An axons (from the Greek ÃÆ'áx? N , axis) or nerve fibers , is the long and slender projection of a nerve cells, or neurons, which typically perform electrical impulses known as action potential, away from the body of nerve cells. The axon function is to transmit information to various neurons, muscles, and glands. In certain sensory neurons (pseudounipolar neurons), such as for touch and warmth, axons are called afferent nerve fibers and electrical impulses travel along this from the periphery to the body cell, and from the body of the cell to the spinal cord along with the other. branch of the same axon. Axon dysfunction has caused many inherited and acquired neurological disorders that can affect peripheral and central neurons. Nerve fibers are classified into three types - groups of nerve fibers, Group B nerve fibers, and Group C fibers. Group A and B are myelinated, and group C is unmyelinated. These groups include sensory fibers and motor fibers. Another classification, grouping only sensory fibers, is classified as Type I, Type II, Type III, and Type IV.
Axons are one of two types of cytoplasmic protrusions from the body cells of neurons; the other type is dendrites. Axons are distinguished from dendrites by some features, including forms (dendrites are often tapered while axons usually maintain a constant radius), length (dendrites are confined to small areas around the cell body while axons can be longer), and functions (dendrites receive signals whereas axons send them). Some types of neurons do not have axons and transmit signals from their dendrites. No neurons have more than one axon; but in invertebrates such as insects or leeches, axons sometimes consist of several regions that function more or less independently of each other.
Axons are covered by membranes known as axolemma; Axoplasma cytoplasm is called axoplasma. Most axon branches, in some cases are very heavy. The end of the axon is called a telodendria. The swollen end of the telodendron is known as an axon terminal that joins the dendron or body cell of another neuron that forms a synaptic connection. Axons make contact with other cells - usually other neurons but sometimes muscle cells or glands - at the junction called synapses. In some circumstances, a single axon neuron may form a synapse with dendrites of the same neuron, producing autaps. In synapses, the adjacent axon membrane is adjacent to the target cell membrane, and the special molecular structure serves to transmit electrical or electrochemical signals across the gap. Some synaptic connections appear along the axons as they extend - these are called "pass through" synapses and can reach hundreds or even thousands along an axon. Other synapses appear as terminals at the end of axonal branches.
A single axon, with all its branches taken together, can supply several parts of the brain and produce thousands of synaptic terminals. A pair of axons make the nervous system in the central nervous system, and the wicked in the peripheral nervous system. The largest white matter duct in the brain is a corpus callosum formed from about 20 million axons.
Video Axon
Anatomy
Axons are the main transmission path of the nervous system, and as their bundles form nerves. Some axons may extend up to one meter or more while others extend as small as one millimeter. The longest axon in the human body is sciatic nerve, which runs from the base of the spinal cord to the big toe of each. The axon diameter also varies. Most individual axons are microscopic in diameter (usually about one micrometer (Ãμm) across). The largest mammalian axons can reach up to 20 Ãμm in diameter. The giant squid axon, which is devoted to performing signals very quickly, is approaching 1 millimeter in diameter, the size of a small pencil. The amount of axonal telodendria (branching structure at the other end of the axon) may also differ from one nerve fiber to the next. Axons in the central nervous system (CNS) usually show multiple telodendria, with many synaptic endpoints. For comparison, the seronar granular cell axon is characterized by a single T-shaped branch node from which two parallel fibers are elongated. The complicated branch allows simultaneous message transmission to a large number of target neurons within a single region of the brain.
There are two types of axons in the nervous system: myelin and unmyelinated axons. Myelin is a layer of fatty insulating substance, formed by two types of Schwann cell glial cells and oligodendrocytes. In the peripheral nervous system Schwann cells form the myelin sheath of the myelin axon. In oligodendrocytes the central nervous system forms myelin sealing. Throughout the myelin nerve fibers, the gaps in the myelin sheath known as Ranvier nodes occur at equal intervals. Myelination allows a very fast mode of electrical impulse propagation called salt conduction.
