neuron

ニューロン 　

WordNet

1. easily agitated; "a nervous addict"; "a nervous thoroughbred"
2. of or relating to the nervous system; "nervous disease"; "neural disorder" (同)neural
3. any bundle of nerve fibers running to various organs and tissues of the body (同)nervus
4. of or relating to neurons; "neural network" (同)neuronal, neuronic

PrepTutorEJDIC

1. 神経単位,神経細胞,ニューロン,ノイロン
2. 『神経の』 / 『神経質な』,神経過敏な / 『心配な』,不安な
3. 〈C〉『神経』 / 〈C〉《複数形で》『神経過敏』,いらだち / 〈U〉『勇気』,度胸(courage) / 〈U〉《時にa ~》厚かましさ,ずぶとさ;無礼 / 〈C〉葉脈;(昆虫の)翅脈(しみゃく) / 〈人,特に自分〉‘に'力を与える,‘を'元気づける
4. 神経[系統]の

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/11/24 08:42:55」(JST)

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This article is about cells in the nervous system. For other uses, see Neuron (disambiguation).

"Brain cells" redirects here. For other uses, see Glial cell.

nerve cell
Drawing of neurons in the pigeon cerebellum. (A) Denotes Purkinje cells, an example of a multipolar neuron. (B) Denotes granule cells, which are also multipolar.
Identifiers
NeuroLex ID	sao1417703748
TA	A14.0.00.002
FMA	FMA:54527
Anatomical terminology

A neuron (/ˈnjʊərɒn/ NYEWR-on or /ˈnʊərɒn/ NEWR-on; also known as a neurone or nerve cell) is an electrically excitable cell that processes and transmits information through electrical and chemical signals. These signals between neurons occur via synapses, specialized connections with other cells. Neurons can connect to each other to form neural networks. Neurons are the core components of the nervous system, which includes the brain, spinal cord and the ganglia of the peripheral nervous system (PNS) which comprises the central nervous system (CNS). Specialized types of neurons include: sensory neurons which respond to touch, sound, light and all other stimuli affecting the cells of the sensory organs that then send signals to the spinal cord and brain, motor neurons that receive signals from the brain and spinal cord to cause muscle contractions and affect glandular outputs, and interneurons which connect neurons to other neurons within the same region of the brain or spinal cord in neural networks.

A typical neuron possesses a cell body (soma), dendrites, and an axon. The term neurite is used to describe either a dendrite or an axon, particularly in its undifferentiated stage. Dendrites are thin structures that arise from the cell body, often extending for hundreds of micrometres and branching multiple times, giving rise to a complex "dendritic tree". An axon is a special cellular extension that arises from the cell body at a site called the axon hillock and travels for a distance, as far as 1 meter in humans or even more in other species. The cell body of a neuron frequently gives rise to multiple dendrites, but never to more than one axon, although the axon may branch hundreds of times before it terminates. At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another. There are, however, many exceptions to these rules: neurons that lack dendrites, neurons that have no axon, synapses that connect an axon to another axon or a dendrite to another dendrite, etc.

All neurons are electrically excitable, maintaining voltage gradients across their membranes by means of metabolically driven ion pumps, which combine with ion channels embedded in the membrane to generate intracellular-versus-extracellular concentration differences of ions such as sodium, potassium, chloride, and calcium. Changes in the cross-membrane voltage can alter the function of voltage-dependent ion channels. If the voltage changes by a large enough amount, an all-or-none electrochemical pulse called an action potential is generated, which travels rapidly along the cell's axon, and activates synaptic connections with other cells when it arrives.

Neurons do not undergo cell division. In most cases, neurons are generated by special types of stem cells. A type of glial cell, called astrocytes (named for being somewhat star-shaped), have also been observed to turn into neurons by virtue of the stem cell characteristic pluripotency. In humans, neurogenesis largely ceases during adulthood—but in two brain areas, the hippocampus and olfactory bulb, there is strong evidence for generation of substantial numbers of new neurons.

