Neuron

Source: Wikipedia, the free encyclopedia.
Neuron
Anatomy of a multipolar neuron
Identifiers
MeSHD009474
NeuroLex IDsao1417703748
TA98A14.0.00.002
THH2.00.06.1.00002
FMA54527
Anatomical terms of neuroanatomy

Within a

action potentials across a neural network. 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

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.[1]

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,

axon terminals, where the neuron can transmit a signal across the synapse to another cell. Neurons may lack dendrites or have no axon. 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 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

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

Schematic of an anatomically accurate single pyramidal neuron, the primary excitatory neuron of the cerebral cortex, with a synaptic connection from an incoming axon onto a dendritic spine

Neurons are the primary components of the nervous system, along with the

glial cells that give them structural and metabolic support.[4] 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.[5] 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 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

Diagram of the components of a 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 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]

Neuron cell body

The accepted view of the neuron attributes dedicated functions to its various anatomical components; however, dendrites and axons often act in ways contrary to their so-called main function.[8]

Diagram of a typical myelinated vertebrate motor neuron
Neurology video

Axons and dendrites in the central nervous system are typically only about one micrometer thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon of a 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

pluripotency
.

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.[10] 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 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 ion materials include sodium, potassium, chloride, and calcium
. 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.

Histology and internal structure

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) 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

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

GABAergic interneurons.[17]
SMI32-stained pyramidal neurons in cerebral cortex

Neurons vary in shape and size and can be classified by their

synaptic cleft between the terminals and the dendrites of the next neuron.[citation needed
]

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:[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
  • motoneurons
    located 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,

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,

glutamate receptors and instead express a class of inhibitory metabotropic glutamate receptors.[20] 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.[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
    retinal ganglion cells.[23][24]

Neurotransmitter

Synaptic vesicles containing neurotransmitters

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

  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 depend 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
glutamate synthase
.

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

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.[28]

When an action potential reaches the axon terminal, it opens

energy metabolism to produce ATP to support continuous neurotransmission.[29]

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]

An annotated diagram of the stages of an action potential propagating down an axon including the role of ion concentration and pump and channel proteins

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

gut microbiome is also connected with the brain.[37]
Neurons also communicate with microglia, the brain's main immune cells via specialised contact sites, called "somatic junctions". These connections enable microglia to constantly monitor and regulate neuronal functions, and exert neuroprotection, when needed.[38]

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

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 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

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

Neural coding

ensemble neuronal responses, and the relationships among the electrical activities of the neurons within the ensemble.[42] It is thought that neurons can encode both digital and analog information.[43]

All-or-none principle

As long as the stimulus reaches the threshold, the full response would be given. Larger stimulus does not result in a larger response, vice versa.[44]: 31 

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

Drawing by Camillo Golgi of a hippocampus stained using the silver nitrate method
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.[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

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.[54] Also, electrical synapses are more common than previously thought,[55] 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.[56]

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.

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.[60]

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

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.[63]

neuropsychiatric symptoms or behavioral changes.[64] 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.[65][66]

cognitive dysfunction
and subtle language problems. PD is both chronic and progressive.

immunosuppressants, cholinesterase inhibitors and, in selected cases, thymectomy
.

Demyelination

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

Although most injury responses include a calcium influx signaling to promote resealing of severed parts, axonal injuries initially lead to acute

proteases
(caused by the influx of calcium ion), 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.

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

5-hydroxymethylcytosine) and enzymes of the DNA base excision repair (BER) pathway.[72]

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,

Llinás' law).[22]

