Aneuron,neurone,[1] ornerve cell is anexcitablecell that fires electric signals calledaction potentials across aneural network in thenervous system. They are located in the brain and spinal cord and help to receive and conduct impulses. Neurons communicate with other cells viasynapses, which are specialized connections that commonly use minute amounts of chemicalneurotransmitters to pass the electric signal from the presynaptic neuron to the target cell through the synaptic gap.
Neurons are the main components ofnervous tissue in allanimals exceptsponges andplacozoans.Plants andfungi 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 theTonian period. Predecessors of neurons were thepeptidergic 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 the evolution of the nervous system.[2]
Neurons are typically classified into three types based on their function.Sensory neurons respond tostimuli such as touch, sound, or light that affect the cells of thesensory organs, and they send signals to thespinal cord orbrain.Motor neurons receive signals from the brain and spinal cord to control everything frommuscle contractions[3] toglandular output.Interneurons connect neurons to other neurons within the same region of the brain or spinal cord. When multiple neurons are functionally connected together, they form what is called aneural circuit.
A neuron contains all the structures of other cells such as anucleus,mitochondria, andGolgi bodies but has additional unique structures such as anaxon, anddendrites.[4] The soma is a compact structure, and the axon and dendrites are filaments extruding from the soma. Dendrites typically branch profusely and extend a few hundred micrometers from the soma. The axon leaves the soma at a swelling called theaxon 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 areaxon terminals, where the neuron can transmit a signal across thesynapse to another cell. Neurons may lack dendrites or have no axons. The termneurite is used to describe either a dendrite or an axon, particularly when the cell isundifferentiated.
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 a dendrite to another dendrite. The signaling process is partly electrical and partly chemical. Neurons are electrically excitable, due to the maintenance ofvoltage gradients across theirmembranes. If the voltage changes by a large enough amount over a short interval, the neuron generates anall-or-nothingelectrochemical pulse called anaction potential. This potential travels rapidly along the axon and activates synaptic connections as it reaches them. Synaptic signals may beexcitatory orinhibitory, increasing or reducing the net voltage that reaches the soma.
In most cases, neurons are generated byneural stem cells during brain development and childhood.Neurogenesis largely ceases during adulthood in most areas of the brain.
Axons may bundle intonerve fascicles that make up thenerves in the peripheral nervous system (like strands of wire that make up a cable). In the central nervous system bundles of axons are callednerve tracts.
Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed in different parts of 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 100micrometers in diameter.[7]
Thesoma is the body of the neuron. As it contains thenucleus, mostprotein synthesis occurs here. The nucleus can range from 3 to 18 micrometers in diameter.[8]
Thedendrites of a neuron are cellular extensions with many branches. This overall shape and structure are referred to metaphorically as a dendritic tree. This is where the majority of input to the neuron occurs via thedendritic spine.
Theaxon is a finer, cable-like projection that can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon primarily carriesnerve signals away from the soma and carries some types of information back to it. Many neurons have only one axon, but this axon may—and usually will—undergo extensive branching, enabling communication with many target cells. The part of the axon where it emerges from the soma is called theaxon hillock. Besides being an anatomical structure, the axon hillock also has the greatest density ofvoltage-dependent sodium channels. This makes it the most easily excited part of the neuron and the spike initiation zone for the axon. In electrophysiological terms, it has the most negativethreshold potential.
While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons.
Theaxon terminal is found at the end of the axon farthest from the soma and containssynapses. Synaptic boutons are specialized structures whereneurotransmitter chemicals are released to communicate with target neurons. In addition to synaptic boutons at the axon terminal, a neuron may haveen passant boutons, which are located along the length of the axon.
Neuron cell body
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.[9]
Diagram of a typical myelinated vertebrate motor neuronNeurology video
Axons and dendrites in 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 humanmotor 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 the toes to theposterior 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 thesquid giant axon, an ideal experimental preparation because of its relatively immense size (0.5–1 millimeter thick, several centimeters long).