Overview

Structure of a typical neuron

Neuron (peripheral nervous system)
Dendrite Soma Axon Nucleus Node of Ranvier Axon terminal Schwann cell Myelin sheath

Anatomy of a multipolar neuron

A neuron is a specialized type of cell found in the bodies of all eumetozoans. Only sponges and a few other simpler animals lack neurons. The features that define a neuron are electrical excitability and the presence of synapses, which are complex membrane junctions that transmit signals to other cells. The body's neurons, plus the glial cells that give them structural and metabolic support, together constitute the nervous system. 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.

Although neurons are very diverse and there are exceptions to nearly every rule, it is convenient to begin with a schematic description of the structure and function of a "typical" neuron. A typical neuron is divided into three parts: the soma or cell body, dendrites, and axon. The soma is usually compact; the axon and dendrites are filaments that extrude from it. Dendrites typically branch profusely, getting thinner with each branching, and extending their farthest branches a few hundred micrometers from the soma. The axon leaves the soma at a swelling called the axon hillock, and can extend for great distances, giving rise to hundreds of branches. Unlike dendrites, an axon usually maintains the same diameter as it extends. The soma may give rise to numerous dendrites, but never to more than one axon. Synaptic signals from other neurons are received by the soma and dendrites; signals to other neurons are transmitted by the axon. A typical synapse, then, is a contact between the axon of one neuron and a dendrite or soma of another. Synaptic signals may be excitatory or inhibitory. If the net excitation received by a neuron over a short period of time is large enough, the neuron generates a brief pulse called an action potential, which originates at the soma and propagates rapidly along the axon, activating synapses onto other neurons as it goes. This is called saltatory conduction.

Many neurons fit the foregoing schema in every respect, but there are also exceptions to most parts of it. There are no neurons that lack a soma, but there are neurons that lack dendrites, and others that lack an axon. Furthermore, in addition to the typical axodendritic and axosomatic synapses, there are axoaxonic (axon-to-axon) and dendrodendritic (dendrite-to-dendrite) synapses.

The key to neural function is the synaptic signaling process, which is partly electrical and partly chemical. The electrical aspect depends on properties of the neuron's membrane. Like all animal cells, the cell body of every neuron is enclosed by a plasma membrane, a bilayer of lipid molecules with many types of protein structures embedded in it. A lipid bilayer 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 actively transport ions from one side of the membrane to the other. Most ion channels are permeable only to specific types of ions. Some ion channels are voltage gated, meaning that they can be switched between open and closed states by altering 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 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.

Neurons communicate by chemical and electrical synapses in a process known as neurotransmission, also called synaptic transmission. The fundamental process that triggers the release of neurotransmitters is the action potential, a propagating electrical signal that is generated by exploiting the electrically excitable membrane of the neuron. This is also known as a wave of depolarization.

Anatomy and histology

Diagram of a typical myelinated vertebrate motor neuron

Neurons are highly specialized for the processing and transmission of cellular signals. Given their diversity of functions performed in different parts of the nervous system, there is, as expected, a wide variety in their shape, size, and electrochemical properties. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter.

• The 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.
• The dendrites of a neuron are cellular extensions with many branches. This overall shape and structure is referred to metaphorically as a dendritic tree. This 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 carries nerve signals away from the soma (and also 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 is also the part of the neuron that 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 action potential threshold. While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons.
• The axon terminal contains synapses, specialized structures where neurotransmitter chemicals are released to communicate with target neurons.

Although the canonical view of the neuron attributes dedicated functions to its various anatomical components, dendrites and axons often act in ways contrary to their so-called main function.

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 motoneuron can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the dorsal columns, 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 millimeters thick, several centimeters long).

Fully differentiated neurons are permanently postmitotic; however, recent research shows that additional neurons throughout the brain can originate from neural stem cells found throughout the brain but in particularly high concentrations in the subventricular zone and subgranular zone through the process of neurogenesis.

Histology and internal structure

Golgi-stained neurons in human hippocampal tissue

Nerve cell bodies stained with basophilic dyes show numerous microscopic clumps of Nissl substance (named after German psychiatrist and neuropathologist Franz Nissl, 1860–1919), which consists of rough endoplasmic reticulum and associated ribosomal RNA. The prominence of the Nissl substance can be explained by the fact that nerve cells are metabolically very active, and hence are involved in large amounts of protein synthesis.

