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Encyclopedia > Neuron

Neurons (IPA: /njˈɒɹɒns/, also known as neurones and nerve cells) are electrically excitable cells in the nervous system that In neuroscience, granule cells are tiny cells found within the granular layer of the cerebellum. ... Drawing of the structure of cork as it appeared under the microscope to Robert Hooke from Micrographia which is the origin of the word cell being used to describe the smallest unit of a living organism Cells in culture, stained for keratin (red) and DNA (green) The cell is the... The nervous system is a highly specialized network whose principal components are cells called neurons. ...

Contents

Overview

Neurons are usually considered pertly amitotic[1] (they do not divide); however, recent research shows that they do indeed undergo adult neurogenesis.[2][3][4][5][6][7] Neurons are typically composed of a soma, or cell body, a dendritic tree and an axon. The majority of vertebrate neurons receive input on the cell body and dendritic tree, and transmit output via the axon. However, there is great heterogeneity throughout the nervous system and the animal kingdom, in the size, shape and function of neurons. Mitosis divides genetic information during cell division. ... Neurogenesis (birth of neurons) is the process by which neurons are created. ... The soma, or perikaryon, is the bulbous end of a neuron, containing the cell nucleus. ... Dendrites (from Greek dendron, “tree”) are the branched projections of a neuron that act to conduct the electrical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project. ... An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ...


Neurons communicate via chemical and electrical synapses, in a process known as synaptic transmission. The fundamental process that triggers synaptic transmission 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. Illustration of the major elements in a prototypical synapse. ... An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. ... Synapses allow nerve cells to communicate with one another through axons and dendrites, converting electrical signals into chemical ones. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cells plasma membrane. ...


History

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.[8] Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells.[8] This became known as the neuron doctrine, one of the central tenets of modern neuroscience.[8] To observe the structure of individual neurons, Cajal used a silver staining method developed by his rival, Camillo Golgi.[8] The Golgi stain is 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 microstructure of individual neurons without much overlap from other cells in the densely packed brain.[9] Santiago Ramón y Cajal Santiago Ramón y Cajal (May 1, 1852 – October 17, 1934) was a famous Spanish histologist, physician, and Nobel laureate. ... Ramón y Cajals drawing of the cells of the chick cerebellum, from Estructura de los centros nerviosos de las aves, Madrid, 1905. ... Drawing by Camillo Golgi of a hippocampus stained with the silver nitrate method Drawing of a Purkinje cell in the cerebellum cortex done by Santiago Ramón y Cajal, clearly demonstrating the power of Golgis staining method to reveal fine detail Golgis method is a nervous tissue staining... Camillo Golgi, 1906. ...


Anatomy and histology

Diagram of a typical myelinated vertebrate neuron.

Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed by neurons in different parts of the nervous system, there is, as expected, a wide variety in the shape, size, and electrochemical properties of neurons. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter.[10] Image File history File links Complete_neuron_cell_diagram. ... Image File history File links Complete_neuron_cell_diagram. ... Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. ... This article does not cite any references or sources. ...

  • The soma is the central part of the neuron. It contains the nucleus of the cell, and therefore is where most protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter.[11]
  • The dendrites of a neuron are cellular extensions with many branches, and metaphorically this overall shape and structure is referred to as a dendritic tree. This is where the majority of input to the neuron occurs. Information outflow (i.e. from dendrites to other neurons) can also occur, but not across chemical synapses; there, the backflow of a nerve impulse is inhibited by the fact that an axon does not possess chemoreceptors and dendrites cannot secrete neurotransmitter chemicals. This unidirectionality of a chemical synapse explains why nerve impulses are conducted only in one direction.
  • The axon is a finer, cable-like projection which 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 carry 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 neurological terms it has the most negative hyperpolarized 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 in order 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. The soma, or perikaryon, is the bulbous end of a neuron, containing the cell nucleus. ... HeLa cells stained for DNA with the Blue Hoechst dye. ... Protein biosynthesis (synthesis) is the process in which cells build proteins. ... In biology, a dendrite is a slender, typically branched projection of a nerve cell, or neuron, which conducts the electrical stimulation received from other cells to the body or soma of the cell from which it projects. ... An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... The arrow labeled axon is pointing directly at the axon hillock. ... In biology, hyperpolarization is any change in a cells membrane potential that makes it more polarized. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... Illustration of the major elements in a prototypical synapse. ... Chemical structure of D-aspartic acid, a common amino acid neurotransmitter. ...