The myelin axons of cortical neurons make up most of the neural network called white matter in the brain. Mielin gives a white appearance to the tissue in contrast to the gray matter of the cerebral cortex that contains the body of a nerve cell. Similar arrangements are seen in the cerebellum. A collection of myelin axons forms a neural channel on the CNS. Where these channels cross the midline of the brain to connect the opposite region they are called commissures . The biggest of these is the corpus callosum that connects the two hemispheres of the brain, and it has about 20 million axons.
The structure of the visible neuron consists of two separate functional regions, or cell compartments together with dendrites as a region, and axonal regions as the other. The Nissl body of soma and dendrites, in which proteins are synthesized does not exist in the axonal region that includes an axon hill.
Axonal region
Axon region or axon compartments, including axon hillocks, initial segments, axon residuals, and telodendric axons, and axon terminals. This also includes the myelin sheath. The Nissl body that produces neuronal proteins does not exist in the axonal region. The proteins required for the growth of axons, and removal of waste materials, require a framework for transport. This axial transport is provided in axoplasm.
Axon Hill
Axon hill is an area formed from the cell body of a neuron as it extends into an axon. This precedes the initial segment. The accepted action potential added to the neuron is transmitted to axon hillock to generate an action potential from the initial segment.
Initial segment
The initial segment of the axon - the thick and non-bermyelin part of the axon that connects directly to the cell body - is composed of a protein-specific complex. Its length is about 25 Ãμm and serves as a potential initiation site. The density of the voltage-gated sodium channel is much higher in the initial segment than in the remaining axons or in adjacent body cells, except for axon hill. The voltage-gated ion channel is known to be found within a certain area of ââthe axonal membrane and initiates the synaptic action, conduction, and transmission potential.
Axonal transport
Axoplasm is the equivalent of cytoplasm inside the cell. Microtubules are formed in axoplasm in axon hillock. They are arranged along the axons, in overlapping parts, and all points in the same direction - toward the axon terminal. This is noted by a positive microtubule tip. This overlapping arrangement provides a route for transporting different materials from the cell body. The study of axoplasm has shown the movement of many vesicles of all sizes to be seen along the cytoskeletal filaments - microtubules, and neurofilaments, in both directions between axons and terminals and body cells.
Anterograde transport out of the body cells along the axon, carrying mitochondria and membrane proteins necessary for growth to the axon terminal. Ingoing retrograde transport carries cell waste material from the axon terminal to the body cell. The incoming and outgoing tracks use different sets of motor proteins. Outbound transport is provided by kinesin, and incoming traffic is provided by dynein. Dynein directed minus-end. There are many forms of kinesis and protein dynein motors, and each is thought to carry a different cargo. The study of transport in axons led to the naming of kinesin.
Myelination
In the nervous system, axons may be myelin or unmyelinated. This is the provision of an insulating layer, called the myelin sheath. In axon the peripheral nervous system is myelinated by glial cells known as Schwann cells. In the central nervous system the myelin sheath is provided by another type of glial cell, oligodendrocyte. Schwann cells myelinate one axon. An oligodendrocyte can myelinate up to 50 axons.
Ranvier Node
The Ranvier knots (also known as the myelin sheath sheath) are the short non-myelin segments of the myelin axon, which are found periodically interspersed between the segments of the myelin sheath. Therefore, at the point of Ranvier's node, the axon decreases in diameter. This node is an area where potential action can be generated. In saltatory conduction, the electric current generated at each Ranvier node is performed with a slight attenuation to the next node in the line, where they remain strong enough to produce other action potentials. Thus in a myelin axis, the effective action potential "jumps" from the node to the node, passing through the myelinated stretches between, so the propagation speed is much faster than the fastest unmyelinated axons can sustain.