The cell body of a neuron is supported by a complex mesh of structural proteins called neurofilaments, which are assembled into larger neurofibrils. Some neurons also contain pigment granules, such as neuromelanin (a brownish-black pigment, byproduct of synthesis of catecholamines) and lipofuscin (yellowish-brown pigment that accumulates with age).

There are different internal structural characteristics between axons and dendrites. Typical axons almost never contain ribosomes, except some in the initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes, with diminishing amounts with distance from the cell body.

Classification

Image of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The red staining indicates GABAergic interneurons.

SMI32-stained pyramidal neurons in cerebral cortex

Neurons exist in a number of different shapes and sizes and can be classified by their morphology and function. The anatomist Camillo Golgi grouped neurons into two types; type I with long axons used to move signals over long distances and type II with short axons, which can often be confused with dendrites. Type I cells can be further divided by where the cell body or soma is located. 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 the myelin sheath. Around the cell body is a branching dendritic tree that receives signals from other neurons. The end of the axon has branching terminals (axon terminal) that release neurotransmitters into a gap called the synaptic cleft between the terminals and the dendrites of the next neuron.

Structural classification

Polarity

Different kinds of neurons:
1 Unipolar neuron
2 Bipolar neuron
3 Multipolar neuron

4 Pseudounipolar neuron

Most neurons can be anatomically characterized as:

• Unipolar or pseudounipolar: dendrite and axon emerging from same process.
• Bipolar: axon and single dendrite on opposite ends of the soma.
• Multipolar: two or more dendrites, separate from the axon:
  • 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 axon cannot be distinguished from dendrites.

Other

Furthermore, some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:

• Basket cells, interneurons that form a dense plexus of terminals around the soma of target cells, found in the cortex and cerebellum.
• Betz cells, large motor neurons.
• Lugaro cells, interneurons of the cerebellum.
• Medium spiny neurons, most neurons in the corpus striatum.
• Purkinje cells, huge neurons in the cerebellum, a type of Golgi I multipolar neuron.
• Pyramidal cells, neurons with triangular soma, a type of Golgi I.
• Renshaw cells, neurons with both ends linked to alpha motor neurons.
• Unipolar brush cells, interneurons with unique dendrite ending in a brush-like tuft.
• Granule cells, a type of Golgi II neuron.
• Anterior horn cells, motoneurons located in the spinal cord.
• Spindle cells, interneurons that connect widely separated areas of the brain

Functional classification

Direction

• Afferent neurons convey information from tissues and organs into the central nervous system and are sometimes also called sensory neurons.
• Efferent neurons transmit signals from the central nervous system to the effector cells and are sometimes called motor neurons.
• 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 region.

Action on other neurons

A neuron affects other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect upon the postsynaptic neuron is determined not by the presynaptic neuron or by the neurotransmitter, but by the type of receptor that is activated. A neurotransmitter can be thought of as a key, and a receptor as a lock: the same type of key can here be used to open many different types of locks. Receptors can be 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).

The two most common neurotransmitters in the brain, glutamate and GABA, have actions that are largely consistent. Glutamate acts on several different types of receptors, and have effects that are excitatory at ionotropic receptors and a modulatory effect at metabotropic receptors. Similarly GABA acts on several different types of receptors, but all of them have effects (in adult animals, at least) that are inhibitory. Because of this consistency, it is common for neuroscientists to simplify the terminology by referring to cells that release glutamate as "excitatory neurons", and cells that release GABA as "inhibitory neurons". Since over 90% of the neurons in the brain release either glutamate or GABA, these labels encompass the great majority of neurons. There are also other types of neurons that have consistent effects on their targets, for example "excitatory" motor neurons in the spinal cord that release acetylcholine, and "inhibitory" spinal neurons that release glycine.