Fully differentiated neurons are permanentlypostmitotic[10] however, stem cells present in the adult brain may regenerate functional neurons throughout the life of an organism (seeneurogenesis).Astrocytes are star-shapedglial cells that have been observed to turn into neurons by virtue of their stem cell-like characteristic ofpluripotency.[11]
Like all animal cells, the cell body of every neuron is enclosed by aplasma membrane, a bilayer oflipid molecules with many types of protein structures embedded in it.[12] A lipid bilayer is a powerful electricalinsulator, 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 arevoltage gated, meaning that they can be switched between open and closed states by altering 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. Theion materials includesodium,potassium,chloride, andcalcium. The interactions between ion channels and ion pumps produce a voltage difference across the membrane, typically 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.
Golgi-stained neurons in humanhippocampal tissueActin filaments in a mouse cortical neuron in culture
Numerous microscopic clumps calledNissl bodies (or Nissl substance) are seen when nerve cell bodies are stained with a basophilic ("base-loving") dye. These structures consist ofrough endoplasmic reticulum and associatedribosomal RNA. Named after German psychiatrist and neuropathologistFranz 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 asaniline or (weakly)hematoxylin[13] highlight negatively charged components, and so bind to the phosphate backbone of the ribosomal RNA.
The cell body of a neuron is supported by a complex mesh of structural proteins calledneurofilaments, which together withneurotubules (neuronal microtubules) are assembled into larger neurofibrils.[14] Some neurons also contain pigment granules, such asneuromelanin (a brownish-black pigment that is byproduct of synthesis ofcatecholamines), andlipofuscin (a yellowish-brown pigment), both of which accumulate with age.[15][16][17] Other structural proteins that are important for neuronal function areactin and thetubulin ofmicrotubules.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.[18]
There are different internal structural characteristics between axons and dendrites. Typical axons seldom containribosomes, except some in 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 theirmorphology and function.[20] The anatomistCamillo 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 the location of the soma. The basic morphology of type I neurons, represented by spinalmotor neurons, consists of a cell body called the soma and a long thin axon covered by amyelin sheath. The dendritic tree wraps around the cell body and receives signals from other neurons. The end of the axon has branchingaxon terminals that release neurotransmitters into a gap called thesynaptic cleft between the terminals and the dendrites of the next neuron.[citation needed]
Most neurons can be anatomically characterized as:[4]
Unipolar: single process. Unipolar cells are exclusively sensory neurons. Their dendrites receive sensory information, sometimes directly from the stimulus itself. The cell bodies of unipolar neurons are always found in ganglia. Sensory reception is a peripheral function, so the cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell body in a ganglion, and into the central nervous system.
Bipolar: 1 axon and 1 dendrite. They are found mainly in theolfactory epithelium, and as part of the retina.
A neuron affects other neurons by releasing a neurotransmitter that binds tochemical 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 asexcitatory (causing an increase in firing rate),inhibitory (causing a decrease in firing rate), ormodulatory (causing long-lasting effects not directly related to firing rate).[citation needed]
The two most common (90%+) neurotransmitters in the brain,glutamate andGABA, have largely consistent actions. Glutamate acts on several types of receptors and has effects that are excitatory ationotropic receptors and a modulatory effect atmetabotropic 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 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 releaseacetylcholine, and "inhibitory"spinal neurons that releaseglycine.[citation needed]
The distinction between excitatory and inhibitory neurotransmitters 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 OFFbipolar cells are, like most neurons, excited by the released glutamate. However, neighboring target neurons called ON bipolar cells are instead inhibited by glutamate, because they lack typicalionotropicglutamate receptors and instead express a class of inhibitorymetabotropic glutamate receptors.[21] When light is present, the photoreceptors cease releasing glutamate, which relieves the ON bipolar cells from inhibition, activating them; this simultaneously removes the excitation from the OFF bipolar cells, silencing them.[citation needed]
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 thevisual cortex, whereassomatostatin-expressing neurons typically block dendritic inputs to the postsynaptic neuron.[22]
Neurons have intrinsic electroresponsive properties like intrinsic transmembrane voltageoscillatory patterns.[23] So neurons can be classified according to theirelectrophysiological characteristics:
Tonic or regular spiking. Some neurons are typically constantly (tonically) active, typically firing at a constant frequency. Example: interneurons inneurostriatum.
