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Neuron
Neuron
Neuron
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Neuron
Anatomy of a multipolar neuron
Identifiers
MeSHD009474
NeuroLex IDsao1417703748
TA98A14.0.00.002
THH2.00.06.1.00002
FMA54527
Anatomical terms of neuroanatomy

A neuron (American English), neurone (British English),[1] 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 the nervous system 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 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 the evolution of the nervous system.[2]

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 and then to the sensorial area in the brain. Motor neurons receive signals from the brain and spinal cord to control everything from muscle contractions[3] to glandular 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 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.[4] The soma or cell body, 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 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 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 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 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.

Nervous system

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Neurons are the primary components of the nervous system, along with the glial cells that give them structural and metabolic support.[5] 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.[6] 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 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.

Anatomy and histology

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Diagram of a typical myelinated vertebrate motor neuron
Schematic of a single pyramidal neuron, with a synapse from an incoming axon onto a dendritic spine

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 100 micrometers in diameter.[7]

  • The soma is the body of the neuron. As it contains the nucleus, most protein synthesis occurs here. The nucleus can range from 3 to 18 micrometers in diameter.[8]
  • The dendrites of a neuron are cellular extensions with many branches. This overall shape and structure are referred to metaphorically as a dendritic tree. The branches form fractal patterns that repeat at multiple size scales.[9] This fractal tree is where the majority of input to the neuron occurs via the dendritic spine.
  • The axon 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 carries nerve 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 the axon hillock. Besides being an anatomical structure, the axon hillock also has the greatest density of voltage-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 negative threshold potential.
    • While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons.
  • The axon terminal is found at the end of the axon farthest from the soma and contains synapses. Synaptic boutons are specialized structures where neurotransmitter chemicals are released to communicate with target neurons. In addition to synaptic boutons at the axon terminal, a neuron may have en passant boutons, which are located along the length of the axon.

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.[10]

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 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 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[11] however, stem cells present in 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.[12]

Membrane

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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 embedded protein structures.[13] A lipid bilayer 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 gated, 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 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 ions include sodium, potassium, chloride, and calcium. The interactions between ion channels and ion pumps produce a voltage difference across the membrane, typically a little 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 machineries that are embedded in the membrane; second, it provides a basis for electrical signal transmission between different parts of the membrane.

Histology and internal structure

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Golgi-stained neurons in human hippocampal tissue
Actin filaments in a mouse cortical neuron in culture

Numerous microscopic clumps called Nissl bodies (or Nissl substance) are seen when nerve cell bodies are stained with 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[14] 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 called neurofilaments, which together with neurotubules (neuronal microtubules) are assembled into larger neurofibrils.[15] 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.[16][17][18] Other structural proteins that are important for neuronal function are actin and 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.[19]

There are different internal structural characteristics between axons and dendrites. Typical axons seldom contain ribosomes, except some in the initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes, in diminishing amounts as the distance from the cell body increases.

Classification

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Image of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The red staining indicates GABAergic interneurons.[20]
See also: List of distinct cell types in the adult human body § Nervous system

Neurons vary in shape and size and can be classified by their morphology and function.[21] 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 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 the terminals and the dendrites of the next neuron.[citation needed]

Structural classification

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Polarity

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Different kinds of neurons

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 the olfactory epithelium, and as part of the retina.
  • Multipolar: 1 axon and 2 or more dendrites
    • Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells
    • Golgi II: neurons whose axonal process projects locally; the best example is the granule cell
  • Anaxonic: where the axon cannot be distinguished from the dendrite(s)
  • Pseudounipolar: 1 process which then serves as both an axon and a dendrite

Other

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Some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:[citation needed]

Functional classification

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Direction

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  • Afferent neurons convey information from tissues and organs into the central nervous system and are also called sensory neurons.
  • Efferent neurons (motor neurons) transmit signals from the central nervous system to the effector cells.
  • Interneurons connect neurons within specific regions of the central nervous system.

Afferent and efferent also refer generally to neurons that, respectively, bring information to or send information from the brain.

Action on other neurons

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A neuron affects other neurons by releasing 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. Receptors are classified broadly as excitatory (causing an increase in firing rate), inhibitory (causing a decrease in firing rate), or modulatory (causing long-lasting effects not directly related to firing rate).[citation needed]

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 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.[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 OFF bipolar 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 typical ionotropic glutamate receptors and instead express a class of inhibitory metabotropic glutamate receptors.[22] 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 the visual cortex, whereas somatostatin-expressing neurons typically block dendritic inputs to the postsynaptic neuron.[23]

Discharge patterns

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Neurons have intrinsic electroresponsive properties like intrinsic transmembrane voltage oscillatory patterns.[24] So neurons can be classified according to their electrophysiological characteristics:

  • Tonic or regular spiking. Some neurons are typically constantly (tonically) active, typically firing at a constant frequency. Example: interneurons in neurostriatum.
  • Phasic or bursting. 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 in globus pallidus, retinal ganglion cells.[25][26]

Neurotransmitter

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Synaptic vesicles containing neurotransmitters
Main article: Neurotransmitter

Neurotransmitters are chemical messengers passed from one neuron to another neuron or to a muscle cell or gland cell.

