two dendrites, each of which receives thousands of synapses. Synapses can be excitatory or inhibitory, either increasing or decreasing activity in the target neuron, respectively. Some neurons also communicate via electrical synapses, which are direct, electrically conductive junctions between cells. When an action potential reaches

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6200-509: The dark . Smell allows the body to recognize chemical molecules in the air through inhalation. Olfactory organs located on either side of the nasal septum consist of olfactory epithelium and lamina propria . The olfactory epithelium, which contains olfactory receptor cells, covers the inferior surface of the cribiform plate , the superior portion of the perpendicular plate, the superior nasal concha. Only roughly two percent of airborne compounds inhaled are carried to olfactory organs as

6300-429: The dendrites is known as a local graded potential and causes the membrane voltage to change from a negative resting potential to a more positive voltage, a process known as depolarization . The opening of sodium channels allows nearby sodium channels to open, allowing the change in permeability to spread from the dendrites to the cell body . If a graded potential is strong enough, or if several graded potentials occur in

6400-551: The environment and hormones released from other parts of the organism, which could be influenced more or less directly by neurons. This also applies to neurotrophins such as BDNF . The gut microbiome is also connected with the brain. Neurons also communicate with microglia , the brain's main immune cells via specialized contact sites, called "somatic junctions". These connections enable microglia to constantly monitor and regulate neuronal functions, and exert neuroprotection when needed. In 1937 John Zachary Young suggested that

6500-477: The evolution of the nervous system. Neurons are typically classified into three types based on their function. Sensory neurons respond to stimuli such as touch, sound, or light that affect the cells of the sensory organs , and they send signals to the spinal cord or brain . Motor neurons receive signals from the brain and spinal cord to control everything from muscle contractions to glandular output . Interneurons connect neurons to other neurons within

6600-474: The excitation from the OFF bipolar cells, silencing them. It is possible to identify the type of inhibitory effect a presynaptic neuron will have on a postsynaptic neuron, based on the proteins the presynaptic neuron expresses. Parvalbumin -expressing neurons typically dampen the output signal of the postsynaptic neuron in the visual cortex , whereas somatostatin -expressing neurons typically block dendritic inputs to

6700-406: The fluid medium that surrounds it causes the cupula itself to move. The ampulla communicates to the brain information about the head's horizontal rotation. Neurons of the adjacent vestibular ganglia monitor the hair cells in these ducts. These sensory fibers form the vestibular branch of the cranial nerve VIII . In general, cellular response to stimuli is defined as a change in state or activity of

6800-421: The head or when the body undergoes linear acceleration, these crystals move disturbing the cilia of the hair cells and, consequently, affecting the release of neurotransmitter to be taken up by surrounding sensory nerves. In other areas of the semi circular canal, specifically the ampulla, a structure known as the cupula—analogous to the gelatinous material in the maculae—distorts hair cells in a similar fashion when

6900-409: The incoming nutrients; by starting the digestive process before food reaches the stomach, the body is able to more effectively and efficiently metabolize food into necessary nutrients. Once food hits the mouth, taste and information from receptors in the mouth add to the digestive response. Chemoreceptors and mechanorceptors , activated by chewing and swallowing, further increase the enzyme release in

7000-486: The initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes, in diminishing amounts as the distance from the cell body increases. Neurons vary in shape and size and can be classified by their morphology and function. The anatomist Camillo Golgi grouped neurons into two types; type I with long axons used to move signals over long distances and type II with short axons, which can often be confused with dendrites. Type I cells can be further classified by

7100-404: The inner ear. Hair cells in the cochlear duct, specifically the organ of Corti , are deflected as waves of fluid and membrane motion travel through the chambers of the cochlea. Bipolar sensory neurons located in the center of the cochlea monitor the information from these receptor cells and pass it on to the brainstem via the cochlear branch of cranial nerve VIII . Sound information is processed in

7200-411: The location of the soma. The basic morphology of type I neurons, represented by spinal motor neurons , consists of a cell body called the soma and a long thin axon covered by a myelin sheath . The dendritic tree wraps around the cell body and receives signals from other neurons. The end of the axon has branching axon terminals that release neurotransmitters into a gap called the synaptic cleft between

7300-567: The metabolism of glucose. All of these responses to a single stimuli aid in protecting the individual, whether the decision is made to stay and fight, or run away and avoid danger. The digestive system can respond to external stimuli, such as the sight or smell of food, and cause physiological changes before the food ever enters the body. This reflex is known as the cephalic phase of digestion . The sight and smell of food are strong enough stimuli to cause salivation, gastric and pancreatic enzyme secretion, and endocrine secretion in preparation for