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). A micrometre (American spelling: micrometer, symbol µm) is an SI unit of length equal to one millionth of a metre, or about a tenth of the diameter of a droplet of mist or fog. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ... The posterior horn (posterior column, posterior cornu, dorsal horn) of the spinal cord is dorsal (more towards the back) to the anterior horn. ... Binomial name Linnaeus, 1758 Range map The giraffe (Giraffa camelopardalis) is an African even-toed ungulate mammal, the tallest of all land-living animal species. ... The squid giant axon is the very large (up to 1 mm in diameter; typically around 0. ...


Classes

Image of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The red staining indicates GABAergic interneurons. Source PLoS Biology [1]
SMI32-stained pyramidal neurons in cerebral cortex.
SMI32-stained pyramidal neurons in cerebral cortex.

Image File history File links GFPneuron. ... Image File history File links GFPneuron. ... For other uses, see Cortex. ... It has been suggested that mGFP be merged into this article or section. ... Gaba may refer to: Gabâ or gabaa (Philippines), the concept of negative karma of the Cebuano people GABA, the gamma-amino-butyric acid neurotransmitter GABA receptor, in biology, receptors with GABA as their endogenous ligand Gaba 1 to 1, an English conversational school in Japan Marianne Gaba, a US model... Image File history File linksMetadata Smi32neuron. ... Image File history File linksMetadata Smi32neuron. ... For other uses, see Cortex. ...

Structural classification

Polarity

Most neurons can be anatomically characterized as:

Pseudounipolar cells (Pseudo- false, uni- one) are sensory neurons in the peripheral nervous system. ... As a part of the retina, the bipolar cell exists between photoreceptors (rod cells and cone cells) and ganglion cells. ... The multipolar neuron possesses a single (usually long) axon and many dendrites, allowing for the integration of a great deal of information from other neurons. ... A pyramidal cell (or pyramidal neuron, or projection neuron) is a multipolar neuron located in the hippocampus and cerebral cortex. ... Drawing of pigeon Purkinje cells (A) by Santiago Ramon y Cajal Purkinje cells are a class of GABAergic neuron located in the cerebellar cortex. ... The anterior horn of the spinal cord (or anterior cornu, or anterior column) is the ventral (front) grey matter section of the spinal cord. ... In neuroscience, granule cells are tiny cells found within the granular layer of the cerebellum. ...

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 are inhibitory GABAergic interneurons found in the molecular layer of the cerebellum. ... The cerebellum (Latin: little brain) is a region of the brain that plays an important role in the integration of sensory perception and motor control. ... Betz cells are pyramidal cell neurons located within the fifth layer of the grey matter in the primary motor cortex. ... The medium spiny neurons are a special type of inhibitory cells representing approximately 75% of the neurons within the corpus striatum of the basal ganglia. ... The corpus striatum is composed of the caudate nucleus and the putamen. ... Drawing of pigeon Purkinje cells (A) by Santiago Ramon y Cajal Purkinje cells are a class of GABAergic neuron located in the cerebellar cortex. ... The cerebellum (Latin: little brain) is a region of the brain that plays an important role in the integration of sensory perception and motor control. ... A pyramidal cell (or pyramidal neuron, or projection neuron) is a multipolar neuron located in the hippocampus and cerebral cortex. ... Renshaw cells are located in the spinal cord horn. ... Alpha motor neurons (α-MNs) are large lower motor neurons of the brainstem and spinal cord. ... In neuroscience, granule cells are tiny cells found within the granular layer of the cerebellum. ... The anterior horn of the spinal cord (or anterior cornu, or anterior column) is the ventral (front) grey matter section of the spinal cord. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ...

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 can also refer generally to neurons which, respectively, bring information to or send information from the brain region. An afferent neuron is a neuron that carries information from sensory receptors at its peripheral endings to the central nervous system. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ... Effector cells are a type of lymphocyte that are actively engaged in secreting antibodies. ... An interneuron (also called relay neuron,association neuron or bipolar neuron) is a term used to describe a neuron which has two different common meanings. ...