  • Cholinergic neurons – acetylcholine. Acetylcholine is released from presynaptic neurons into the synaptic cleft. It acts as a ligand for both ligand-gated ion channels and metabotropic (GPCRs) muscarinic receptors. Nicotinic receptors are pentameric ligand-gated ion channels composed of alpha and beta subunits that bind nicotine. Ligand binding opens the channel causing the influx of Na+ depolarization and increases the probability of presynaptic neurotransmitter release. Acetylcholine is synthesized from choline and acetyl coenzyme A.
  • Adrenergic neurons – noradrenaline. Noradrenaline (norepinephrine) is released from most postganglionic neurons in the sympathetic nervous system onto two sets of GPCRs: alpha adrenoceptors and beta adrenoceptors. Noradrenaline is one of the three common catecholamine neurotransmitters, and the most prevalent of them in the peripheral nervous system; as with other catecholamines, it is synthesized from tyrosine.
  • GABAergic neurons – gamma aminobutyric acid. GABA is one of two neuroinhibitors in the central nervous system (CNS), along with glycine. GABA has a homologous function to ACh, gating anion channels that allow Cl− 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 enzyme glutamate decarboxylase.
  • Glutamatergic neurons – glutamate. Glutamate is one of two primary excitatory amino acid neurotransmitters, along with aspartate. Glutamate receptors are one of four categories, three of which are ligand-gated ion channels and one of which is a G-protein coupled receptor (often referred to as GPCR).
  1. AMPA and Kainate receptors function as cation channels permeable to Na+ cation channels mediating fast excitatory synaptic transmission.
  2. NMDA receptors are another cation channel that is more permeable to Ca2+. 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.
  3. Metabotropic receptors, GPCRs modulate synaptic transmission and postsynaptic excitability.
Glutamate can cause excitotoxicity when blood flow to the brain is interrupted, resulting in brain 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 enzyme glutamate synthase.

Multimodel classification

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Since 2012 there has been a push from the cellular and computational 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 called patch-sequencing in which all three qualities can be measured at once is used extensively by the Allen Institute for Brain Science.[28] 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.[29]

Connectivity

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Main articles: Synapse and Chemical synapse
A signal propagating down an axon to the cell body and dendrites of the next cell
Chemical synapse

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 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.[30]

When an action potential reaches 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.[31]

An autapse is a synapse in which a neuron's axon connects to its dendrites.

The human brain has some 8.6 x 1010 (eighty six billion) neurons.[32][33] 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).[34]

Nonelectrochemical signaling

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Beyond electrical and chemical signaling, studies suggest neurons in healthy human brains can also communicate through:

  • force generated by the enlargement of dendritic spines[35]
  • the transfer of proteins – transneuronally transported proteins (TNTPs)[36][37]

They can also get modulated by input from the environment and hormones released from other parts of the organism,[38] 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.[39] 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.[40]

Mechanisms for propagating action potentials

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Main article: Action potential

In 1937 John Zachary Young suggested that the squid giant axon could be used to study neuronal electrical properties.[41] 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 (Ca2+).

Several stimuli can activate a neuron leading to electrical activity, including pressure, stretch, chemical transmitters, and changes in the electric potential across the cell membrane.[42] 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.[43]

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 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 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

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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 the stimulus and the individual or ensemble neuronal responses and the relationships among the electrical activities of the neurons within the ensemble.[44] It is thought that neurons can encode both digital and analog information.[45]

All-or-none principle

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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.[46]: 31 
Main article: All-or-none law

The conduction of nerve impulses is an example of an all-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.[46]: 31  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 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 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 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.[47]

Although neurons have long been assumed to always give a stereotyped maximal response or none at all, there is a body of research that argues that this is only partially correct, and that while it is true that Neurons either fire an Action Potential or do not, the amplitude and duration of the Action Potentials that a Neuron fires can vary greatly, allowing the Neuron to encode information in at least the strength of the Action Potential. Additionally, the analog information carried in the Action Potential has been shown to be able to survive and travel distances originally not thought to be possible. This has been proposed to be a highly effective way to encode information compared to the usual rate and temporal coding theories commonly seen in the literature, with the ability to transfer around 4 times more information than current wisdom would suggest.[48][49][50][51][52]

Etymology and spelling

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The German anatomist Heinrich Wilhelm Waldeyer introduced the term neuron in 1891,[53] based on the ancient Greek νεῦρον neuron 'sinew, cord, nerve'.[54]

The word was adopted in French with the spelling neurone. That spelling was also used by many writers in English,[55] but has now become rare in American usage and uncommon in British usage.[56][54]