The distinction between excitatory and inhibitory neurotransmitters is not absolute, however. 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 the typical ionotropic glutamate receptors and instead express a class of inhibitory metabotropic glutamate receptors. When light is present, the photoreceptors cease releasing glutamate, which relieves the ON bipolar cells from inhibition, activating them; this simultaneously removes the excitation from the OFF bipolar cells, silencing them.

It is possible to identify the type of inhibitory effect a presynaptic neuron will have on a postsynaptic neuron, based on the proteins the presynaptic neuron expresses. Parvalbumin-expressing neurons typically dampen the output signal of the postsynaptic neuron in the visual cortex, whereas somatostatin-expressing neurons typically block dendritic inputs to the postsynaptic neuron.

Discharge patterns

Neurons can be classified according to their electrophysiological characteristics:

• Tonic or regular spiking. Some neurons are typically constantly (or tonically) active. 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.

Classification by neurotransmitter production

• 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 influx of Na⁺ depolarization and increases the probability of presynaptic neurotransmitter release. Acetylcholine is synthesized from choline and acetyl coenzyme A.
• GABAergic neurons—gamma aminobutyric acid. GABA is one of two neuroinhibitors in the CNS, the other being 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 (recall that 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 neurotransmitter, the other being 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 both 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 is dependant 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 NMDA and AMPA receptor activation more so than would normally be the case 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.

• 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 the substantia nigra has been linked to Parkinson's disease. Dopamine is synthesized from the amino acid tyrosine. Tyrosine is catalyzed into levadopa (or L-DOPA) by tyrosine hydroxlase, and levadopa is then converted into dopamine by amino acid decarboxylase.
• Serotonergic neurons—serotonin. Serotonin (5-Hydroxytryptamine, 5-HT) can act as excitatory or inhibitory. Of the four 5-HT receptor classes, 3 are GPCR and 1 is ligand gated cation channel. Serotonin is synthesized from tryptophan by tryptophan hydroxylase, and then further by aromatic acid decarboxylase. A lack of 5-HT at postsynaptic neurons has been linked to depression. Drugs that block the presynaptic serotonin transporter are used for treatment, such as Prozac and Zoloft.

Connectivity

Main articles: Synapse and Chemical synapse

Neurons communicate with one another via synapses, where the axon terminal or en passant boutons (terminals located along the length of the axon) of one cell impinges upon 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 and either increase or decrease activity in the target neuron. Some neurons also communicate via electrical synapses, which are direct, electrically conductive junctions between cells.

In a chemical synapse, the process of synaptic transmission is as follows: 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 also triggers mitochondrial calcium uptake, which, in turn, activates mitochondrial energy metabolism to produce ATP to support continuous neurotransmission.

The human brain has a huge number of synapses. Each of the 10¹¹ (one hundred billion) neurons 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).

Mechanisms for propagating action potentials

A signal propagating down an axon to the cell body and dendrites of the next cell

In 1937, John Zachary Young suggested that the squid giant axon could be used to study neuronal electrical properties. Being larger than but similar in nature to human neurons, squid cells were easier to study. By inserting electrodes into the giant squid 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). These signals are generated and propagated by charge-carrying ions including sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), and calcium (Ca²⁺).

There are several stimuli that can activate a neuron leading to electrical activity, including pressure, stretch, chemical transmitters, and changes of the electric potential across the cell membrane. Stimuli cause specific ion-channels within the cell membrane to open, leading to a flow of ions through the cell membrane, changing the membrane potential.

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 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 nonspiking neurons tend to be sensory neurons or interneurons, because they cannot carry signals long distances.

Neural coding

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 amongst the electrical activities of the neurons within the ensemble. It is thought that neurons can encode both digital and analog information.

All-or-none principle

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 does not produce a stronger signal but can produce a higher frequency of firing. There are different types of receptor response to stimulus, slowly adapting or tonic receptors respond to steady stimulus and produce a steady rate of firing. These tonic receptors most often respond to increased intensity of stimulus 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 to get greater intensity of a specific frequency (color) there have to be more photons, as the photons can't become "stronger" for a specific frequency.