Action on other neurons

  • Excitatory neurons excite their target neurons. Excitatory neurons in the central nervous system, including the brain, are often glutamatergic. Neurons of the peripheral nervous system, such as spinal motoneurons that synapse onto muscle cells, often use acetylcholine as their excitatory neurotransmitter. However, this is just a general tendency that may not always be true. It is not the neurotransmitter that decides excitatory or inhibitory action, but rather it is the postsynaptic receptor that is responsible for the action of the neurotransmitter.
  • Inhibitory neurons inhibit their target neurons. Inhibitory neurons are often interneurons. The output of some brain structures (neostriatum, globus pallidus, cerebellum) are inhibitory. The primary inhibitory neurotransmitters are GABA and glycine.
  • Modulatory neurons evoke more complex effects termed neuromodulation. These neurons use such neurotransmitters as dopamine, acetylcholine, serotonin and others.

Excitatory Postsynaptic Potential is generally abbreviated to Much information is available under the heading synapse, but this is a different concept. ... Glutamate is the anion of glutamic acid. ... The Spinal cord nested in the vertebral column. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ... The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... Inhibitory Postsynaptic Potential is commonly abbreviated to Currently there is more information available under the heading synapse. ... Gaba may refer to: Gabâ or gabaa (Philippines), the concept of negative karma of the Cebuano people GABA, the gamma-amino-butyric acid neurotransmitter GABA receptor, in biology, receptors with GABA as their endogenous ligand Gaba 1 to 1, an English conversational school in Japan Marianne Gaba, a US model... For the plant, see Glycine (plant). ... In neuroscience, Neuromodulation is the process in which several classes of neurotransmitters in the nervous system regulate diverse populations of neurons. ... For other uses, see Dopamine (disambiguation). ... The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... For the professional wrestling stable, see Ravens Nest#Serotonin. ...

Discharge patterns

Neurons can be classified according to their electrophysiological characteristics: Current Clamp is a common technique in electrophysiology. ...

  • 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 fast firing rates, for example some types of cortical inhibitory interneurons, cells in globus pallidus.
  • Thin-spike. Action potentials of some neurons are more narrow compared to the others. For example, interneurons in prefrontal cortex are thin-spike neurons.

Neurotransmitter released

Some examples are

  • cholinergic neurons
  • GABAergic neurons
  • glutamatergic neurons
  • dopaminergic neurons
  • 5-hydroxytryptamine neurons (5-HT; serotonin)

Serotonin (5-hydroxytryptamine, or 5-HT) is a monoamine neurotransmitter synthesised in serotonergic neurons in the central nervous system and enterochromaffin cells in the gastrointestinal tract. ...

Connectivity

Main article: Synapse

Neurons communicate with one another via synapses, where the axon terminal 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 will either increase or decrease activity in the target neuron. Some neurons also communicate via electrical synapses, which are direct, electrically-conductive junctions between cells. Illustration of the major elements in a prototypical synapse. ... Illustration of the major elements in a prototypical synapse. ... Drawing of pigeon Purkinje cells (A) by Santiago Ramon y Cajal Purkinje cells are a class of GABAergic neuron located in the cerebellar cortex. ... The cerebellum (Latin: little brain) is a region of the brain that plays an important role in the integration of sensory perception and motor control. ... The supraoptic nucleus (SON) is a nucleus of magnocellular neurosecretory cells in the hypothalamus of the mammalian brain. ... Excitatory Postsynaptic Potential is generally abbreviated to Much information is available under the heading synapse, but this is a different concept. ... Inhibitory Postsynaptic Potential is commonly abbreviated to Currently there is more information available under the heading synapse. ... An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. ... A gap junction is a junction between certain animal/plant cell-types that allows different molecules and ions to pass freely 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. Voltage-dependent calcium channels (VDCC) are a group of voltage-gated ion channels found in excitable cells (neurons, glial cells, muscle cells, etc. ... Calcium (Ca2+) plays a vital role in the anatomy, physiology and biochemistry of organisms and of the cell, particularly in signal transduction pathways. ... In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm or cell nucleus that binds to a specific molecule (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates the cellular response to the ligand. ...


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 (0.1 to 0.5 quadrillion).[12] The human brain controls the central nervous system (CNS), by way of the cranial nerves and spinal cord, the peripheral nervous system (PNS) and regulates virtually all human activity. ...