Some previous works used nerve cell (cellule nervose), as adopted in Camillo Golgi's 1873 paper on the discovery of the silver staining technique used to visualize nervous tissue under light microscopy.[57]

History

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Further information: History of neuroscience
Drawing by Camillo Golgi of a hippocampus stained using the silver nitrate method
Drawing of a Purkinje cell in the cerebellar cortex done by Santiago Ramón y Cajal, demonstrating the ability of Golgi's staining method to reveal fine detail

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.[58]

To make the structure of individual neurons visible, Ramón y Cajal improved a silver staining process that had been developed by Camillo Golgi.[58] 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 for anastomosis between axons and dendrites and called each nervous element "an autonomous canton."[58][53] This became known as the neuron doctrine, one of the central tenets of modern neuroscience.[58]

In 1891, the German anatomist Heinrich Wilhelm Waldeyer wrote a highly influential review of the neuron doctrine in which he introduced the term neuron to describe the anatomical and physiological unit of the nervous system.[59][60]

The silver impregnation stains are a useful method for neuroanatomical 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.[61]

Neuron doctrine

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Drawing of neurons in the pigeon cerebellum, by Spanish neuroscientist Santiago Ramón y Cajal in 1899. (A) denotes Purkinje cells and (B) denotes granule cells, both of which are multipolar.

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.[62] Also, electrical synapses are more common than previously thought,[63] 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.[64]

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.[65] The Law of Dynamic Polarization has important exceptions; dendrites can serve as synaptic output sites of neurons[66] and axons can receive synaptic inputs.[67]

Compartmental modelling of neurons

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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 for Drosophila melanogaster.[68]

Neurons in the brain

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The number of neurons in the brain varies dramatically from species to species.[69] In a human, there are an estimated 10–20 billion neurons in the cerebral cortex and 55–70 billion neurons in the cerebellum.[70] By contrast, the nematode worm Caenorhabditis elegans has just 302 neurons, making it an ideal model organism as scientists have been able to map all of its neurons. The fruit fly Drosophila 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.

Neurological disorders

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Main article: Neurological disorders

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 inherited neurological disorders, affecting 36 in 100,000 people.[71]

Alzheimer's disease (AD), also known simply as Alzheimer's, is a neurodegenerative disease characterized by progressive cognitive deterioration, together with declining activities of daily living and neuropsychiatric symptoms or behavioral changes.[72] 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.[73][74]

Parkinson's disease (PD), also known as Parkinson's, is a degenerative disorder of the central nervous system that often impairs motor skills and speech.[75] Parkinson's disease belongs to a group of conditions called movement disorders.[76] 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 the motor cortex by the basal ganglia, normally caused by the insufficient formation and action of dopamine, which is produced in the dopaminergic neurons of the brain. Secondary symptoms may include high-level cognitive dysfunction and subtle language problems. PD is both chronic and progressive.

Myasthenia gravis is a neuromuscular disease leading to fluctuating muscle weakness and fatigability during simple activities. Weakness is typically caused by circulating antibodies that block acetylcholine receptors at the postsynaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated with immunosuppressants, cholinesterase inhibitors and, in selected cases, thymectomy.

Demyelination

[edit]
Further information: Demyelinating disease
Guillain–Barré syndrome – demyelination

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 as multiple sclerosis, Guillain-Barré syndrome, and chronic inflammatory demyelinating polyneuropathy. Although demyelination is often caused by an autoimmune reaction, it may also be caused by viral infections, metabolic disorders, trauma, and some medications.

Axonal degeneration

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Although most injury responses include a calcium influx signaling to promote resealing of severed parts, axonal injuries initially lead to acute axonal degeneration, which is the rapid separation of the proximal and distal ends, occurring within 30 minutes of injury.[77] Degeneration follows with swelling of the axolemma, and eventually leads to bead-like formation. Granular disintegration of the axonal cytoskeleton and inner organelles occurs after axolemma degradation. Early changes include accumulation of mitochondria in the paranodal regions at the site of injury. The endoplasmic reticulum degrades and mitochondria swell up and eventually disintegrate. The disintegration is dependent on ubiquitin and calpain proteases (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.

Development

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Main article: Neurogenesis

Neurons develop through the process of neurogenesis, in which neural stem cells divide to produce differentiated neurons. Once fully differentiated they are no longer capable of undergoing mitosis. Neurogenesis primarily occurs during embryonic development.