See also

References

  1. ^ "Stepwise emergence of the neuronal gene expression program in early animal evolution: Cell".
  2. ^ Zayia LC, Tadi P. Neuroanatomy, Motor Neuron. [Updated 2022 Jul 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK554616/
  3. ^ a b Zedalis J. and Eggebrecht J. (2018, Mar 8)  Biology for AP® Courses 26.1 Neurons and Glial Cells. OpenStax https://openstax.org/books/biology-ap-courses/pages/26-1-neurons-and-glial-cells (Accessed 2023, Aug 15).
  4. . A bundle of nerve fibers (axons) connecting neighboring or distant nuclei of the CNS is a tract.
  5. ^ "What are the parts of the nervous system?". October 2018. Retrieved 2022-07-08.
  6. ^ Davies, Melissa (2002-04-09). "The Neuron: size comparison". Neuroscience: A journey through the brain. Retrieved 2009-06-20.
  7. ^ Chudler EH. "Brain Facts and Figures". Neuroscience for Kids. Retrieved 2009-06-20.
  8. ^ "16.7: Nervous System". Biology LibreTexts. 2021-01-14. Retrieved 2022-02-28.
  9. S2CID 12908713
    .
  10. .
  11. ^ State Hospitals Bulletin. State Commission in Lunacy. 1897. p. 378.
  12. ^ "Medical Definition of Neurotubules". www.merriam-webster.com.
  13. S2CID 31301135
    .
  14. .
  15. .
  16. .
  17. .
  18. ISBN 978-971-23-4807-5.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  19. .
  20. .
  21. .
  22. ^ .
  23. ^ Kolodin YO, Veselovskaia NN, Veselovsky NS, Fedulova SA. Ion conductances related to shaping the repetitive firing in rat retinal ganglion cells. Acta Physiologica Congress. Archived from the original on 2012-10-07. Retrieved 2009-06-20.
  24. ^ "Ionic conductances underlying excitability in tonically firing retinal ganglion cells of adult rat". Ykolodin.50webs.com. 2008-04-27. Retrieved 2013-02-16.
  25. PMID 30239935
    .
  26. ^ "Patch-seq technique helps depict the variation of neural cells in the brain". News-medical.net. 3 December 2020. Retrieved 26 August 2021.
  27. ^ Science AAAS. "BRAIN CELL CENSUS". Retrieved 2023-10-17.
  28. .
  29. .
  30. .
  31. ^ "Why is the human brain so difficult to understand? We asked 4 neuroscientists". Allen Institute. Retrieved 17 October 2023.
  32. S2CID 38482114
    .
  33. . Retrieved 21 February 2022.
  34. ^ "Researchers discover new type of cellular communication in the brain". The Scripps Research Institute. Retrieved 12 February 2022.
  35. PMID 35081342
    .
  36. .
  37. .
  38. .
  39. ^ Chudler EH. "Milestones in Neuroscience Research". Neuroscience for Kids. Retrieved 2009-06-20.
  40. ^ Patlak J, Gibbons R (2000-11-01). "Electrical Activity of Nerves". Action Potentials in Nerve Cells. Archived from the original on August 27, 2009. Retrieved 2009-06-20.
  41. PMID 21646013
    .
  42. .
  43. (PDF) on 2012-02-15.
  44. ^ .
  45. .
  46. ^ . 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.'
  47. ^ a b Oxford English Dictionary, 3rd edition, 2003, s.v.
  48. PMID 31844876
    .
  49. ^ "Google Books Ngram Viewer". books.google.com. Retrieved 19 December 2020.
  50. ^
    S2CID 11273256
    .
  51. . ... 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.
  52. ^ "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.
  53. S2CID 24331507
    .
  54. .
  55. .
  56. .
  57. ^ Sabbatini RM (April–July 2003). "Neurons and Synapses: The History of Its Discovery". Brain & Mind Magazine: 17.
  58. PMID 15282273
    .
  59. .
  60. .
  61. .
  62. .
  63. .
  64. ^ "About Alzheimer's Disease: Symptoms". National Institute on Aging. Archived from the original on 15 January 2012. Retrieved 28 December 2011.
  65. S2CID 8570146
    .
  66. .
  67. ^ "Parkinson's Disease Information Page". NINDS. 30 June 2016. Archived from the original on 4 January 2017. Retrieved 18 July 2016.
  68. ^ "Movement Disorders". The International Neuromodulation Society.
  69. S2CID 25287010
    .
  70. .
  71. . 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.
  72. ^ .
  73. .
  74. .
  75. .

Further reading

External links

This page is based on the copyrighted Wikipedia article: Neuron. Articles is available under the CC BY-SA 3.0 license; additional terms may apply.Privacy Policy