Mechanisms for propagating action potentials

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

The cell membrane in the axon and soma contain voltage-gated ion channels which allow the neuron to generate and propagate an electrical impulse (an action potential). Substantial early knowledge of neuron electrical activity came from experiments with squid giant axons. In 1937, John Zachary Young suggested that the giant squid axon can be used to study neuronal electrical properties.[13] As they are much larger than human neurons, but similar in nature, it was easier to study them with the technology of that time. By inserting electrodes into the giant squid axons, accurate measurements could be made of the membrane potential. Voltage-gated ion channel is a ion channel that is specifically activated, or gated, by the surrounding potential difference near the channel (or near the cell, neuron or synapse). ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... The squid giant axon is the very large (up to 1 mm in diameter; typically around 0. ... John Zachary Young (18 March 1907 – 4 July 1997), generally known as JZ, was an English zoologist and neurophysiologist, described as one of the most influential biologists of the 20th century . ... Current Clamp is a common technique in electrophysiology. ... Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cells plasma membrane. ...


Electrical activity can be produced in neurons by a number of stimuli. Pressure, stretch, chemical transmitters, and electrical current passing across the nerve membrane as a result of a difference in voltage can all initiate nerve activity.[14] A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. ...


The narrow cross-section of axons lessens the metabolic expense of carrying 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. Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. ... Neuroglia cells of the brain shown by Golgis method. ... Oligodendrocytes (from Greek literally meaning few tree cells), or oligodendroglia (Greek, few tree glue),[1] are a variety of neuroglia. ... Named after the German physiologist Theodor Schwann, Schwann cells are a variety of neuroglia that mainly provide myelin insulation to axons in the peripheral nervous system of jawed vertebrates. ... Saltatory conduction (from the Latin saltare, to hop or leap) is a means by which action potentials are transmitted along myelinated nerve fibers. ... Nodes of Ranvier are regularly spaced gaps in the myelin sheath around an axon or nerve fiber. ... A diagram showing the CNS: 1. ...


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.


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. It is important to note that a greater intensity of stimulation produces more impulses per second, not a stronger impulse.[citation needed] All or None (AON) is a stock market term usually mentioned when trading securities. ...


Histology and internal structure

Golgi-stained neurons in human hippocampal tissue.
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 ribosomes. 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. Image File history File linksMetadata Download high-resolution version (1344x1024, 376 KB) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Neuron Metadata This file contains additional information, probably added from the digital camera or scanner used to create... Image File history File linksMetadata Download high-resolution version (1344x1024, 376 KB) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Neuron Metadata This file contains additional information, probably added from the digital camera or scanner used to create... For other uses, see Hippocampus (disambiguation). ... Franz Nissl Franz Nissl (1860-1919) was born in Frankenthal in the Bavarian Palatinate, the son of Theodor Nissl and Maria Haas. ... The endoplasmic reticulum or ER is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae that is responsible for several specialized functions: Protein translation, folding, and transport of proteins to be used in the cell membrane (e. ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ...


The cell body of a neuron is supported by a complex meshwork 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). Intermediate filaments are one component of the cytoskeleton - important structural components of living cells. ... tyrosine is the precursor of catecholamines epinephrine norepinephrine dopamine Synthesis Catecholamines are chemical compounds derived from the amino acid tyrosine containing catechol and amine groups. ... Lipofuscin is the name given to brown pigment granules composed of lipid-containing residues of lysosomal digestion. ...


There are different internal structural characteristics between axons and dendrites. Axons typically 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. Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ... The endoplasmic reticulum or ER is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae that is responsible for several specialized functions: Protein translation, folding, and transport of proteins to be used in the cell membrane (e. ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ...


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. Cajal further extended this to the Law of Dynamic Polarization, which states that neural transmission goes only in one direction, from axons toward dendrites.[15] As with all doctrines, there are some exceptions. For example glial cells may also play a role in information processing.[16] Also, electrical synapses are more common than previously thought,[17] 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 neurons that retain individual cell bodies and the crayfish giant axon consists of a series of neurons with high conductance septate junctions. The Law of Dynamic Polarization also has important exceptions; dendrites can serve as synaptic output sites of neurons[18] and axons can receive synaptic inputs. Ramón y Cajals drawing of the cells of the chick cerebellum, from Estructura de los centros nerviosos de las aves, Madrid, 1905. ... The nervous system is a highly specialized network whose principal components are cells called neurons. ... Santiago Ramón y Cajal Santiago Ramón y Cajal (May 1, 1852 – October 17, 1934) was a famous Spanish histologist, physician, and Nobel laureate. ... Neuroglia of the brain shown by Golgis method. ... An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. ...