Neurons initially develop from the neural tube in the embryo. The neural tube has three layers – a ventricular zone, an intermediate zone, and a marginal zone. The ventricular zone surrounds the tube's central canal and becomes the ependyma. Dividing cells of the ventricular zone form the intermediate zone which stretches to the outermost layer of the neural tube called the pial layer. The gray matter of the brain is derived from the intermediate zone. The extensions of the neurons in the intermediate zone make up the marginal zone when myelinated becomes the brain's white matter.[78]

Differentiation of the neurons is ordered by their size. Large motor neurons are first. Smaller sensory neurons together with glial cell differentiate at birth.[78]

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 the neocortex 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.[79]

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 using transdifferentiation, in which "cells are forced to adopt new identities".[80]

During neurogenesis in the mammalian brain, progenitor and stem cells progress from proliferative divisions to differentiative divisions. This progression leads to the neurons and glia that populate cortical layers. Epigenetic modifications play a key role in regulating gene expression in differentiating neural stem cells, and are critical for cell fate determination in the developing and adult mammalian brain. Epigenetic modifications include DNA cytosine methylation to form 5-methylcytosine and 5-methylcytosine demethylation.[81] DNA cytosine methylation is catalyzed by DNA methyltransferases (DNMTs). Methylcytosine demethylation is catalyzed in several stages by TET enzymes that carry out oxidative reactions (e.g. 5-methylcytosine to 5-hydroxymethylcytosine) and enzymes of the DNA base excision repair (BER) pathway.[81]

At different stages of mammalian nervous system development, two DNA repair processes are employed in the repair of DNA double-strand breaks. These pathways are homologous recombinational repair used in proliferating neural precursor cells, and non-homologous end joining used mainly at later developmental stages[82]

Intercellular communication between developing neurons and microglia is also indispensable for proper neurogenesis and brain development.[83]

Nerve regeneration

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Main article: Neuroregeneration

Peripheral axons can regrow if they are severed,[84] but one neuron cannot be functionally replaced by one of another type (Llinás' law).[24]

See also

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References

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Further reading

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Neuron

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A neuron, also known as a nerve cell, is the fundamental structural and functional unit of the , responsible for receiving, processing, and transmitting information through electrical and chemical signals to enable sensory perception, , and cognitive functions throughout the body. Neurons are electrically excitable cells that maintain a resting of approximately -70 mV due to ion gradients, primarily involving sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻) ions, and generate action potentials to propagate signals along their length. The typical neuron consists of three main components: the soma (cell body), which houses the nucleus and organelles for protein synthesis and metabolic activities; dendrites, branched extensions that receive incoming signals from other neurons or sensory receptors; and the , a long, slender projection that conducts outgoing electrical impulses away from the soma toward target cells. Axons are often insulated by a sheath formed by glial cells, which accelerates via , and terminate in synaptic terminals that release neurotransmitters to communicate across synapses. Structurally, neurons are classified into several types based on the number and arrangement of processes extending from the soma: multipolar neurons, which have one axon and multiple dendrites and are the most common type in the central nervous system; bipolar neurons, featuring one axon and one dendrite, typically found in sensory pathways like the retina and olfactory epithelium; pseudounipolar (or unipolar) neurons, with a single process that bifurcates into axon-like and dendrite-like branches, common in sensory ganglia; and anaxonic neurons, lacking a distinct axon, such as those in the retina or cerebellum. Functionally, neurons are categorized into three primary groups: sensory (afferent) neurons, which convey information from peripheral sensory organs to the central nervous system; motor (efferent) neurons, which transmit commands from the central nervous system to muscles or glands for action; and interneurons, which integrate signals within the central nervous system to facilitate processing and coordination between sensory and motor pathways. In the and , there are approximately 86 to 100 billion neurons, forming vast networks that underpin complex behaviors and cognition, with most generated during though limited persists in regions like the hippocampus into adulthood. Once mature, neurons generally do not divide but can undergo plasticity, strengthening or weakening connections through synaptic remodeling in response to experience or injury.

Role in the Nervous System

Definition and Basic Functions

A neuron is an electrically excitable cell specialized for communication through the transmission of electrochemical signals across the . These cells form the fundamental building blocks of the , enabling rapid information processing and coordination of bodily functions. The primary functions of neurons involve receiving inputs from other cells via synapses, integrating these signals through of excitatory and inhibitory influences, and generating outputs in the form of action potentials that propagate along the to influence target cells such as muscles or other neurons. This process allows neurons to act as decision-making units, firing only when integrated inputs reach a threshold, thereby ensuring efficient signal relay without constant activity. Through interconnected networks, neurons collectively facilitate essential processes including sensory perception by converting environmental stimuli into neural signals, by directing muscle contractions and glandular secretions, and higher cognitive functions such as learning and formation. Neurons are among the longest-lived cells in the , with many post-mitotic neurons persisting for the entire lifespan, often exceeding a century in long-lived individuals.