7400-411: The more explicit interpretation of external stimuli. Effectively, these localized graded potentials trigger action potentials that communicate, in their frequency, along nerve axons eventually arriving in specific cortexes of the brain. In these also highly specialized parts of the brain, these signals are coordinated with others to possibly trigger a new response. If a signal from the presynaptic neuron

7500-432: The nature of the stimulus. In response to a mechanical stimulus, cellular sensors of force are proposed to be extracellular matrix molecules, cytoskeleton, transmembrane proteins, proteins at the membrane-phospholipid interface, elements of the nuclear matrix, chromatin, and the lipid bilayer. Response can be twofold: the extracellular matrix, for example, is a conductor of mechanical forces but its structure and composition

7600-402: The nervous system, there is a wide variety in their shape, size, and electrochemical properties. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter. The accepted view of the neuron attributes dedicated functions to its various anatomical components; however, dendrites and axons often act in ways contrary to their so-called main function. Axons and dendrites in

7700-535: The neuron. Depending on the type of stimulus, a neuron can be either excitatory or inhibitory. Nerves in the peripheral nervous system spread out to various parts of the body, including muscle fibers . A muscle fiber and the motor neuron to which it is connected. The spot at which the motor neuron attaches to the muscle fiber is known as the neuromuscular junction . When muscles receive information from internal or external stimuli, muscle fibers are stimulated by their respective motor neuron. Impulses are passed from

7800-406: The object maintains even pressure, the neurons stop firing. The neurons of the skin and muscles that are responsive to pressure and vibration have filtering accessory structures that aid their function. The pacinian corpuscle is one such structure. It has concentric layers like an onion, which form around the axon terminal. When pressure is applied and the corpuscle is deformed, mechanical stimulus

7900-461: The opening of sodium channels resulting in a localized potential. The absolute threshold for smell is the minimum amount of sensation needed to elicit a response from receptors in the nose. This amount of sensation has a definable value and is often considered to be a single drop of perfume in a six-room house. This value will change depending on what substance is being smelled. Taste records flavoring of food and other materials that pass across

8000-525: The pancreas and liver can be secreted to aid in metabolism and breakdown of food. Intracellular measurements of electrical potential across the membrane can be obtained by microelectrode recording. Patch clamp techniques allow for the manipulation of the intracellular or extracellular ionic or lipid concentration while still recording potential. In this way, the effect of various conditions on threshold and propagation can be assessed. Positron emission tomography (PET) and magnetic resonance imaging (MRI) permit

8100-564: The phosphate backbone of the ribosomal RNA. The cell body of a neuron is supported by a complex mesh of structural proteins called neurofilaments , which together with neurotubules (neuronal microtubules) are assembled into larger neurofibrils. Some neurons also contain pigment granules, such as neuromelanin (a brownish-black pigment that is byproduct of synthesis of catecholamines ), and lipofuscin (a yellowish-brown pigment), both of which accumulate with age. Other structural proteins that are important for neuronal function are actin and

8200-404: The posterior pituitary and is the storage site of oxytocin and vasopressin. Sometimes (incorrectly) considered synonymous with the posterior pituitary, the pars nervosa includes Herring bodies and pituicytes . Also known as the infundibulum or pituitary stalk, the infundibular stalk bridges the hypothalamic and hypophyseal systems. The median eminence is only occasionally included as part of

8300-456: The posterior pituitary. Other sources specifically exclude it from the pituitary. A few sources include the pars intermedia as part of the posterior lobe, but this is a minority view. It is based upon the gross anatomical separation of the posterior and anterior pituitary along the cystic remnants of Rathke's pouch, causing the pars intermedia to remain attached to the neurohypophysis. Two hormones are classically considered as being related to

8400-409: The posterior pituitary: oxytocin and vasopressin. These hormones are created in the hypothalamus and released in the posterior pituitary. After creation, they are stored in neurosecretory vesicles regrouped into Herring bodies before being secreted in the posterior pituitary via the bloodstream. Insufficient secretion of vasopressin underlies diabetes insipidus , a condition in which the body loses

8500-467: The postsynaptic neuron. Neurons have intrinsic electroresponsive properties like intrinsic transmembrane voltage oscillatory patterns. So neurons can be classified according to their electrophysiological characteristics: Neurotransmitters are chemical messengers passed from one neuron to another neuron or to a muscle cell or gland cell . Since 2012 there has been a push from the cellular and computational neuroscience community to come up with

8600-425: The release of calcium ions within the cell. Calcium ions bind to proteins within the muscle cell to allow for muscle contraction; the ultimate consequence of a stimulus. The endocrine system is affected largely by many internal and external stimuli. One internal stimulus that causes hormone release is blood pressure . Hypotension , or low blood pressure, is a large driving force for the release of vasopressin ,