Neurons in the brain

The number of neurons in the brain varies dramatically from species to species.[19] One estimate puts the human brain at about 100 billion (1011) neurons and 100 trillion (1014) synapses.[19] 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. By contrast, the fruit fly Drosophila melanogaster has around 300,000 neurons (which do spike) 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. Binomial name Maupas, 1900 Caenorhabditis elegans (IPA: ) is a free-living nematode (roundworm), about 1 mm in length, which lives in temperate soil environments. ... Binomial name Meigen, 1830[1] Drosophila melanogaster (from the Greek for black-bellied dew-lover) is a two-winged insect that belongs to the Diptera, the order of the flies. ... Ion channels are pore-forming proteins that help to establish and control the small voltage gradient that exists across the plasma membrane of all living cells (see cell potential) by allowing the flow of ions down their electrochemical gradient. ...


Neurologic diseases

Alzheimer's disease: 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, 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), recognition (agnosia), and functions such as decision-making and planning get impaired.


Parkinson's disease: Parkinson's disease (also known as Parkinson disease or PD) 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: Myasthenia gravis is a neuromuscular disease leading to fluctuating muscle weakness and fatigability. 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, 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 (AAD), 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.


References

  1. ^ Nature Reviews Neuroscience 8, 368-378 (May 2007) | doi:10.1038/nrn2124
  2. ^ WSU | Ask Dr. Universe | The BIG Questions
  3. ^ http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fneuroscience%2Fans_006.html
  4. ^ Sciam Observations Scientific American Community
  5. ^ Brain Cell Regeneration Studies
  6. ^ Princeton - PWB 040599 - Do brain cells regenerate?
  7. ^ Past Peak: Neurons Regenerate After All
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  9. ^ Grant, Gunnar (9 January 2007 (online)). "How the 1906 Nobel Prize in Physiology or Medicine was shared between Golgi and Cajal". Brain Research Reviews. doi:doi:10.1016/j.brainresrev.2006.11.004. PMID 17027775. Retrieved on 2007-04-02.
  10. ^ The Neuron: Size Comparison
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A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 92nd day of the year (93rd in leap years) in the Gregorian calendar. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 92nd day of the year (93rd in leap years) in the Gregorian calendar. ... is the 78th day of the year (79th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ...

Sources

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  • 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.
  • 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