Distribution Across Nervous System Components

Neurons are distributed throughout the (CNS), which comprises the and , and the peripheral nervous system (PNS), which includes cranial and spinal nerves as well as ganglia. The vast majority of neurons reside in the CNS, where they form complex networks for processing and integration, while PNS neurons primarily serve as conduits for sensory input and motor output between the CNS and the body's periphery. In humans, the alone houses approximately 86 billion neurons, underscoring the CNS's role as the primary site of neural computation. Neuron density varies significantly across CNS regions to support specialized functions; for instance, the contains about 80% of the brain's total neurons despite comprising only 10% of its mass, due to the high packing of small granule cells. In the , motor neurons are concentrated in the ventral horn, where their cell bodies give rise to axons that innervate skeletal muscles via the ventral roots. , which facilitate local circuit integration, are abundant in the , comprising 20-30% of its neuronal population and providing essential inhibitory and excitatory connections. Within the PNS, sensory neurons are clustered in dorsal root ganglia adjacent to the , with their cell bodies pseudounipolar and axons bifurcating to convey peripheral stimuli to the CNS. Adaptations in myelination reflect functional demands: many PNS axons, particularly large-diameter motor and proprioceptive sensory fibers, are myelinated by Schwann cells to enable and rapid signal transmission over long distances. In contrast, unmyelinated fibers predominate in the , such as postganglionic sympathetic and parasympathetic axons, allowing slower, modulatory influences on visceral organs like and glandular secretion.

Anatomy

Somatic Structure

The soma, or cell body, of a neuron is a compact, spherical or polygonal structure that serves as the central hub of the cell, housing the nucleus and various organelles essential for cellular maintenance and function. It typically constitutes about one-tenth of the neuron's total volume and contains the nucleus, which encloses the genetic material (DNA) required for cellular operations, along with organelles such as the (both smooth and rough), Golgi apparatus, mitochondria, , and lysosomes. A prominent feature within the soma are the Nissl bodies, specialized clusters of rough (rER) studded with ribosomes, which are rich in and responsible for synthesizing proteins critical to neuronal structure and activity, including secretory, membrane, and cytoskeletal proteins. These Nissl bodies, named after Franz Nissl, appear as dense granules or blocks in the —larger in motor neurons and more dust-like in sensory cells—and are absent from the , facilitating the neuron's high metabolic demands for . Extending from the soma are dendrites, which form a highly branched, tree-like network of short, tapering processes that increase the neuron's surface area for receiving inputs. These structures, often numbering several from each neuron, consist of dendritic shafts covered in numerous small protrusions called dendritic spines, which provide sites for synaptic contacts and compartmentalize signaling. Dendrites contain organelles similar to those in the soma, including Nissl bodies in their proximal regions, supporting local protein synthesis and structural integrity. The integration of inputs occurs primarily at the soma, where signals from dendrites converge to influence the neuron's overall activity. The axon emerges as a single, elongated fiber from the soma, specialized for transmitting signals over distances, and originates at the —a cone-shaped junction devoid of large organelles like Nissl bodies or the Golgi apparatus. Adjacent to the hillock is the initial segment, a short (approximately 30-40 micrometers long) that extends from the soma and contains dense concentrations of neurofilaments, , mitochondria, and possibly residual Nissl substance, marking it as a key structural zone for signal initiation. Axons vary widely in length, from millimeters in to up to 1 meter in projection neurons such as those in the human , which spans from the to the foot. Their diameters, ranging from 1 to 25 micrometers, also differ significantly, with larger diameters generally permitting faster signal conduction along the axon.