There are a number of other receptor types that are called quickly adapting or phasic receptors, where firing decreases or stops with steady stimulus; examples include: skin when touched by an object causes the neurons to fire, but if the object maintains even pressure against the skin, 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, there is no more stimulus; thus, typically these neurons 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 a number of other neurons.

History

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 term neuron was coined by the German anatomist Heinrich Wilhelm Waldeyer. The neuron's place as the primary functional unit of the nervous system was first recognized in the early 20th century through the work of the Spanish anatomist Santiago Ramón y Cajal. Ramón y Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells. This became known as the neuron doctrine, one of the central tenets of modern neuroscience. To observe the structure of individual neurons, Ramón y Cajal improved a silver staining process known as Golgi's method, which had been developed by his rival, Camillo Golgi. Cajal's improvement, which involved a technique he called "double impregnation", is still in use. The silver impregnation stains are an extremely useful method for neuroanatomical investigations because, for reasons unknown, it stains a very small percentage of cells in a tissue, so one is able to see the complete micro structure of individual neurons without much overlap from other cells in the densely packed brain.

The neuron doctrine

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 a few refinements to the simplest form of the doctrine. For example, glial cells, which are not considered neurons, play an essential role in information processing. Also, electrical synapses are more common than previously thought, meaning that there are direct, cytoplasmic connections between neurons. In fact, there are examples of neurons forming even tighter coupling: the squid giant axon arises from the fusion of multiple axons.

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

Neurons in the brain

The number of neurons in the brain varies dramatically from species to species. One estimate (published in 1988) puts the human brain at about 100 billion (10¹¹) neurons and 100 trillion (10¹⁴) synapses. A lower estimate (published in 2009) is 86 billion neurons, of which 16.3 billion are in the cerebral cortex, and 69 billion in the cerebellum. By contrast, the nematode worm Caenorhabditis elegans has just 302 neurons, making it an ideal experimental subject as scientists have been able to map all of the organism's 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

Main article: Neurology

Charcot–Marie–Tooth disease (CMT), also known as hereditary motor and sensory neuropathy (HMSN), hereditary sensorimotor neuropathy and peroneal muscular atrophy, 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 but also in the hands and arms in the advanced stages of disease. Presently incurable, this disease is one of the most common inherited neurological disorders, with 37 in 100,000 affected.

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. 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.

Parkinson's disease (PD), also known as Parkinson disease, is a degenerative disorder of the central nervous system that often impairs the sufferer's motor skills and speech. Parkinson's disease belongs to a group of conditions called movement disorders. 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 post-synaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated with immunosuppressants, cholinesterase inhibitors and, in selected cases, thymectomy.

Demyelination

Demyelination is the act of demyelinating, or the loss of the myelin sheath insulating the nerves. When myelin degrades, conduction of signals along the nerve can be impaired or lost, and the nerve eventually withers. This leads to certain neurodegenerative disorders like multiple sclerosis and chronic inflammatory demyelinating polyneuropathy.

Axonal degeneration

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 rapid separation of the proximal and distal ends within 30 minutes of injury. 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. Endoplasmic reticulum degrades and mitochondria swell up and eventually disintegrate. The disintegration is dependent on ubiquitin and calpain proteases (caused by influx of calcium ion), suggesting that axonal degeneration is an active process. Thus the axon undergoes complete fragmentation. The process takes about roughly 24 hrs in the PNS, and longer in the CNS. The signaling pathways leading to axolemma degeneration are currently unknown.

Nerve regeneration

Main article: Neuroregeneration

It has been demonstrated that neurogenesis can sometimes occur in the adult vertebrate brain, a finding that led to controversy in 1999. However, more recent studies of the age of human neurons suggest that this process occurs only for a minority of cells, and the overwhelming majority of neurons comprising the neocortex were formed before birth and persist without replacement.

It is often possible for peripheral axons to regrow if they are severed. Recent studies have also shown that the body contains a variety of stem cell types that have the capacity to differentiate into neurons. A report in Nature suggested that researchers had found a way to transform human skin cells into working nerve cells using a process called transdifferentiation in which "cells are forced to adopt new identities".