Phasic orbursting. Neurons that fire in bursts are called phasic.
Fast-spiking. Some neurons are notable for their high firing rates, for example, some types of cortical inhibitory interneurons, cells inglobus pallidus,retinal ganglion cells.[24][25]
Cholinergic neurons – acetylcholine.Acetylcholine is released from presynaptic neurons into the synaptic cleft. It acts as aligand for both ligand-gated ion channels andmetabotropic (GPCRs)muscarinic receptors.Nicotinic receptors are pentameric ligand-gated ion channels composed of alpha and beta subunits that bindnicotine. Ligand binding opens the channel causing the influx ofNa+ depolarization and increases the probability of presynaptic neurotransmitter release. Acetylcholine is synthesized fromcholine andacetyl coenzyme A.
GABAergic neurons –gamma aminobutyric acid. GABA is one of two neuroinhibitors in thecentral nervous system (CNS), along with glycine. GABA has a homologous function toACh, gating anion channels that allowCl− ions to enter the post synaptic neuron. Cl− causes hyperpolarization within the neuron, decreasing the probability of an action potential firing as the voltage becomes more negative (for an action potential to fire, a positive voltage threshold must be reached). GABA is synthesized from glutamate neurotransmitters by the enzymeglutamate decarboxylase.
Glutamatergic neurons – glutamate.Glutamate is one of two primary excitatory amino acid neurotransmitters, along withaspartate. Glutamate receptors are one of four categories, three of which are ligand-gated ion channels and one of which is aG-protein coupled receptor (often referred to as GPCR).
AMPA andKainate receptors function ascation channels permeable to Na+ cation channels mediating fast excitatory synaptic transmission.
NMDA receptors are another cation channel that is more permeable toCa2+. The function of NMDA receptors depends on glycine receptor binding as a co-agonist within the channel pore. NMDA receptors do not function without both ligands present.
Metabotropic receptors, GPCRs modulate synaptic transmission and postsynaptic excitability.
Glutamate can cause excitotoxicity when blood flow to the brain is interrupted, resulting inbrain damage. When blood flow is suppressed, glutamate is released from presynaptic neurons, causing greater NMDA and AMPA receptor activation than normal outside of stress conditions, leading to elevated Ca2+ and Na+ entering the post synaptic neuron and cell damage. Glutamate is synthesized from the amino acid glutamine by the enzymeglutamate synthase.
Dopaminergic neurons—dopamine.Dopamine is a neurotransmitter that acts on D1 type (D1 and D5) Gs-coupled receptors, which increase cAMP and PKA, and D2 type (D2, D3, and D4) receptors, which activate Gi-coupled receptors that decrease cAMP and PKA. Dopamine is connected to mood and behavior and modulates both pre- and post-synaptic neurotransmission. Loss of dopamine neurons in thesubstantia nigra has been linked toParkinson's disease. Dopamine is synthesized from the amino acidtyrosine. Tyrosine is catalyzed into levodopa (orL-DOPA) bytyrosine hydroxlase, and levodopa is then converted into dopamine by the aromatic amino aciddecarboxylase.
Serotonergic neurons—serotonin.Serotonin (5-Hydroxytryptamine, 5-HT) can act as excitatory or inhibitory. Of its four 5-HT receptor classes, 3 are GPCR and 1 is a ligand-gated cation channel. Serotonin is synthesized fromtryptophan bytryptophan hydroxylase, and then further by decarboxylase. A lack of 5-HT at postsynaptic neurons has been linked to depression. Drugs that block the presynapticserotonin transporter are used for treatment, such asProzac andZoloft.
Purinergic neurons—ATP.ATP is a neurotransmitter acting at both ligand-gated ion channels (P2X receptors) and GPCRs (P2Y) receptors. ATP is, however, best known as acotransmitter. Suchpurinergic signaling can also be mediated by otherpurines likeadenosine, which particularly acts at P2Y receptors.