Membrane Properties and Ion Channels

The plasma membrane of a neuron consists of a fluid bilayer, formed by two layers of amphipathic molecules with hydrophilic heads facing the aqueous environments inside and outside the cell, and hydrophobic tails forming the inner core. This structure creates a semipermeable barrier that restricts passive diffusion, while embedded proteins, including transporters and channels, facilitate selective ion movement essential for electrical signaling. The resting of neurons is typically around -70 mV, with the cell interior negative relative to the exterior, primarily due to higher permeability to K⁺ and the active maintenance of gradients by the Na⁺/K⁺-ATPase pump. This electrogenic pump hydrolyzes ATP to export three Na⁺ and import two K⁺ per cycle, counteracting passive leaks and sustaining the unequal distributions (high intracellular K⁺ ~140 mM and low Na⁺ ~10 mM; high extracellular Na⁺ ~145 mM and low K⁺ ~4 mM). Neuronal excitability arises from specialized ion channels that regulate membrane permeability to ions like Na⁺, K⁺, and Ca²⁺. Voltage-gated ion channels open or close in response to changes in , enabling rapid (via Na⁺ influx through voltage-gated Na⁺ channels) and (via K⁺ efflux through voltage-gated K⁺ channels), while voltage-gated Ca²⁺ channels support processes like synaptic transmission by allowing Ca²⁺ entry. Ligand-gated channels, activated by binding, mediate synaptic integration by permitting ion fluxes that either depolarize (e.g., Na⁺ or Ca²⁺ permeable) or hyperpolarize (e.g., Cl⁻ or K⁺ permeable) the membrane. Leak channels, which are constitutively open, provide background permeability—predominantly to K⁺—that stabilizes the and sets the baseline for excitability. The Goldman-Hodgkin-Katz (GHK) equation provides a quantitative model for the under steady-state conditions, incorporating relative permeabilities (P) and concentrations of multiple permeant s, unlike the which applies to single s. Derived from the constant-field theory, it assumes a uniform across the and derives fluxes as functions of concentration gradients and voltage, setting net current to zero at equilibrium. The unidirectional flux of an species (e.g., cation X) through the is given by the Goldman flux : JX=−PXzXFVmRT(1−ezXFVm/RT)([X]i−[X]oezXFVm/RT)J_X = -P_X \frac{z_X F V_m}{RT (1 - e^{z_X F V_m / RT})} \left( [X]_i - [X]_o e^{z_X F V_m / RT} \right) where PXP_X is permeability, zXz_X is valence, FF is Faraday's constant, RR is the gas constant, TT is temperature, VmV_m is membrane potential, and subscripts i/o denote intra/extracellular concentrations. The net flux is JXnet=JXi−JXoJ_X^{net} = J_X^i - J_X^o, but at steady state with no net current, the sum of fluxes for all ions (considering opposite signs for anions) equals zero. For monovalent cations (Na⁺, K⁺) and anions (Cl⁻), this simplifies to the GHK voltage equation: Vm=RTFln⁡(PK[K+]o+PNa[Na+]o+PCl[Cl−]iPK[K+]i+PNa[Na+]i+PCl[Cl−]o)V_m = \frac{RT}{F} \ln \left( \frac{P_K [K^+]_o + P_{Na} [Na^+]_o + P_{Cl} [Cl^-]_i}{P_K [K^+]_i + P_{Na} [Na^+]_i + P_{Cl} [Cl^-]_o} \right) Here, parameters reflect typical neuronal values: PK:PNa:PCl≈1:0.04:0.45P_K : P_{Na} : P_{Cl} \approx 1 : 0.04 : 0.45, yielding Vm≈−70V_m \approx -70 mV when combined with measured concentrations. This equation highlights how permeability ratios, rather than concentrations alone, determine the potential. Mutations in genes encoding neuronal ion channels, termed channelopathies, alter channel function and thereby disrupt membrane excitability; for instance, such genetic defects have been linked to through impaired voltage-gated Na⁺ or K⁺ channel activity.

Internal Histology

The internal of neurons reveals a highly organized rich in specialized structures that support cellular architecture, energy production, and intracellular trafficking. The neuronal forms the foundational scaffold, comprising , neurofilaments, and filaments that provide mechanical support, maintain axonal and dendritic integrity, and facilitate the transport of organelles and molecules along the neuron's extensive processes. , composed of dimers, are polar structures essential for intracellular transport and neuronal polarity, extending throughout the soma, dendrites, and axons. Neurofilaments, as type IV intermediate filaments primarily consisting of three subunits (NF-L, NF-M, NF-H), are abundant in axons and contribute to and , particularly in long-projection neurons like motor neurons. filaments, or microfilaments, form dynamic networks concentrated in the dendritic spines and growth cones, enabling motility, , and local cytoskeletal remodeling. Key organelles within the neuronal include mitochondria, the Golgi apparatus, and the , each adapted to meet the high metabolic demands of neurons. Mitochondria, distributed along axons and dendrites, generate ATP through to power energy-intensive processes such as pumping and synaptic transmission. The Golgi apparatus, often appearing as stacked cisternae in the soma, modifies and packages proteins and lipids received from the into vesicles for and synaptic delivery. The rough , studded with ribosomes and visible as Nissl bodies, synthesizes proteins, while the smooth serves as a major intracellular store for calcium s, regulating signaling and buffering during neuronal activity. Histological techniques have been instrumental in visualizing these internal components. Nissl staining, using basic dyes like , selectively highlights the rough in the neuronal soma and dendrites, allowing differentiation of neuronal populations based on their metabolic activity. Silver impregnation methods, such as Bielschowsky's technique, impregnate neurofibrils and axons to reveal their fine branching and connectivity in tissue sections. Electron microscopy provides ultrastructural detail, resolving individual (approximately 25 nm in diameter), mitochondrial cristae, and Golgi stacks, which are critical for understanding distribution in three dimensions. Axoplasmic transport, powered by motor proteins and along the microtubule tracks of the , moves organelles, proteins, and other cargoes bidirectionally at rates ranging from 1 to 400 mm/day, ensuring the maintenance of distant axonal terminals. drives anterograde transport toward the tip, while mediates retrograde movement back to the soma.