Computational power

Historically it has been thought that neurons are relatively simple devices and that the immense computational power of the brain comes from having a great many of them. Indeed, artificial intelligence research has followed this line. However, it is now becoming increasingly apparent that even single neurons can perform complex computations.

See also

• Artificial neuron
• Bidirectional cell
• Biological neuron model
• List of neuroscience databases
• Neuroplasticity

References

Further reading

• Kandel E.R., Schwartz, J.H., Jessell, T.M. 2000. Principles of Neural Science, 4th ed., McGraw-Hill, New York.
• Bullock, T.H., Bennett, M.V.L., Johnston, D., Josephson, R., Marder, E., Fields R.D. 2005. The Neuron Doctrine, Redux, Science, V.310, p. 791–793.
• Ramón y Cajal, S. 1933 Histology, 10th ed., Wood, Baltimore.
• Richard S. Snell: Clinical neuroanatomy (Lippincott Williams & Wilkins, Ed.6th 2006) Philadelphia, Baltimore, New York, London. ISBN 978-963-226-293-2
• Roberts A., Bush B.M.H. 1981. Neurones Without Impulses. Cambridge University Press, Cambridge.
• Peters, A., Palay, S.L., Webster, H, D., 1991 The Fine Structure of the Nervous System, 3rd ed., Oxford, New York

External links

• IBRO (International Brain Research Organization). Fostering neuroscience research especially in less well-funded countries.
• NeuronBank an online neuromics tool for cataloging neuronal types and synaptic connectivity.
• High Resolution Neuroanatomical Images of Primate and Non-Primate Brains.
• The Department of Neuroscience at Wikiversity, which presently offers two courses: Fundamentals of Neuroscience and Comparative Neuroscience.
• NIF Search – Neuron via the Neuroscience Information Framework
• Cell Centered Database – Neuron
• Complete list of neuron types according to the Petilla convention, at NeuroLex.
• NeuroMorpho.Org an online database of digital reconstructions of neuronal morphology.
• Immunohistochemistry Image Gallery: Neuron
• Khan Academy: Anatomy of a neuron

UpToDate Contents

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• 1. 筋萎縮性側索硬化症およびその他の運動ニューロン疾患の診断 diagnosis of amyotrophic lateral sclerosis and other forms of motor neuron disease
• 2. 筋萎縮性側索硬化症およびその他の運動ニューロン疾患の臨床的特徴 clinical features of amyotrophic lateral sclerosis and other forms of motor neuron disease
• 3. 重症筋無力症の鑑別診断 differential diagnosis of myasthenia gravis
• 4. 成人における詳細な神経学的検査 the detailed neurologic examination in adults
• 5. 筋萎縮性側索硬化症の疫学および病因 epidemiology and pathogenesis of amyotrophic lateral sclerosis

English Journal

• pH-dependent inhibition of tetrodotoxin-resistant Na(+) channels by diclofenac in rat nociceptive neurons.

• Nakamura M1, Jang IS2.
• Progress in neuro-psychopharmacology & biological psychiatry.Prog Neuropsychopharmacol Biol Psychiatry.2016 Jan 4;64:35-43. doi: 10.1016/j.pnpbp.2015.07.003. Epub 2015 Jul 12.
• Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used for the treatment of inflammatory pain. It is well established that NSAIDs exert their analgesic effects by inhibiting cyclooxygenase to prevent the production of prostaglandins; however, several NSAIDs including diclofenac also modulate
• PMID 26176424

• Potential of the cannabinoid CB2 receptor as a pharmacological target against inflammation in Parkinson's disease.

• Gómez-Gálvez Y1, Palomo-Garo C1, Fernández-Ruiz J2, García C3.
• Progress in neuro-psychopharmacology & biological psychiatry.Prog Neuropsychopharmacol Biol Psychiatry.2016 Jan 4;64:200-8. doi: 10.1016/j.pnpbp.2015.03.017. Epub 2015 Apr 9.
• Inflammation is an important pathogenic factor in Parkinson's disease (PD), so that it can contribute to kill dopaminergic neurons of the substantia nigra and to enhance the dopaminergic denervation of the striatum. The cannabinoid type-2 (CB2) receptor has been investigated as a potential anti-infl
• PMID 25863279

• Elevated Cytoplasmic Free Zinc and Increased Reactive Oxygen Species Generation in the Context of Brain Injury.