Since 2012 there has been a push from the cellular andcomputational neuroscience community to come up with 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 calledpatch-sequencing in which all three qualities can be measured at once is used extensively by theAllen Institute for Brain Science.[27] 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.[28]
A signal propagating down an axon to the cell body and dendrites of the next cellChemical synapse
Neurons communicate with each other viasynapses, where either theaxon terminal of one cell contacts another neuron's dendrite, soma, or, less commonly, axon. Neurons such as Purkinje cells in 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 thesupraoptic 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 conductivejunctions between cells.[29]
When an action potential reaches the axon terminal, it opensvoltage-gated calcium channels, allowingcalcium ions to enter the terminal. Calcium causessynaptic 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 theaxon terminal triggers mitochondrial calcium uptake, which, in turn, activates mitochondrialenergy metabolism to produceATP to support continuous neurotransmission.[30]
Anautapse is a synapse in which a neuron's axon connects to its dendrites.
Thehuman brain has some 8.6 x 1010 (eighty six billion) neurons.[31][32] 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 1015 synapses (1 quadrillion).This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 1014 to 5 x 1014 synapses (100 to 500 trillion).[33]
Beyond electrical and chemical signaling, studies suggest neurons in healthy human brains can also communicate through:
force generated by the enlargement of dendritic spines[34]
the transfer ofproteins – transneuronally transported proteins (TNTPs)[35][36]
They can also get modulated by input from the environment andhormones released from other parts of the organism,[37] which could be influenced more or less directly by neurons. This also applies toneurotrophins such asBDNF. Thegut microbiome is also connected with the brain.[38]Neurons also communicate withmicroglia, 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.[39]
In 1937John Zachary Young suggested that thesquid giant axon could be used to study neuronal electrical properties.[40] 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 themembrane 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 generatesubthreshold membrane potential oscillations. These signals are generated and propagated by charge-carryingions including sodium (Na+), potassium (K+), chloride (Cl−), andcalcium (Ca2+).
Several stimuli can activate a neuron leading to electrical activity, includingpressure, stretch, chemical transmitters, and changes in the electric potential across the cell membrane.[41] 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.[42]
Thin neurons and axons require lessmetabolic 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 ofmyelin around their axons. The sheaths are formed byglial cells:oligodendrocytes in the central nervous system andSchwann cells in the peripheral nervous system. The sheath enables action potentials to travelfaster 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 unsheathednodes of Ranvier, which contain a high density of voltage-gated ion channels.Multiple sclerosis 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 agraded electrical signal, which in turn causes graded neurotransmitter release. Suchnon-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 is represented in the brain by neurons. The main goal of studying neural coding is to characterize the relationship between thestimulus and the individual orensemble neuronal responses and the relationships among the electrical activities of the neurons within the ensemble.[43] It is thought that neurons can encode bothdigital andanalog information.[44]
As long as the stimulus reaches the threshold, the full response will be given. A larger stimulus does not result in a larger response, and vice versa.[45]: 31
The conduction of nerve impulses is an example of anall-or-none response. In other words, if 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.[45]: 31 Receptors respond in different ways to stimuli. Slowly adapting ortonic 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 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 includeskin which, when touched causes neurons to fire, but if 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.
Thepacinian 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 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.[46]
The word was adopted in French with the spellingneurone. That spelling was also used by many writers in English,[49] but has now become rare in American usage and uncommon in British usage.[50][48]
Some previous works usednerve cell (cellule nervose), as adopted inCamillo Golgi's 1873 paper on the discovery of the silver staining technique used to visualize nervous tissue under light microscopy.[51]
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 anatomistSantiago Ramón y Cajal.[52]
To make the structure of individual neurons visible, Ramón y Cajal improved asilver staining process that had been developed byCamillo Golgi.[52] The improved process involves 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 foranastomosis between axons and dendrites and called each nervous element "an autonomous canton."[52][47] This became known as theneuron doctrine, one of the central tenets of modernneuroscience.[52]
In 1891, the German anatomistHeinrich Wilhelm Waldeyer wrote a highly influential review of the neuron doctrine in which he introduced the termneuron to describe the anatomical and physiological unit of the nervous system.[53][54]
The silver impregnation stains are a useful method forneuroanatomical investigations because, for reasons unknown, it stains only a small percentage of cells in a tissue, exposing the complete micro structure of individual neurons without much overlap from other cells.[55]
The neuron doctrine is the now fundamental idea that neurons are the basic structural and functional units of the nervous system. The theory was put forward by Santiago Ramón y Cajal in the late 19th century. It held that neurons are discrete cells (not connected in a meshwork), acting as metabolically distinct units.