Classification

Structural Types

Neurons are classified into structural types primarily based on their polarity, which refers to the number and arrangement of processes—dendrites and axons—extending from the cell body (soma). This morphological reflects adaptations for receiving and transmitting signals within neural circuits. Unipolar neurons feature a single process emerging from the soma, which typically branches into both dendritic and axonal components; they are common in sensory systems but rare in s. Bipolar neurons possess two distinct processes: one and one , extending from opposite poles of the soma, facilitating direct sensory transduction; representative examples include bipolar cells and neurons. Multipolar neurons, characterized by one and multiple radiating from the soma, dominate nervous systems and enable complex integration of inputs; they include pyramidal cells in the and Purkinje cells in the . Pseudounipolar neurons, a variant found in peripheral nervous systems, have a single process that bifurcates shortly after leaving the soma into a peripheral branch (functioning as a ) and a central branch (as an ); these are typical of sensory neurons in dorsal ganglia. In addition to these polar types, anaxonic neurons lack a clearly identifiable , with multiple short processes that may serve both input and output functions; prominent examples are amacrine cells in the , which form local connections within the inner plexiform layer. Axons in many neuron types, particularly multipolar and pseudounipolar, often give rise to collaterals—side branches that extend from the main to form additional synaptic connections, enhancing signal distribution to multiple targets. These collaterals typically terminate in arborizations, intricate bush-like networks that increase the axon's reach and connectivity within specific brain regions, such as the axonal arborizations of lateral geniculate neurons in the . Dendritic branching patterns vary across structural types to optimize input reception: bipolar neurons show simple, elongated dendrites for localized sensory capture, while multipolar neurons display highly branched, tree-like s—often with basal tufts and a prominent apical in pyramidal cells—to integrate diverse synaptic inputs over larger areas. Multipolar neurons predominate in mammalian brains, comprising over 99% of central nervous system neurons and underscoring their role in the majority of signal relay processes.

Functional Types

Neurons are functionally classified based on the direction of signal transmission relative to the central nervous system (CNS). Afferent neurons, also termed sensory neurons, convey sensory information from peripheral receptors toward the CNS, enabling the detection of environmental stimuli. Efferent neurons transmit commands from the CNS to peripheral effectors, such as skeletal muscles or glands, to elicit motor responses or secretory actions. Interneurons, comprising the majority of neurons in the CNS, integrate signals between afferent and efferent neurons, supporting complex information processing and reflex modulation within neural circuits. Another key functional classification distinguishes neurons by their postsynaptic effects and the neurotransmitters they employ. Excitatory neurons depolarize target cells to increase the likelihood of generation, with neurons—predominantly using glutamate as their transmitter—serving as the primary mediators of excitation in the mammalian brain. Inhibitory neurons hyperpolarize postsynaptic membranes to suppress firing, chiefly through GABAergic neurons that release gamma-aminobutyric acid (GABA) or glycinergic neurons using , thereby maintaining neural balance and preventing hyperexcitability. Neuromodulatory neurons exert prolonged, modulatory influences on circuit dynamics rather than direct excitation or inhibition, as seen in neurons that release to regulate reward processing, motivation, and plasticity in regions like the . Neurotransmitters further delineate functional neuron types into categories based on chemical structure. Small-molecule neurotransmitters include amino acids such as and GABA, which enable rapid synaptic transmission; monoamines like , serotonin, and norepinephrine, which often mediate slower modulatory effects; and , involved in both excitatory and modulatory roles at neuromuscular junctions and autonomic synapses. Neuropeptides, consisting of short chains (e.g., or ), typically co-released with small molecules, provide additional layers of signaling for pain modulation or stress responses. Many neurons exhibit multimodal functionality by co-releasing multiple transmitters from distinct vesicular pools, such as neurons that simultaneously release glutamate and GABA to fine-tune . A specialized functional class, mirror neurons, exemplifies integrated sensory-motor processing unique to . Located in the ventral , these neurons discharge both during the execution of goal-directed actions, like grasping, and during the observation of similar actions performed by others, facilitating action understanding and . This dual activation underscores their role in bridging perception and intention, with evidence primarily from monkeys and homologous systems in humans.