• Stork CJ1, Li YV2.
• Acta neurochirurgica. Supplement.Acta Neurochir Suppl.2016;121:347-53. doi: 10.1007/978-3-319-18497-5_60.
• Intracellular zinc release and the generation of reactive oxygen species (ROS) have been reported to be common ingredients in numerous toxic signaling mechanisms in neurons. A key source for intracellular zinc release is its liberation from metallothionein-III (MT-III). MT-III binds and regulates in
• PMID 26463973

Japanese Journal

• Therapeutic Application of Stem Cell Technology toward the Treatment of Parkinson's Disease (Stem Cell Research for Regenerative Medicine/Personalized Medicine)

• Nishimura Kaneyasu,Takahashi Jun
• Biological & pharmaceutical bulletin 36(2), 171-175, 2013-02
• NAID 40019558324

• 下オリーブニューロンのVelarde-Llinasモデルにおける信号応答性能

• 信川 創,西村 治彦
• 電子情報通信学会論文誌. A, 基礎・境界 96(1), 1-11, 2013-01
• NAID 40019552810

Related Links

• Neuron

Neuroscience journal published by Cell press. Abstracts since 1988 available for free. Fulltext issues available for 1996-current issues.

• Neuron | Vol 77, Iss 5, Pgs 801-994, (6 March, 2013) | ScienceDirect ...

The online version of Neuron at ScienceDirect.com, the world's leading platform for high quality peer-reviewed full-text journals.

Related Pictures

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★リンクテーブル★

リンク元	「100Cases 13」「ニューロン」「nervous」「neural」「neuronal」
拡張検索	「neuronal ceroid lipofuscinosis」
関連記事	「neuro」

「100Cases 13」

　　[★]

☆case13　手の筋力の低下

■glossary

brisk adj. (人・態度が)活発な、元気のよい、きびきびした。ぶっきらぼうな、素っ気ない。(商売が)活況の(⇔dull)。小気味のよい、(大気など)爽快な、気持ちのよい。(味などが)ピリッとする。鋭い。(飲料が)盛んに泡立つ

dysarthria n. difficulty in articulating words due to disease of the central nervous system　構音障害

dysphasia n. loss of or deficiency in the power to use or understand language as a result of injury to or disease of the brain　失語症、言語障害

multidisciplinary adj. 集学的な

gastrostomy 胃瘻造設術

feeding gastrostomy n. Surgery A procedure in which an opening is created in the anterior wall of the stomach to allow suction decompression and improved respiratory function by eliminating the need for a nasogastric feeding tube

■症例

67歳、男性　元大学講師(retired university lecturer)

主訴：左手の筋力低下と筋萎縮

現病歴：左手を使った労作後に左手の筋力が低下する(例えば、ドライバーを使った後など)。前腕の筋に疝痛をみとめる。発話はわずかに流暢であり(slight slurred)、水を飲んだあとで窒息しだす。服用薬はシンバスタチン、アスピリン、アテノロール。喫煙はしない。飲酒は週にワインのボトルを1本あける。

既往歴：高血圧が15年間続いている。3年前に心筋梗塞。

家族歴：妻と暮らしている。大きくなった2人の子供がいる。

・診察 examination

　血圧：146/88 mmHg。心血管系、呼吸器系、腹部に異常を認めない。上肢に萎縮を認める。特に左手に著しい萎縮を認める。両側の上腕の筋にいくらか線維性筋攣縮を認める。筋力は左で全般的に低下(globaly reduced)、右手でわずかに低下。筋緊張は正常。上腕二頭筋反射・上腕三頭筋反射は両側ともに活発に認められる(brisk)。感覚喪失(sensory loss)は認めない。わずかに構音障害を認める