Later discoveries yielded refinements to the doctrine. For example,glial cells, which are non-neuronal, play an essential role in information processing.[56] Also, electrical synapses are more common than previously thought,[57] comprising direct, cytoplasmic connections between neurons; In fact, neurons can form even tighter couplings: the squid giant axon arises from the fusion of multiple axons.[58]
Ramón y Cajal also postulated the Law of Dynamic Polarization, which states that a neuron receives signals at its dendrites and cell body and transmits them, as action potentials, along the axon in one direction: away from the cell body.[59] The Law of Dynamic Polarization has important exceptions; dendrites can serve as synaptic output sites of neurons[60] and axons can receive synaptic inputs.[61]
Although neurons are often described as "fundamental units" of the brain, they perform internal computations. Neurons integrate input within dendrites, and this complexity is lost in models that assume neurons to be a fundamental unit. Dendritic branches can be modeled as spatial compartments, whose activity is related to passive membrane properties, but may also be different depending on input from synapses.Compartmental modelling of dendrites is especially helpful for understanding the behavior of neurons that are too small to record with electrodes, as is the case forDrosophila melanogaster.[62]
The number of neurons in the brain varies dramatically from species to species.[63] In a human, there are an estimated 10–20 billion neurons in thecerebral cortex and 55–70 billion neurons in thecerebellum.[64] By contrast, thenematode wormCaenorhabditis elegans has just 302 neurons, making it an idealmodel organism as scientists have been able to map all of its neurons. The fruit flyDrosophila melanogaster, a common subject in biological experiments, has around 100,000 neurons and exhibits many complex behaviors. Many properties of neurons, from the type of neurotransmitters used to ion channel composition, are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems.
Charcot–Marie–Tooth disease (CMT) is a heterogeneous inherited disorder of nerves (neuropathy) that is characterized by loss of muscle tissue and touch sensation, predominantly in the feet and legs extending to the hands and arms in advanced stages. Presently incurable, this disease is one of the most common inheritedneurological disorders, affecting 36 in 100,000 people.[65]
Alzheimer's disease (AD), also known simply asAlzheimer's, is aneurodegenerative disease characterized by progressivecognitive deterioration, together with declining activities of daily living andneuropsychiatric symptoms or behavioral changes.[66] The most striking early symptom is loss of short-term memory (amnesia), which usually manifests as minor forgetfulness that becomes steadily more pronounced with illness progression, with relative preservation of older memories. As the disorder progresses, cognitive (intellectual) impairment extends to the domains of language (aphasia), skilled movements (apraxia), and recognition (agnosia), and functions such as decision-making and planning become impaired.[67][68]
Parkinson's disease (PD), also known asParkinson's, is a degenerative disorder of the central nervous system that often impairs motor skills and speech.[69] Parkinson's disease belongs to a group of conditions calledmovement disorders.[70] It is characterized by muscle rigidity,tremor, a slowing of physical movement (bradykinesia), and in extreme cases, a loss of physical movement (akinesia). The primary symptoms are the results of decreased stimulation of themotor cortex by thebasal ganglia, normally caused by the insufficient formation and action of dopamine, which is produced in thedopaminergic neurons of the brain. Secondary symptoms may include high-levelcognitive dysfunction and subtle language problems. PD is both chronic and progressive.
Demyelination is a process characterized by the gradual loss of the myelin sheath enveloping nerve fibers. When myelin deteriorates, signal conduction along nerves can be significantly impaired or lost, and the nerve eventually withers. Demyelination may affect both central and peripheral nervous systems, contributing to various neurological disorders such asmultiple sclerosis,Guillain-Barré syndrome, andchronic inflammatory demyelinating polyneuropathy. Although demyelination is often caused by anautoimmune reaction, it may also be caused by viral infections, metabolic disorders, trauma, and some medications.