Electrophysiology

Action Potential Generation

Action potentials in neurons are brief, stereotyped electrical events lasting approximately 1-2 ms, first quantitatively modeled in using squid giant . These events are initiated at the , the junction between the neuronal soma and , where excitatory and inhibitory synaptic inputs are spatially and temporally integrated as graded postsynaptic potentials. If the net reaches the of about -55 mV, voltage-gated sodium channels open rapidly, triggering the action potential. This threshold serves as a critical detection mechanism, where the density and properties of voltage-gated ion channels, particularly sodium channels like Nav1.6 clustered at the , determine the sensitivity to depolarizing inputs. The action potential unfolds in distinct phases driven by the sequential activation of voltage-gated ion channels. The rising phase begins with the influx of Na⁺ ions through opening sodium channels, rapidly depolarizing the from threshold to a peak of around +40 mV. This is followed by the falling phase, where sodium channels inactivate and voltage-gated channels open, allowing K⁺ efflux that repolarizes the toward the potassium equilibrium potential. The process concludes with an , a brief period of hyperpolarization below the due to lingering potassium conductance, which helps reset the . The biophysical basis of these phases is captured by the Hodgkin-Huxley model, which mathematically describes the currents underlying generation. The model treats the neuronal as a with parallel ionic conductances, governed by the equation: CmdVdt=−gˉNam3h(V−ENa)−gˉKn4(V−EK)−gL(V−EL)+IC_m \frac{dV}{dt} = - \bar{g}_\mathrm{Na} m^3 h (V - E_\mathrm{Na}) - \bar{g}_\mathrm{K} n^4 (V - E_\mathrm{K}) - g_\mathrm{L} (V - E_\mathrm{L}) + I where VV is the , CmC_m is the (1 μF/cm²), II is the applied current, and the ionic currents depend on voltage-sensitive gating variables mm, hh, and nn for activation and inactivation of sodium and potassium channels. The sodium conductance is given by gNa=gˉNam3hg_\mathrm{Na} = \bar{g}_\mathrm{Na} m^3 h, with maximum conductance gˉNa=120\bar{g}_\mathrm{Na} = 120 mS/cm², reversal potential ENa=50E_\mathrm{Na} = 50 mV (adjusted from intracellular recordings); potassium by gK=gˉKn4g_\mathrm{K} = \bar{g}_\mathrm{K} n^4, gˉK=36\bar{g}_\mathrm{K} = 36 mS/cm², EK=−77E_\mathrm{K} = -77 mV; and leak gL=0.3g_\mathrm{L} = 0.3 mS/cm², EL=−54.4E_\mathrm{L} = -54.4 mV. The gating variables follow first-order kinetics: dmdt=αm(1−m)−βmm,dhdt=αh(1−h)−βhh,dndt=αn(1−n)−βnn\frac{dm}{dt} = \alpha_m (1 - m) - \beta_m m, \quad \frac{dh}{dt} = \alpha_h (1 - h) - \beta_h h, \quad \frac{dn}{dt} = \alpha_n (1 - n) - \beta_n n with voltage-dependent rate constants α\alpha and β\beta defined as: αm=0.1(V+40)1−exp⁡(−(V+40)/10),βm=4exp⁡(−(V+65)/18)\alpha_m = \frac{0.1 (V + 40)}{1 - \exp(-(V + 40)/10)}, \quad \beta_m = 4 \exp(-(V + 65)/18) αh=0.07exp⁡(−(V+65)/20),βh=11+exp⁡(−(V+35)/10)\alpha_h = 0.07 \exp(-(V + 65)/20), \quad \beta_h = \frac{1}{1 + \exp(-(V + 35)/10)} αn=0.01(V+55)1−exp⁡(−(V+55)/10),βn=0.125exp⁡(−(V+65)/80)\alpha_n = \frac{0.01 (V + 55)}{1 - \exp(-(V + 55)/10)}, \quad \beta_n = 0.125 \exp(-(V + 65)/80) (Here, VV is in mV, shifted from the original squid axon values for standardization.) These equations, solved numerically, simulate the rapid Na⁺ activation (m rises quickly near threshold) and delayed K⁺ activation (n slower), reproducing the observed phases and enabling predictions of excitability under varying conditions. The model highlights how threshold detection emerges from the nonlinear voltage dependence of channel gating, ensuring regenerative depolarization once initiated.

Propagation Mechanisms

In unmyelinated axons, action potentials propagate via continuous conduction, where local circuit currents depolarize adjacent segments sequentially along the entire length, resulting in conduction velocities of 0.5 to 10 m/s. This mechanism relies on the passive spread of voltage changes through the axoplasm, limited by the axon's and resistance. Myelinated axons employ saltatory conduction, in which the action potential regenerates only at the nodes of Ranvier—short, periodic gaps (typically 1-2 μm long) in the myelin sheath that concentrate voltage-gated sodium channels—allowing the impulse to "jump" between nodes at speeds up to 150 m/s. This leaping process minimizes the energy and time required for ion channel activation across the insulated internodal segments, which can span 0.2 to 2 mm depending on axon diameter. The sheath enabling is a multilayered -rich , comprising 70-85% such as (about 40%), phospholipids (40%), and glycolipids (20%), with 15-30% proteins including myelin basic protein (MBP), which stabilizes the compact spiral wrapping by interacting with lipid bilayers. In nervous system (PNS), Schwann cells form by extending and spiraling their plasma around a single axonal segment, producing one internode per cell. In the (CNS), similarly wrap axons but extend processes to myelinate multiple internodes across different axons, up to 50 or more. Demyelination disrupts this insulation, forcing continuous conduction and slowing velocities by 50-100 fold, as the loss of exposes the to excessive current leakage and requires reactivation along the full length, a hallmark of disorders like . models the passive electrotonic spread of signals in axons as a leaky cable, where the λ quantifies the distance over which a steady voltage decays to 1/e (about 37%) of its initial value. The is derived from the core conductor model and given by λ=rmri,\lambda = \sqrt{\frac{r_m}{r_i}},
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