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96-621: 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 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

192-428: 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 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

288-456: 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

384-426: 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

480-543: 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

576-406: 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

672-401: 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 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

768-746: 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

864-463: 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

960-460: 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

1056-450: A number of glial cells (although the white matter contains more), which are often referred to as supporting cells of 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

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1152-465: 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

1248-403: 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 the axon. During early development of the vertebrate embryo, a longitudinal groove on the neural plate gradually deepens and

1344-459: 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 . Nervous tissue Nervous tissue , also called neural tissue ,

1440-456: 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

1536-447: 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

1632-496: 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

1728-618: 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

1824-439: 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 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

1920-418: 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

2016-657: Is composed of neurons , also called nerve cells, and neuroglial cells . Four types of neuroglia found in the CNS are astrocytes , microglial cells , ependymal cells , and oligodendrocytes . Two types of neuroglia found in the PNS are satellite glial cells and Schwann cells . In the central nervous system (CNS), the tissue types found are grey matter and white matter . The tissue is categorized by its neuronal and neuroglial components. Neurons are cells with specialized features that allow them to receive and facilitate nerve impulses, or action potentials , across their membrane to

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2112-478: 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

2208-457: 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

2304-451: 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 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

2400-475: Is the main tissue component of the nervous system . The nervous system regulates and controls body functions and activity. It consists of two parts: the central nervous system (CNS) comprising the brain and spinal cord , and the peripheral nervous system (PNS) comprising the branching peripheral nerves . It is composed of neurons , also known as nerve cells, which receive and transmit impulses, and neuroglia , also known as glial cells or glia, which assist

2496-423: Is the part of the nervous system consisting primarily of the brain and spinal cord . The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all parts of the bodies of bilaterally symmetric and triploblastic animals —that is, all multicellular animals except sponges and diploblasts . It is a structure composed of nervous tissue positioned along

2592-667: Is to form the communication network of the nervous system by conducting electric signals across tissue. In the CNS, grey matter, which contains the synapses , is important for information processing. White matter, containing myelinated axons, connects and facilitates nerve impulse between grey matter areas in the CNS. In the PNS, the ganglion tissue, containing the cell bodies and dendrites, contain relay points for nerve tissue impulses. The nerve tissue, containing myelinated axons bundles, carry action potential nerve impulses. Neoplasms (tumours) in nervous tissue include: Central nervous system The central nervous system ( CNS )

2688-431: 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

2784-641: 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

2880-443: The axon terminal , is separated from the dendrite of the following neuron by a small gap called a synaptic cleft . When the action potential travels to the axon terminal, neurotransmitters are released across the synapse and bind to the post-synaptic receptors , continuing the nerve impulse. Neurons are classified both functionally and structurally. Functional classification: Structural classification: Neuroglia encompasses

2976-415: The brain and spinal cord . The brain is encased in 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

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3072-488: The dorsal body cavity , while the brain is housed in the cranial cavity within 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

3168-433: 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 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,

3264-470: 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

3360-434: 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 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

3456-459: 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 the mesencephalic duct (cerebral aqueduct). The metencephalon becomes, among other things,

3552-412: The olfactory nerves and olfactory epithelium . As parts of 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:

3648-418: 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 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

3744-467: 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

3840-402: 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

3936-446: The rostral (nose end) to caudal (tail end) axis of the body and may have an enlarged section at the rostral end which is a brain. Only arthropods , cephalopods and vertebrates have a true brain, though precursor structures exist in onychophorans , gastropods and lancelets . The rest of this article exclusively discusses the vertebrate central nervous system, which is radically distinct from all other animals. In vertebrates ,

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4032-681: 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

4128-444: 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

4224-427: 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

4320-408: 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

4416-449: 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

4512-410: 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 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;

4608-454: 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

4704-489: The aforementioned reticular system the thalamus is involved in wakefulness and consciousness, such as though the SCN . The hypothalamus engages in functions of 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

4800-461: 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 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

4896-429: 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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4992-583: 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

5088-415: 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,

5184-416: The brain and spinal cord are both enclosed in the meninges . The meninges provide 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

5280-463: 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 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

5376-413: 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 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

5472-438: 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

5568-420: 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

5664-429: 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

5760-547: 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

5856-433: The cognitive capabilities of the brain. Connecting each of the hemispheres is the corpus callosum as well as several additional commissures. One of 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,

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5952-548: 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

6048-476: 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

6144-432: 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 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

6240-431: The individual. The cerebrum of cerebral hemispheres make up the largest visual portion of the human brain. Various structures combine to form 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

6336-485: 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

6432-411: 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

6528-1079: 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

6624-439: 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 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

6720-471: 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 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

6816-400: 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

6912-407: The next neuron. They possess a large cell body ( soma ), with cell projections called dendrites and an axon . Dendrites are thin, branching projections that receive electrochemical signaling ( neurotransmitters ) to create a change in voltage in the cell. Axons are long projections that carry the action potential away from the cell body toward the next neuron. The bulb-like end of the axon, called

7008-408: The non-neural cells in nervous tissue that provide various crucial supportive functions for neurons. They are smaller than neurons, and vary in structure according to their function. Neuroglial cells are classified as follows: In the central nervous system : In the peripheral nervous system : The three layers of connective tissue surrounding each nerve are: The function of nervous tissue

7104-406: 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

7200-423: 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

7296-423: 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 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

7392-560: 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

7488-399: 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

7584-400: 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 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

7680-465: 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

7776-416: 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 the front, the mesencephalon , and, between the mesencephalon and the spinal cord, the rhombencephalon . (By six weeks in the human embryo)

7872-506: The propagation of the nerve impulse as well as provide nutrients to the neurons. Nervous tissue is made up of different types of neurons, all of which have an axon . An axon is the long stem-like part of the cell that sends action potentials to the next cell. Bundles of axons make up the nerves in the PNS and tracts in the CNS. Functions of the nervous system are sensory input , integration, control of muscles and glands , homeostasis , and mental activity . Nervous tissue

7968-416: 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, the striatum , the hippocampus and

8064-473: 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

8160-454: 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 a region called the ventricular zone . The neural stem cells, principally radial glial cells , multiply and generate neurons through

8256-420: 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

8352-509: 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

8448-399: 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

8544-460: 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 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

8640-413: 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

8736-690: 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

8832-419: 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

8928-488: 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

9024-429: The upper sections of the vertebral canal . Microscopically, there are differences between 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

9120-404: 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

9216-499: Was a key innovation in 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

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