8700-535: The release of neurotransmitters stored in synaptic vesicles , which enter the synapse between two neurons known as the presynaptic and postsynaptic neurons; if the signal from the presynaptic neuron is excitatory, it will cause the release of an excitatory neurotransmitter, causing a similar response in the postsynaptic neuron. These neurons may communicate with thousands of other receptors and target cells through extensive, complex dendritic networks. Communication between receptors in this fashion enables discrimination and

8800-471: The released glutamate. However, neighboring target neurons called ON bipolar cells are instead inhibited by glutamate, because they lack typical ionotropic glutamate receptors and instead express a class of inhibitory metabotropic glutamate receptors. When light is present, the photoreceptors cease releasing glutamate, which relieves the ON bipolar cells from inhibition, activating them; this simultaneously removes

8900-402: The same cell or in an adjacent one. Sensitivity to stimuli is obtained by chemical amplification through second messenger pathways in which enzymatic cascades produce large numbers of intermediate products, increasing the effect of one receptor molecule. Though receptors and stimuli are varied, most extrinsic stimuli first generate localized graded potentials in the neurons associated with

9000-421: The same region of the brain or spinal cord. When multiple neurons are functionally connected together, they form what is called a neural circuit . A neuron contains all the structures of other cells such as a nucleus , mitochondria , and Golgi bodies but has additional unique structures such as an axon , and dendrites . The soma is a compact structure, and the axon and dendrites are filaments extruding from

9100-399: The signal to have enough strength to reach the CNS. If the stimulus does not warrant a strong enough response, it is said to not reach absolute threshold , and the body does not react. However, if the stimulus is strong enough to create an action potential in neurons away from the photoreceptor, the body will integrate the information and react appropriately. Visual information is processed in

9200-507: The silver staining technique used to visualize nervous tissue under light microscopy. The neuron's place as the primary functional unit of the nervous system was first recognized in the late 19th century through the work of the Spanish anatomist Santiago Ramón y Cajal . To make the structure of individual neurons visible, Ramón y Cajal improved a silver staining process that had been developed by Camillo Golgi . The improved process involves

9300-399: The soma. Dendrites typically branch profusely and extend a few hundred micrometers from the soma. The axon leaves the soma at a swelling called the axon hillock and travels for as far as 1 meter in humans or more in other species. It branches but usually maintains a constant diameter. At the farthest tip of the axon's branches are axon terminals , where the neuron can transmit a signal across

9400-421: The specific sensory organ or tissue. In the nervous system , internal and external stimuli can elicit two different categories of responses: an excitatory response, normally in the form of an action potential , and an inhibitory response. When a neuron is stimulated by an excitatory impulse, neuronal dendrites are bound by neurotransmitters which cause the cell to become permeable to a specific type of ion;

9500-405: The stomach and intestine. The digestive system is also able to respond to internal stimuli. The digestive tract, or enteric nervous system alone contains millions of neurons. These neurons act as sensory receptors that can detect changes, such as food entering the small intestine, in the digestive tract. Depending on what these sensory receptors detect, certain enzymes and digestive juices from

9600-669: The term neuron in 1891, based on the ancient Greek νεῦρον neuron 'sinew, cord, nerve'. The word was adopted in French with the spelling neurone . That spelling was also used by many writers in English, but has now become rare in American usage and uncommon in British usage. Some previous works used nerve cell ( cellule nervose ), as adopted in Camillo Golgi 's 1873 paper on the discovery of

9700-422: The terminals and the dendrites of the next neuron. Most neurons can be anatomically characterized as: Some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are: Afferent and efferent also refer generally to neurons that, respectively, bring information to or send information from the brain. A neuron affects other neurons by releasing

9800-489: The toes to the posterior column of the spinal cord, over 1.5 meters in adults. Giraffes have single axons several meters in length running along the entire length of their necks. Much of what is known about axonal function comes from studying the squid giant axon , an ideal experimental preparation because of its relatively immense size (0.5–1 millimeter thick, several centimeters long). Fully differentiated neurons are permanently postmitotic however, stem cells present in

9900-401: The type of neurotransmitter determines to which ion the neurotransmitter will become permeable. In excitatory postsynaptic potentials , an excitatory response is generated. This is caused by an excitatory neurotransmitter, normally glutamate binding to a neuron's dendrites, causing an influx of sodium ions through channels located near the binding site. This change in membrane permeability in

10000-404: The voltage difference across the membrane. Others are chemically gated, meaning that they can be switched between open and closed states by interactions with chemicals that diffuse through the extracellular fluid. The ion materials include sodium , potassium , chloride , and calcium . The interactions between ion channels and ion pumps produce a voltage difference across the membrane, typically

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