Axon

Axon
Blausen 0657 MultipolarNeuron.png
An axon of a multipolar neuron
Identifiers
MeSHD001369
Anatomical terminology

An axon (from Greek ἄξων áxōn, axis) or nerve fiber, is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action potentials, away from the nerve cell body. The function of the axon is to transmit information to different neurons, muscles, and glands. In certain sensory neurons (pseudounipolar neurons), such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body, and from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction has caused many inherited and acquired neurological disorders which can affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, and group C nerve fibers. Groups A and B are myelinated, and group C are unmyelinated. These groups include both sensory fibers and motor fibers. Another classification, groups only the sensory fibers, and these are grouped as Type I, Type II, Type III, and Type IV.

An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron; the other type is a dendrite. Axons are distinguished from dendrites by several features, including shape (dendrites often taper while axons usually maintain a constant radius), length (dendrites are restricted to a small region around the cell body while axons can be much longer), and function (dendrites receive signals whereas axons transmit them). Some types of neurons have no axon and transmit signals from their dendrites. In some species, axons can emanate from dendrites and these are known as axon-carrying dendrites.[1] No neuron ever has more than one axon; however in invertebrates such as insects or leeches the axon sometimes consists of several regions that function more or less independently of each other.[2]

Axons are covered by a membrane known as an axolemma; the cytoplasm of an axon is called axoplasm. Most axons branch, in some cases very profusely. The end branches of an axon are called telodendria. The swollen end of a telodendron is known as the axon terminal which joins the dendron or cell body of another neuron forming a synaptic connection. Axons make contact with other cells—usually other neurons but sometimes muscle or gland cells—at junctions called synapses. In some circumstances, the axon of one neuron may form a synapse with the dendrites of the same neuron, resulting in an autapse. At a synapse, the membrane of the axon closely adjoins the membrane of the target cell, and special molecular structures serve to transmit electrical or electrochemical signals across the gap. Some synaptic junctions appear along the length of an axon as it extends—these are called en passant ("in passing") synapses and can be in the hundreds or even the thousands along one axon.[3] Other synapses appear as terminals at the ends of axonal branches.

A single axon, with all its branches taken together, can innervate multiple parts of the brain and generate thousands of synaptic terminals. A bundle of axons make a nerve tract in the central nervous system,[4] and a fascicle in the peripheral nervous system. In placental mammals the largest white matter tract in the brain is the corpus callosum, formed of some 20 million axons in the human brain.[4]

Anatomy

A typical myelinated axon
A dissected human brain, showing grey matter and white matter

Axons are the primary transmission lines of the nervous system, and as bundles they form nerves. Some axons can extend up to one meter or more while others extend as little as one millimeter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot. The diameter of axons is also variable. Most individual axons are microscopic in diameter (typically about one micrometer (µm) across). The largest mammalian axons can reach a diameter of up to 20 µm. The squid giant axon, which is specialized to conduct signals very rapidly, is close to 1 millimetre in diameter, the size of a small pencil lead. The numbers of axonal telodendria (the branching structures at the end of the axon) can also differ from one nerve fiber to the next. Axons in the central nervous system (CNS) typically show multiple telodendria, with many synaptic end points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate branching allows for the simultaneous transmission of messages to a large number of target neurons within a single region of the brain.

There are two types of axons in the nervous system: myelinated and unmyelinated axons.[5] Myelin is a layer of a fatty insulating substance, which is formed by two types of glial cells Schwann cells and oligodendrocytes. In the peripheral nervous system Schwann cells form the myelin sheath of a myelinated axon. In the central nervous system oligodendrocytes form the insulating myelin. Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly spaced intervals. The myelination enables an especially rapid mode of electrical impulse propagation called saltatory conduction.

The myelinated axons from the cortical neurons form the bulk of the neural tissue called white matter in the brain. The myelin gives the white appearance to the tissue in contrast to the grey matter of the cerebral cortex which contains the neuronal cell bodies. A similar arrangement is seen in the cerebellum. Bundles of myelinated axons make up the nerve tracts in the CNS. Where these tracts cross the midline of the brain to connect opposite regions they are called commissures. The largest of these is the corpus callosum that connects the two cerebral hemispheres, and this has around 20 million axons.[4]

The structure of a neuron is seen to consist of two separate functional regions, or compartments – the cell body together with the dendrites as one region, and the axonal region as the other.[3] Nissl bodies of the soma and dendrites, where protein is synthesised is absent in the axonal region which includes the axon hillock.[3]

Axonal region

The axonal region or compartment, includes the axon hillock, the initial segment, the rest of the axon, and the axon telodendria, and axon terminals. It also includes the myelin sheath. The Nissl bodies that produce the neuronal proteins are absent in the axonal region.[3] Proteins needed for the growth of the axon, and the removal of waste materials, need a framework for transport. This axonal transport is provided for in the axoplasm.

Axon hillock

Detail showing microtubules at axon hillock and initial segment.

The axon hillock is the area formed from the cell body of the neuron as it extends to become the axon. It precedes the initial segment. The received action potentials that are summed in the neuron are transmitted to the axon hillock for the generation of an action potential from the initial segment.