A thin section of lung tissue stained with hematoxylin and eosin. ... Nervous tissue is the fourth major class of vertebrate tissue. ... Grey matter (or gray matter) is a major component of the central nervous system, consisting of nerve cell bodies, glial cells (astroglia and oligodendrocytes), capillaries, and short nerve cell extensions/processes (axons and dendrites). ... The soma, or perikaryon, is the bulbous end of a neuron, containing the cell nucleus. ... An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... The arrow labeled axon is pointing directly at the axon hillock. ... Axoplasm is the cytoplasm of the axon of a neuron. ... The axolemma is the membrane of a neurons axon. ... Intermediate filaments are one component of the cytoskeleton - important structural components of living cells. ... Dendrites (from Greek dendron, “tree”) are the branched projections of a neuron that act to conduct the electrical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project. ... Image of a Nissl-stained histological section through the rodent hippocampus showing various classes of neurons. ... Close up of the dendrite of a striatal medium spiny neuron. ... An apical dendrite is a dendrite that emerges from the apex of a pyramidal cell. ... A basal dendrite is a dendrite that emerges from the base of a pyramidal cell. ... As a part of the retina, the bipolar cell exists between photoreceptors (rod cells and cone cells) and ganglion cells. ... Pseudounipolar cells (Pseudo- false, uni- one) are sensory neurons in the peripheral nervous system. ... The multipolar neuron possesses a single (usually long) axon and many dendrites, allowing for the integration of a great deal of information from other neurons. ... A pyramidal cell (or pyramidal neuron, or projection neuron) is a multipolar neuron located in the hippocampus and cerebral cortex. ... Drawing of pigeon Purkinje cells (A) by Santiago Ramon y Cajal Purkinje cells are a class of GABAergic neuron located in the cerebellar cortex. ... In neuroscience, granule cells are tiny cells found within the granular layer of the cerebellum. ... The mechanism of the reflex arc In the nervous system, afferent neurons--otherwise known as sensory or receptor neurons--carry nerve impulses from receptors or sense organs toward the central nervous system. ... The mechanism of the reflex arc Sensory neurons (or neurones) are nerve cells within the nervous system responsible for converting external stimuli from the organisms environment into internal electrical impulses. ... The mechanism of the reflex arc Sensory neurons (neurones) are nerve cells within the nervous system responsible for converting external stimuli from the organisms environment into internal electrical motor reflex loops and several forms of involuntary behavior, including pain avoidance. ... The general somatic afferent fibers (or somatic sensory fibers), afferent fibers, arise from cells in the spinal ganglia and are found in all the spinal nerves, except occasionally the first cervical, and conduct impulses of pain, touch and temperature from the surface of the body through the posterior roots to... The general visceral afferent fibers (GVA, or sympathetic afferent fibers), conduct sensory impulses from the viscera through the rami communicantes and posterior roots to the spinal cord. ... Special somatic afferent (SSA) refers to efferent nerves which supply muscles derived from ectoderm. ... Special visceral afferent (SVA) refers to afferent nerves supporting the gastrointestinal tract. ... An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... Type Ia Sensory Fiber also called Primary Afferent Type 1A Fiber or Group II sensory fibers is a component of a muscle fibers muscle spindle which keeps track of the how fast a muscle stretch changes (the velocity of the stretch). ... Organ of Golgi (neurotendinous spindle) from the human tendo calcaneus. ... Type II sensory fiber are the second of the two main groups of stretch receptors. ... A delta fibers (Aδ) are the fibers which convey fast pain information. ... C-fibers are unmyeliniated and as a result, have a slower conduction velocity, lower than 2 m/s. ... In the nervous system, efferent nerves otherwise known as motor or effector neuron carry nerve impulses away from the central nervous systemto effectors - either muscles or glands. ... Motor nerves allow the brain to stimulate muscle contraction. ... In vertebrates, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (CNS) which project their axons outside the CNS and directly or indirectly control muscles. ... The general somatic efferent fibers (or somatic motor fibers), efferent fibers, arise from cells in the anterior column of the spinal cord and pass out through the anterior roots to the voluntary muscles. ... The general visceral efferent fibers (GVE or sympathetic efferent fibers), probably arise from cells in the lateral column or the base of the anterior column and emerge through the anterior roots and white rami communicantes. ... Special visceral efferent (SVE) refers to efferent nerves which supply muscles which derived from the branchial arches. ... Upper motor neurons are any neurons that carry motor information down to the final common pathway, that is, any neurons that are not directly responsible for stimulating the target muscle. ... Lower motor neurons (LMNs) are the motoneurons connecting the brainstem and spinal cord to muscle fibers, bringing the nerve impulses from the upper motor neurons out to the muscles. ... Alpha motor neurons (α-MNs) are large lower motor neurons of the brainstem and spinal cord. ... A muscle spindle, with γ motor and Ia sensory fibers Gamma motoneurons (γ-motoneurons), also called gamma motor neurons, are a component of the fusimotor system, the system by which the central nervous system controls muscle spindle sensitivity. ... Illustration of the major elements in a prototypical synapse. ... Neuropil is the feltwork of unmyelinated neuronal processes (axonal and dendritic) within the gray