Axon terminal
Axons can divide into many branches called telodendria (the tip of a Greek tree). At the end of each telodendron is the axon terminal (also called synaptic bouton, or terminal bouton). The axon terminal contains the synaptic vesicles that store the neurotransmitter to be released at the synapse. This makes some synaptic connections with other neurons possible. Sometimes axons of neurons can sneeze into the dendrites of the same neuron, when it is known as autaps.
Maps Axon
Potential action
Most axons carry signals in the form of action potentials, which are discrete electrochemical impulses that travel rapidly along the axon, beginning in the cell body and ending at points where the axon makes synaptic contact with the target cell. The defining characteristic of an action potential is that "all-or-none" - any potential action generated by the axon is essentially of the same size and shape. This all-or-none characteristic allows the action potential to be transmitted from one end of the long axon to the other without reduction in size. However, several types of neurons with short axons carrying multilevel electrochemical signals, of variable amplitude.
When the action potential reaches the presinaptic terminal, it activates the synaptic transmission process. The first step is the rapid opening of calcium ion channels in the axon membrane, allowing calcium ions to flow into the membrane across the membrane. Increased intracellular calcium concentrations cause synaptic vesicles (small containers covered by lipid membranes) filled with neurotransmitter chemistry to fuse with axon membranes and empty their contents into the extracellular space. The neurotransmitter is released from the presynaptic nerve through exocytosis. Chemical neurotransmitters then diffuse across to receptors located in the target cell membrane. The neurotransmitter binds to these receptors and activates them. Depending on the type of receptor being activated, the effect on the target cell may excite the target cell, inhibit it, or alter the metabolism in some way. The whole sequence of events often happens in less than a thousandth of a second. After that, inside the presinaptic terminal, a new vesicle set is moved to the position next to the membrane, ready to be released when the next action potential arrives. Action potential is the last electrical step in synaptic message integration at the neuron scale.
Extracellular recording of potential propagation action in axons has been shown in free-moving animals. While extracellular somatic action potentials have been used to study cellular activity in free-moving animals such as spot cells, axonal activity in both white and gray material can also be recorded. The extracellular tape of the axial action potential propagation differs from the somatic action potential in three ways: 1. The signal has a shorter peak-trough duration (~ 150? S) than a pyramidal cell (~ 500? S) or interneuron (~ 250? S). 2. The voltage change is triphasic. 3. The activity recorded on the tetrode is only visible on one of four recording cables. In recording of free-moving rats, axonal signals have been isolated in white matter including alveus and corpus callosum as well as hippocampal gray matter.
In fact, the in vivo action potential generation is sequential, and this sequential spike is a digital code in neurons. Although previous research has shown the axonal origin of a single spike caused by short-term pulses, the in vivo physiological signals trigger successive spike initiation in the neuronal cell body.
In addition to spreading the action potential to the axonal terminal, the axon is able to amplify the action potential, which ensures the spreading of a safe sequential action potential toward the axonal terminal. In the case of molecular mechanisms, the voltage-gated sodium channel in axon has a lower threshold and a shorter refractory period in response to short-term pulses.
Development and growth
Development
Development of axons to their targets, is one of the six major stages in the overall development of the nervous system. Studies conducted on culture hippocampal neurons show that neurons initially produce many equivalent neurites, but only one of these neurons is destined to become an axon. It is not clear whether the axon specification precedes the axon elongation or vice versa, although recent evidence indicates the latter. If the axons that are not fully developed are cut, the polarity may change and other neurites potentially become axons. This change of polarity occurs only when axons are cut at least 10 m shorter than other neurites. Once the incision is made, the longest neurite will be the future axon and all other neurites, including the original axons, will turn into dendrites. Imposing an external force on the neurite, causing it to elongate, will make it an axon. Nevertheless, axonal development is achieved through a complex interaction between extracellular signaling, intracellular signaling and cytoskeletal dynamics.