Initial segment

The axonal initial segment (AIS) is a structurally and functionally separate microdomain of the axon.[6][7] One function of the initial segment is to separate the main part of an axon from the rest of the neuron; another function is to help initiate action potentials.[8] Both of these functions support neuron cell polarity, in which dendrites (and, in some cases, soma) of a neuron receive input signals and the neuron's axon provides output signals.[9]

The axon initial segment is unmyelinated and contains a specialized complex of proteins. It is between approximately 20 and 60 µm in length and functions as the site of action potential initiation.[10][11] Both the position on the axon and the length of the AIS can change showing a degree of plasticity that can fine-tune the neuronal output.[10][12] A longer AIS is associated with a greater excitability.[12]

The AIS is highly specialized for the fast conduction of nerve impulses. This is achieved by a high concentration of voltage-gated ion channels in the initial segment where the action potential is initiated. The ion channels are accompanied by a high number of cell adhesion molecules and scaffolding proteins that anchor them to the cytoskeleton.[10] Interactions with ankyrin G are important as it is the major organizer in the AIS.[10]

Axonal transport

The axoplasm is the equivalent of cytoplasm in the cell. Microtubules form in the axoplasm at the axon hillock. They are arranged along the length of the axon, in overlapping sections, and all point in the same direction – towards the axon terminals.[13] This is noted by the positive endings of the microtubules. This overlapping arrangement provides the routes for the transport of different materials from the cell body.[13] Studies on the axoplasm has shown the movement of numerous vesicles of all sizes to be seen along cytoskeletal filaments – the microtubules, and neurofilaments, in both directions between the axon and its terminals and the cell body.

Outgoing anterograde transport from the cell body along the axon, carries mitochondria and membrane proteins needed for growth to the axon terminal. Ingoing retrograde transport carries cell waste materials from the axon terminal to the cell body.[14] Outgoing and ingoing tracks use different sets of motor proteins.[13] Outgoing transport is provided by kinesin, and ingoing return traffic is provided by dynein. Dynein is minus-end directed.[14] There are many forms of kinesis and dynein motor proteins, and each is thought to carry a different cargo.[13] The studies on transport in the axon led to the naming of kinesin.[13]

Myelination

Transmission electron micrograph of a myelinated axon in cross-section. Generated by the electron microscopy unit at Trinity College, Hartford CT
Cross section of an axon.
1. Axon
2. Nucleus of Schwann cell
3. Schwann cell
4. Myelin sheath
5. Neurilemma

In the nervous system, axons may be myelinated, or unmyelinated. This is the provision of an insulating layer, called a myelin sheath. In the peripheral nervous system axons are myelinated by glial cells known as Schwann cells. In the central nervous system the myelin sheath is provided by another type of glial cell, the oligodendrocyte. Schwann cells myelinate a single axon. An oligodendrocyte can myelinate up to 50 axons.[15]

Nodes of Ranvier

Nodes of Ranvier (also known as myelin sheath gaps) are short unmyelinated segments of a myelinated axon, which are found periodically interspersed between segments of the myelin sheath. Therefore, at the point of the node of Ranvier, the axon is reduced in diameter.[16] These nodes are areas where action potentials can be generated. In saltatory conduction, electrical currents produced at each node of Ranvier are conducted with little attenuation to the next node in line, where they remain strong enough to generate another action potential. Thus in a myelinated axon, action potentials effectively "jump" from node to node, bypassing the myelinated stretches in between, resulting in a propagation speed much faster than even the fastest unmyelinated axon can sustain.

Axon terminals

An axon can divide into many branches called telodendria (Greek–end of tree). At the end of each telodendron is an axon terminal (also called a synaptic bouton, or terminal bouton). Axon terminals contain synaptic vesicles that store the neurotransmitter for release at the synapse. This makes multiple synaptic connections with other neurons possible. Sometimes the axon of a neuron may synapse onto dendrites of the same neuron, when it is known as an autapse.

Other Languages
Afrikaans: Akson
العربية: محور عصبي
asturianu: Axón
azərbaycanca: Akson
беларуская: Аксон
български: Аксон
bosanski: Akson
català: Axó
čeština: Axon
dansk: Akson
Deutsch: Axon
eesti: Akson
Ελληνικά: Νευράξονας
español: Axón
euskara: Axoi
فارسی: آسه
français: Axone
galego: Axón
한국어: 축삭
Հայերեն: Աքսոն
hrvatski: Akson
Bahasa Indonesia: Akson
italiano: Assone
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қазақша: Аксон
Kreyòl ayisyen: Aksòn
Кыргызча: Аксон
Latina: Axon
lietuvių: Aksonas
македонски: Нервно влакно
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Nederlands: Axon
日本語: 神経繊維
norsk: Akson
norsk nynorsk: Akson
oʻzbekcha/ўзбекча: Akson
polski: Neuryt
português: Axónio
română: Axon
русский: Аксон
Scots: Axon
Simple English: Axon
slovenščina: Nevrit
کوردی: تەوەرە
српски / srpski: Akson
srpskohrvatski / српскохрватски: Akson
svenska: Axon
Tagalog: Akson
тоҷикӣ: Аксон
Türkçe: Akson
українська: Аксон
اردو: محوار
Tiếng Việt: Sợi trục
中文: 轴突