■キーワード＆着目するポイント

　筋萎縮、線維性筋攣縮、感覚喪失なし

■解説

(第1パラグラフ)　疫学

・この男は運動ニューロン疾患。

・この疾患は原因不明の疾患で、脊髄、脳神経核、運動皮質に影響を及ぼす。

・この病気は普通50-70歳で出現する。

(第2パラグラフ) 症状

・筋力低下と筋萎縮は一側の手か腕にあらわれるのが一般的　←　両側性でないということか。

・筋力低下は運動後に最も顕著に表れる

・前腕の疝痛は疾患の初期に一般的である。

・患者は下肢の筋力低下あるいは構音障害や言語障害を訴えてやってくることがある。

・この病態の特徴的な生理的徴候は線維性攣縮(筋束の不規則で早い収縮。下位運動ニューロンの障害を示唆)　→　下肢の運動ニューロン損傷による筋肉の脱神経が原因

・反射は著明　→　皮質運動ニューロンの喪失による。

・感覚喪失はない

(第3パラグラフ)　鑑別

・進行例では診断は容易だが、初期ではmore problematic。

・疲労で悪化する四肢の筋力低下は「重症筋無力症」と混乱する。

・老人における失語症と構音障害の原因は、「脳血管障害による偽性球麻痺」であることがより一般的。

・上肢に感覚障害を伴わない筋萎縮、線維束攣縮を呈する他の疾患として「頚髄症」がある。

・「外傷」や「心尖部肺癌(パンコースト腫瘍)」による腕神経叢の損傷で腕に症状があらわれるかもしれない。

・末梢運動神経優位のニューロパチーは対側性の筋力低下と反射の低下というパターンを起こす。　←　？

(第4パラグラフ)　症状・経過

・運動ニューロン疾患は進行性で不治の病態

・足の痙性麻痺を発現する傾向がある

・球麻痺は失語症と構音障害を起こす

・肛門括約筋は普通影響を受けない

・知能は一般的に影響を受けない

(第5パラグラフ)　治療・管理

・この病態に対して治癒的な治療法はない

・発症から平均生存期間は2-4年

・医師は診断と予後について患者と家族に説明しなければならない。

・集学的なチームはサポートを提供しなければいけない。

・病気が進行して発話が悪くなったとき、コンピュータの使用で会話は補助されうる。

・feeding gastrostomyは十分なカロリーの摂取を可能にするために必要かもしれない。

・非侵襲的な呼吸器は呼吸不全を補助するために使われうる

・普通、気管支肺炎で死亡する

■運動ニューロン疾患とは？

運動ニューロン疾患 motor neuron disease MND

■参考文献

HIM = Harrison's Principles of Internal Medicine 17th Edition

CASES = 100 Cases in Clinical Medicine Second edition

IMD = 内科診断学第2版

「ニューロン」

　　[★]

英

neuron, nerve cell

同

神経細胞、神経元？、神経単位？

組織学

構造

• 細胞体

• ニッスル小体

• 樹状突起
• 軸索

• 起始円錐
• 髄鞘

• ランヴィエ絞輪

• 軸索終末

• シナプス
• シナプス間隙

分類

• 双極性ニューロン
• 偽単極性ニューロン

「nervous」

　　[★]

• adj.

• 神経の、心配な、神経質な

関

anxiety、anxious、anxiously、care、concern、fear、nerve、nervi、nervousness、nervus、neural、neuro、neuron、neuroticism、trouble、uneasy、worrisome、worry

　 　 　

「neural」

　　[★]

• adj.

• 神経の、神経性の、神経型の

関

nerve、nervi、nervosa、nervous、nervus、neurally、neuro、neurogenic、neurologic、neurological、neuron、neuronal、neuronally

　 　

「neuronal」

　　[★]

• 神経細胞の、ニューロンの、神経型の

関

nerve cell、neural、neural cell、neuron、neuronal cell、neurone

「neuronal ceroid lipofuscinosis」

　　[★] 神経性セロイドリボフスチノーシス

「neuro」

　　[★]

• comb form.

• ニューロ、神経

関

nerve、nervi、nervous、nervus、neural、neuron