Extracellular signaling
The extracellular signals that spread through the extracellular matrix surrounding the neurons play an important role in axonal development. These signal molecules include proteins, neurotrophic factors, and extracellular matrix and adhesion molecules. Netrin (also known as UNC-6) is a secreted protein, functioning in the formation of axons. When the UNR-5 neutral receptors mutate, some neurites are projected irregularly from neurons and eventually an axon is extended to the anterior. Neurotropic factors - nerve growth factor (NGF), brain-derived neurotrophic factors (BDNF) and neurotrophin-3 (NTF3) are also involved in the development of axons and binding to Trk receptors.
Ganglioside-converting enzyme plasma membrane ganglioside sialidase (PMGS), which is involved in the activation of TrkA at the tip of the neutrites, is necessary for the lengthening of the axon. Asymmetric PMGs distribute to the ends of neurites that are destined to become the axons of the future.
Giving intracellular signals
During axonal development, PI3K activity increases at the end of the doomed axon. Disrupting PI3K activity inhibits axonal development. Activation of PI3K results in the production of phosphatidylinositol (3,4,5) -trisphosphate (PtdIns) which can cause significant neurite elongation, converting it into an axon. Thus, the excessive expression of phosphatase that damages PtdIns leads to polarization failure.
Cytoskeletal Dynamics
Neurites with the lowest actin filament content will be axons. The concentration of PGMS and the f-actin content is inversely correlated; when PGMS becomes enriched at the tip of the neurite, its f-actin content substantially decreases. In addition, exposure to actin-depolymerizing drugs and toxin B (which inactivate Rho-signaling) causes the formation of some axons. As a result, actin tissue disruption in the growth cones will induce its neurite to become an axon.
Growth
Growing axons move through their environment through a growth cone, which is at the end of the axon. The growth cones have extensions such as a wide sheet called lamellipodium containing a bulge called filopodia. The filopodia is the mechanism by which the entire process is attached to the surface and explores the surrounding environment. Actin plays a major role in the mobility of this system. The environment with high levels of cell adhesion molecules (CAM) creates an ideal environment for axonal growth. This seems to provide a "sticky" surface for the axons growing together. Special examples of CAM for the nervous system include N-CAM, TAG-1 axonal glycoproteins, and MAG all of which are part of the immunoglobulin superfamily. Another set of molecules called extracellular matrix-adhesion molecules also provides a sticky substrate for axons to grow together. Examples of these molecules include laminin, fibronectin, tenascin, and perlecan. Some of these are surfaces that are bound to the cell and thus act as short-distance or repulsive pullers. The other is a ligand that can be diffusible and thus can have a long-range effect.
A cell called a guidepost cell helps in the guidance of nerve axon growth. These cells are usually other, sometimes immature, neurons.
It has also been found through research that if the axons of the neuron are damaged, as long as the soma (the cell body of the neuron) is not damaged, the axon regenerates and re-establishes the synaptic connection with the neuron with the help of the cell stake. This is also referred to as neuroregeneration.
Nogo-A is a type of neuron growth inhibitory component present in the central nervous system of the myelin membrane (found in axons). It has an important role in limiting axonal regeneration in the central nervous system of adult mammals. In a recent study, if Nogo-A was blocked and neutralized, it was possible to induce long-aconal regeneration leading to increased functional recovery in rats and spinal cord mice. This has not been done in humans. A recent study also found that activated macrophages through a specific inflammatory line activated by Dectin-1 receptors are capable of promoting axon recovery, also causing neurotoxicity in neurons.
Classification
The axons of neurons in the human peripheral nervous system can be classified by physical features and the nature of signal conduction. Axons are known to have different thicknesses (from 0.1 to 20 μm) and these differences are thought to correspond to the velocity that the action potential can travel along the axis - conduction velocity . Erlanger and Gasser prove this hypothesis, and identify several types of nerve fibers, building a relationship between the axon diameter and the speed of its neural conduction. They published their findings in 1941 by providing the first axon classification.
