Neuron
Neuron | |
---|---|
Identifiers | |
MeSH | D009474 |
NeuroLex ID | sao1417703748 |
TA98 | A14.0.00.002 |
TH | H2.00.06.1.00002 |
FMA | 54527 |
Anatomical terms of neuroanatomy |
Within a
Neurons are the main components of
Neurons are typically classified into three types based on their function. Sensory neurons respond to stimuli such as touch, sound, or light that affect the cells of the sensory organs, and they send signals to the spinal cord or brain. Motor neurons receive signals from the brain and spinal cord to control everything from muscle contractions[2] 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.
Neurons are special cells which are made up of some structures that are common to all other eukaryotic cells such as the cell body (soma), a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus,
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 a 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 maintenance of
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
Neurons are the primary components of the nervous system, along with the
Axons may bundle into fascicles that make up the nerves in the peripheral nervous system (like strands of wire make up cables). Bundles of axons in the central nervous system are called tracts.
Anatomy and histology
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 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.[6]
- 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.[7]
- The dendrites of a neuron are cellular extensions with many branches. This overall shape and structure are referred to metaphorically as a dendritic tree. This is where the majority of input to the neuron occurs via the dendritic spine.
- The 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 synapses. Synaptic boutons are specialized structures where neurotransmitterchemicals 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.[8]
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
Fully differentiated neurons are permanently
Membrane
Like all animal cells, the cell body of every neuron is enclosed by a
Histology and internal structure
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) haematoxylin[11] 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.[12] 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.[13][14][15] 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.[16]
There are different internal structural characteristics between axons and dendrites. Typical axons almost never contain
Classification
Neurons vary in shape and size and can be classified by their
Structural classification
Polarity
Most neurons can be anatomically characterized as:[19]
- Unipolar: single process. Unipolar cells are exclusively sensory neurons. Their dendrites are receiving 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
Some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:[citation needed]
- 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
- 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
- Rosehip cells, unique human inhibitory neurons that interconnect with Pyramidal cells
- 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
- motoneuronslocated in the spinal cord
- Spindle cells, interneurons that connect widely separated areas of the brain
Functional classification
Direction
- 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
A neuron affects other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect upon the postsynaptic neuron is determined by the type of receptor that is activated, not by the presynaptic neuron or by the neurotransmitter. A neurotransmitter can be thought of as a key, and a receptor as a lock: the same neurotransmitter can activate multiple types of receptors. Receptors can be classified broadly as excitatory (causing an increase in firing rate), inhibitory (causing 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,
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,
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.[21]
Discharge patterns
Neurons have intrinsic electroresponsive properties like intrinsic transmembrane voltage oscillatory patterns.[22] 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
Neurotransmitter
Neurotransmitters are chemical messengers passed from one neuron to another neuron or to a muscle cell or gland cell.
- Cholinergic neurons – acetylcholine. acetyl coenzyme A.
- Adrenergic neurons – noradrenaline. beta adrenoceptors. Noradrenaline is one of the three common catecholamine neurotransmitter, and the most prevalent of them in the peripheral nervous system; as with other catecholamines, it is synthesised 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. coupled receptor (often referred to as GPCR).
- AMPA and Kainate receptors function as cation channels permeable to Na+ cation channels mediating fast excitatory synaptic transmission.
- NMDA receptors are another cation channel that is more permeable to Ca2+. The function of NMDA receptors depend on glycine receptor binding as a co-agonist within the channel pore. NMDA receptors do not function without both ligands present.
- Metabotropic receptors, GPCRs modulate synaptic transmission and postsynaptic excitability.
- Glutamate can cause excitotoxicity when blood flow to the brain is interrupted, resulting in 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 levodopa (or L-DOPA) by tyrosine hydroxlase, and levodopa is then converted into dopamine by the aromatic amino acid decarboxylase.
- Serotonergic neurons—Zoloft.
- Purinergic neurons—ATP. , which particularly acts at P2Y receptors.
- Histaminergic neurons—neuromodulator. Histamine-producing neurons are found in the tuberomammillary nucleus of the hypothalamus.[25] Histamine is involved in arousaland regulating sleep/wake behaviors.
Multimodel classification
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.[26] 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.[27]
Connectivity
Neurons communicate with each other via
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.[28]
When an action potential reaches the axon terminal, it opens
An autapse is a synapse in which a neuron's axon connects to its own dendrites.