matter of the central nervous system Traditionally, when pathologists looked at brain tissue they concentrated on neurons (the active functioning cells of the brain), glial cells and axons (especially in white matter, which is mostly composed... In a neuron, synaptic vesicles, also called neurotransmitter vesicles, store the various neurotransmitters that are released during calcium-regulated exocytosis at the presynaptic terminal into the synaptic cleft of a synapse. ... A neuromuscular junction is the junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscles surface. ... An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. ... An interneuron (also called relay neuron,association neuron or bipolar neuron) is a term used to describe a neuron which has two different common meanings. ... Renshaw cells are located in the spinal cord horn. ... In a sensory system, a sensory receptor is a structure that recognizes a stimulus in the internal or external environment of an organism. ... NERVE ENDINGS SUCK PENIS!!! ... Meissners corpuscles (discovered by the anatomist Georg Meissner (1829-1903) are a type of mechanoreceptor and more specifically, a tactile corpuscle(corpusculum tactus). ... Merkel nerve endings are mechanoreceptors found in the skin and mucosa of vertebrates that provide touch information to the brain. ... A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. ... A Pacinian corpuscle is a structure that functions as a mechanoreceptor. ... Ruffini Endings are one of the four main cutaneous mechanoreceptors. ... Bold text == Headline text == minni hi. ... This article is about cellular photoreceptors. ... Hair cells are the sensory cells of both the auditory system and the vestibular system in all vertebrates. ... Taste buds are small structures on the upper surface of the tongue, soft palate, and epiglottis that provide information about the taste of food being eaten. ... Neuroglia of the brain shown by Golgis method. ... Astrocytes (also known collectively as astroglia) are characteristic star-shaped glial cells in the brain. ... Oligodendrocytes (from Greek literally meaning few tree cells), or oligodendroglia (Greek, few tree glue),[1] are a variety of neuroglia. ... Ependyma is the thin epithelial membrane lining the ventricular system of the brain and the spinal cord canal. ... Microglia cells positive for lectins Microglia are a type of glial cell that act as the immune cells of the Central nervous system (CNS). ... Radial glial cells are a pivotal cell type in the developing CNS involved in key developmental processes, ranging from patterning and neuronal migration to their newly described role as precursors during neurogenesis. ... Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. ... White matter is one of the two main solid components of the central nervous system. ... Named after the German physiologist Theodor Schwann, Schwann cells are a variety of neuroglia that mainly provide myelin insulation to axons in the peripheral nervous system of jawed vertebrates. ... Oligodendrocytes (from Greek literally meaning few tree cells), or oligodendroglia (Greek, few tree glue),[1] are a variety of neuroglia. ... This article is about anatomy; for the musical group see Nodes of Ranvier (band) Nodes of Ranvier are regularly spaced gaps in the myelin sheath around an axon or nerve fiber. ... The portion of nerve fiber between two Nodes of Ranvier is called an internodal segment (or internode). ... Oblique clefts may be seen in the medullary sheath, subdividing it into irregular portions, which are termed Schmidt-Lanterman incisures (or clefts of schmidt-lanterman, segments of Lantermann, medullary segments. ... Neurolemma (spelled also neurolema, neurilemma and neurilema, and used interchangeably with epineurium) is the insulating myelin layer that surrounds an individual peripheral nerve fiber. ... Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, muscle, and nervous tissue. ... Neurolemma (spelled also neurolema, neurilemma and neurilema, and used interchangeably with epineurium) is the insulating myelin layer that surrounds an individual peripheral nerve fiber. ... In a nerve fiber, the tubular sheath of the funiculi, perineurium, is a fine, smooth, transparent membrane, which may be easily separated, in the form of a tube, from the fibers it encloses; in structure it is made up of connective tissue, which has a distinctly lamellar arrangement. ... The nerve fibers are held together and supported within the funiculus by delicate connective tissue, called the endoneurium. ... A small bundle of fibers, enclosed in a tubular sheath, is called a funiculus; if the nerve is of small size, it may consist only of a single funiculus; but if large, the funiculi are collected together into larger bundles or nerve fascicles, which are bound together in a common... The meninges (singular meninx) are the system of membranes that envelop the central nervous system. ...

  Results from FactBites:
 
Neuroscience For Kids - cells of the nervous system (681 words)
One way to classify neurons is by the number of extensions that extend from the neuron's cell body (soma).
Neurons are the oldest and longest cells in the body!
Neurons can be quite large - in some neurons, such as corticospinal neurons (from motor cortex to spinal cord) or primary afferent neurons (neurons that extend from the skin into the spinal cord and up to the brain stem), can be several feet long!
Neuromedia - Our articles - Brain - How does the neuron function? (764 words)
The neuron is different from the other cells in its capacity to generate an electric signal that is called a nervous impulse.
Neurons are separated from their environments by a fine layer (a few thousandths of a millimetre thick) called the cell membrane.
The nervous impulse is transmitted to another neuron or another type of cell (for example a muscular cell) via the synapse (the contact between one neuron and the next).
  More results at FactBites »

 
 

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