Axons are classified into two systems. The first was introduced by Erlanger and Gasser, grouping fibers into three main groups using letters A, B, and C. These groups, group A, group B, and group C include sensory (afferent) fibers and motor fibers. (eferents). The first group A, subdivided into alpha, beta, gamma, and delta fibers - A ?, A?, A?, And A ?. Motor neurons of different motor fibers, are lower motor neurons - alpha motor neurons, beta motor neurons, and gamma motor neurons that have A?, A?, And A? nerve fibers respectively.
The later findings by other researchers, identified two groups of Aa fibers that are motor fibers. These are then fed into systems that include only sensory fibers (although some of them are mixed nerves and also motor fibers). This system refers to sensory groups as Type and uses Roman numerals - Type Ia, Type Ib, Type II, Type III, and Type IV.
Motor
The lower motor neurons have two types of fibers:
Sensoric
Different sensory receptors conserve different types of nerve fibers. Proprioceptors are innervated by type Ia, Ib and II sensory sensors, mechanoreceptors by type II and III sensory fibers and nociceptors and thermoreceptors by type III and IV sensory fibers.
Autonom
The autonomic nervous system has two types of peripheral fibers:
Clinical interests
In the order of severity, nerve injury can be described as neurapraxia, axonotmesis, or neurotmesis. A concussion is considered a mild form of diffuse axonal injury. Axon dysfunction in the nervous system is one of the major causes of many neurological disorders affecting peripheral and central neurons.
Demielination of axons causes many of the neurological symptoms found in multiple sclerosis.
Dysmyelination is an abnormal formation of the myelin sheath. It is involved in some leukodystrophies, as well as schizophrenia.
History
The German anatomist, Otto Friedrich Karl Deiters, is generally credited with the discovery of axons by distinguishing them from dendrites. Swiss RÃÆ'üdolf Albert von KÃÆ'ölliker and Germany Robert Remak are the first to identify and characterize the initial segments of axons. KÃÆ'ölliker named axons in 1896. Alan Hodgkin and Andrew Huxley also used giant squid akon (1939) and in 1952 they had obtained a full quantitative description of the ionic basis of the action potential, leading to the Hodgkin-Model Huxley formulation. Hodgkin and Huxley were awarded the Nobel Prize for this work in 1963. The formula detailing the conductance of axons extends to vertebrates in the Frankenhaeuser-Huxley equation. Louis-Antoine Ranvier was the first to describe the gaps or vertices found on axons and for this contribution the axonal feature is now commonly referred to as Ranvier's knot. Santiago RamÃÆ'ón y Cajal, an expert on Spanish anatomy, proposed that axons are the component of neuron output, describing their function. Joseph Erlanger and Herbert Gasser have previously developed a classification system for peripheral nerve fibers, based on axonal conduction velocity, myelination, fiber size, etc. The basic understanding of biochemistry for action potential propagation has advanced further, and includes many details about individual ion channels.
Other animals
Axons in invertebrates have been studied extensively. Longfin long squid beaches, often used as model organisms have the longest known axons. The giant squid has the largest axons known. Its size ranges from half (usually) to one millimeter in diameter and is used in the control of its jet propulsion system. The fastest recorded conduction velocity of 210 m/s, found in axons that are powered from some Pelaeis Penaeid shrimp and the usual range is between 90 and 200 m/s (cf 100-120 m/s for fastest myelinated vertebrate myks).
See also
- Axons guide
- Electrophysiology
- Nervous guidance channels
- Neuroregeneration
- Neuronal tracking
- Axon pioneers
References
External links
- Histology: 3_09 at Oklahoma University Medical Center - "Slide 3 spinal cord"
- Bialowas, Andrzej, Carlier, Edmond, Campanac, Emilie, Debanne, Dominique, Alcaraz. Axon Physiology, GisÃÆ'èlePHYSIOLOGICAL REVIEWS, V. 91 (2), 04/2011, p.Ã, 555-602.
Source of the article : Wikipedia