The human brain has some 8.6 x 1010 (eighty six billion) neurons.[30][31] 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).[32]
Nonelectrochemical signaling
Beyond electrical and chemical signaling, studies suggest neurons in healthy human brains can also communicate through:
- force generated by the enlargement of dendritic spines[33]
- the transfer of proteins – transneuronally transported proteins (TNTPs)[34][35]
They can also get modulated by input from the environment and
Mechanisms for propagating action potentials
In 1937 John Zachary Young suggested that the squid giant axon could be used to study neuronal electrical properties.[39] 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
Several stimuli can activate a neuron leading to electrical activity, including pressure, stretch, chemical transmitters, and changes of the electric potential across the cell membrane.[40] 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.[41]
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
Neural coding
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, like brighter image/louder sound, does not produce a stronger signal, but can increase firing frequency.[44]: 31 Receptors respond in different ways to stimuli. Slowly adapting or tonic receptors respond to steady stimulus and produce a steady rate of firing. 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 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 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, stimulus ends; 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.[45]
Etymology and spelling
The German anatomist Heinrich Wilhelm Waldeyer introduced the term neuron in 1891,[46] based on the ancient Greek νεῦρον neuron 'sinew, cord, nerve'.[47]
The word was adopted in French with the spelling neurone. That spelling was also used by many writers in English,[48] but has now become rare in American usage and uncommon in British usage.[49][47]
History
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.[50]
To make the structure of individual neurons visible, Ramón y Cajal improved a silver staining process that had been developed by Camillo Golgi.[50] 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 absolutely autonomous canton."[50][46] This became known as the neuron doctrine, one of the central tenets of modern neuroscience.[50]
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.[51][52]
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.[53]
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 refinements to the doctrine. For example,
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.[57] The Law of Dynamic Polarization has important exceptions; dendrites can serve as synaptic output sites of neurons[58] and axons can receive synaptic inputs.[59]
Compartmental modelling of neurons
Although neurons are often described of 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 due to passive membrane properties, but may also be different depending on input from synapses.
Neurons in the brain
The number of neurons in the brain varies dramatically from species to species.[61] In a human, there are an estimated 10–20 billion neurons in the cerebral cortex and 55–70 billion neurons in the cerebellum.[62] 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
This article needs additional citations for verification. (May 2018) |
Demyelination
Axonal degeneration
Although most injury responses include a calcium influx signaling to promote resealing of severed parts, axonal injuries initially lead to acute
Neurogenesis
Neurons are born through the process of neurogenesis, in which neural stem cells divide to produce differentiated neurons. Once fully differentiated neurons are formed, they are no longer capable of undergoing mitosis. Neurogenesis primarily occurs in the embryo of most organisms.
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 forms before birth and persists without replacement. The extent to which adult neurogenesis exists in humans, and its contribution to cognition are controversial, with conflicting reports published in 2018.[70]
The body contains a variety of stem cell types that have the capacity to 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".[71]
During
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[73]
Intercellular communication between developing neurons and microglia is also indispensable for proper neurogenesis and brain development.[74]
Nerve regeneration
Peripheral axons can regrow if they are severed,
See also
References
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Ramon y Cajal's first paper on the Golgi stain was on the bird cerebellum, and it appeared in the Revista in 1888. He acknowledged that he found the nerve fibers to be very intricate, but stated that he could find no evidence for either axons or dendrites undergoing anastomosis and forming nets. He called each nervous element 'an absolutely autonomous canton.'
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... a man who would write a highly influential review of the evidence in favor of the neuron doctrine two years later. In his paper, Waldeyer (1891), ... , wrote that nerve cells terminate freely with end arborizations and that the 'neuron' is the anatomical and physiological unit of the nervous system. The word 'neuron' was born this way.
- ^ "Whonamedit - dictionary of medical eponyms". www.whonamedit.com.
Today, Wilhelm von Waldeyer-Hartz is remembered as the founder of the neurone theory, coining the term "neurone" to describe the cellular function unit of the nervous system and enunciating and clarifying that concept in 1891.
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By transforming cells from human skin into working nerve cells, researchers may have come up with a model for nervous-system diseases and perhaps even regenerative therapies based on cell transplants. The achievement, reported online today in Nature, is the latest in a fast-moving field called transdifferentiation, in which cells are forced to adopt new identities. In the past year, researchers have converted connective tissue cells found in skin into heart cells, blood cells, and liver cells.
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Further reading
- Bullock TH, Bennett MV, Johnston D, Josephson R, Marder E, Fields RD (November 2005). "Neuroscience. The neuron doctrine, redux". Science. 310 (5749): 791–3. S2CID 170670241.
- Kandel ER, Schwartz JH, Jessell TM (2000). Principles of Neural Science (4th ed.). New York: McGraw-Hill. ISBN 0-8385-7701-6.
- Peters A, Palay SL, Webster HS (1991). The Fine Structure of the Nervous System (3rd ed.). New York: Oxford University Press. ISBN 0-19-506571-9.
- Ramón y Cajal S (1933). Histology (10th ed.). Baltimore: Wood.
- Roberts A, Bush BM (1981). Neurones without Impulses. Cambridge: Cambridge University Press. ISBN 0-521-29935-7.
- Snell RS (2010). Clinical Neuroanatomy. Lippincott Williams & Wilkins. ISBN 978-0-7817-9427-5.
External links
- Neurobiology at Curlie
- 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 Archived 2015-01-22 at the Wayback Machine 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
- Neuron images