Although most injury responses include a calcium influx signaling to promote resealing of severed parts, axonal injuries initially lead to acuteaxonal degeneration, which is the rapid separation of the proximal and distal ends, occurring within 30 minutes of injury.[71] Degeneration follows with swelling of theaxolemma, and eventually leads to bead-like formation. Granular disintegration of the axonalcytoskeleton and innerorganelles occurs after axolemma degradation. Early changes include accumulation ofmitochondria in the paranodal regions at the site of injury. The endoplasmic reticulum degrades and mitochondria swell up and eventually disintegrate. The disintegration is dependent onubiquitin andcalpainproteases (caused by the influx of calcium ions), suggesting that axonal degeneration is an active process that produces complete fragmentation. The process takes about roughly 24 hours in the PNS and longer in the CNS. The signaling pathways leading to axolemma degeneration are unknown.
Neurons initially develop from theneural tube in the embryo. The neural tube has three layers – aventricular zone, anintermediate zone, and a marginal zone. The ventricular zone surrounds the tube's central canal and becomes theependyma. Dividing cells of the ventricular zone form the intermediate zone which stretches to the outermost layer of the neural tube called the pial layer. Thegray matter of the brain is derived from the intermediate zone. The extensions of the neurons in the intermediate zone make up the marginal zone whenmyelinated becomes the brain'swhite matter.[72]
Differentiation of the neurons is ordered by their size. Largemotor neurons are first. Smaller sensory neurons together withglial cell differentiate atbirth.[72]
Adult neurogenesis can occur and studies of the age of human neurons suggest that this process occurs only for a minority of cells and that the vast majority of neurons in theneocortex form before birth and persist without replacement. The extent to which adult neurogenesis exists in humans, and its contribution to cognition are controversial, with conflicting reports published in 2018.[73]
The body contains a variety of stem cell types that can differentiate into neurons. Researchers found a way to transform human skin cells into nerve cells usingtransdifferentiation, in which "cells are forced to adopt new identities".[74]
At different stages of mammalian nervous system development, two DNA repair processes are employed in the repair of DNA double-strand breaks. These pathways arehomologous recombinational repair used in proliferating neural precursor cells, andnon-homologous end joining used mainly at later developmental stages[76]
Intercellular communication between developing neurons andmicroglia is also indispensable for proper neurogenesis and brain development.[77]
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^abFinger, Stanley (1994).Origins of neuroscience: a history of explorations into brain function. Oxford University Press. p. 47.ISBN9780195146943.OCLC27151391.Ramon y Cajal's first paper on the Golgi stain was on the bird cerebellum, and it appeared in theRevista in 1888. He acknowledged that he found the nerve fibers to be very intricate, but stated that he could find no evidence for either axons or dendrites undergoing anastomosis and forming nets. He called each nervous element 'an autonomous canton.'
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^Finger, Stanley (1994).Origins of neuroscience: a history of explorations into brain function. Oxford University Press. p. 47.ISBN9780195146943.OCLC27151391.... a man who would write a highly influential review of the evidence in favor of the neuron doctrine two years later. In his paper, Waldeyer (1891), ... , wrote that nerve cells terminate freely with end arborizations and that the 'neuron' is the anatomical and physiological unit of the nervous system. The word 'neuron' was born this way.
^"Whonamedit - dictionary of medical eponyms".www.whonamedit.com.Today, Wilhelm von Waldeyer-Hartz is remembered as the founder of the neurone theory, coining the term "neurone" to describe the cellular function unit of the nervous system and enunciating and clarifying that concept in 1891.
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^Callaway, Ewen (26 May 2011). "How to make a human neuron".Nature.doi:10.1038/news.2011.328.By transforming cells from human skin into working nerve cells, researchers may have come up with a model for nervous-system diseases and perhaps even regenerative therapies based on cell transplants. The achievement, reported online today inNature, is the latest in a fast-moving field called transdifferentiation, in which cells are forced to adopt new identities. In the past year, researchers have converted connective tissue cells found in the skin into heart cells, blood cells, and liver cells.
