An axon is a specialized projection of a neuron that carries signals toward other nerve cells, muscles, or glands. Together with the cell body and dendrites, it forms one of the principal structural regions of a typical neuron. Axons usually transmit information through action potentials, electrical signals that propagate along their membranes, and communicate with target cells at synapses. Their specialized organization allows a single cell to receive information in one region and deliver its output elsewhere. (ncbi.nlm.nih.gov)
Structure and organization
A typical neuron develops one axon and several dendrites. Dendrites mainly receive incoming signals, whereas the axon provides an output pathway. The distinction is functional and molecular as well as anatomical: axonal and somatodendritic regions contain different distributions of membrane components and structural proteins. An axon may branch, allowing its signal to reach multiple targets rather than a single endpoint. (ncbi.nlm.nih.gov)
In many neurons, the axon begins at a tapered region called the axon hillock. Immediately beyond it lies the axon initial segment, a specialized region with a high concentration of voltage-gated ion channels. Its membrane-associated scaffolding helps organize these channels and preserve the distinction between axonal and somatodendritic compartments. In most neurons with this organization, action potentials begin within the initial segment rather than in the hillock itself. (pubmed.ncbi.nlm.nih.gov)
The axonal cell membrane is also called the axolemma, and its internal cytoplasm is called axoplasm. The axon contains a cytoskeleton that supports its shape and provides routes for intracellular transport. Longitudinal microtubules act as tracks along which molecular motors move membrane-bound cargoes and organelles between the cell body and distant axonal regions. (ncbi.nlm.nih.gov)
Electrical signaling
Axonal signaling depends on an electrochemical gradient across the membrane. Changes in the permeability of ion channels allow ions to move into or out of the axon, changing its membrane potential. When depolarization reaches the conditions required for excitation, voltage-gated channels generate an action potential. Activation of neighboring membrane regions allows the signal to propagate along the axon. (ncbi.nlm.nih.gov)
The initial segment and, in myelinated axons, the nodes of Ranvier are particularly important excitable domains. Their specialized channel distributions make signal initiation and propagation more reliable. Axonal function therefore depends not only on the presence of ion channels, but also on their organization into distinct membrane regions. Neuronal scaffolding molecules and interactions with surrounding supporting cells contribute to this organization. (pubmed.ncbi.nlm.nih.gov)
Myelin and supporting cells
Many vertebrate axons are surrounded by myelin, a multilayered membrane sheath produced by glial cells. In the central nervous system, myelin is formed by oligodendrocytes; in the peripheral nervous system, it is formed by Schwann cells. An oligodendrocyte can myelinate segments of several axons, whereas each myelinating Schwann cell forms one segment around one axon. (ncbi.nlm.nih.gov)
The sheath is interrupted by short gaps called nodes of Ranvier. These contain concentrated voltage-gated channels that regenerate the electrical signal. Myelination enables saltatory conduction, in which action potentials are regenerated at successive nodes rather than continuously along every part of the axonal membrane. Myelin also helps establish distinct molecular domains in the underlying axon; it is not simply an inert insulating covering. (ncbi.nlm.nih.gov)
Axonal transport and maintenance
Axons require a supply of materials from the cell body, particularly when their terminals lie far away. Axonal transport moves organelles, vesicle precursors, and proteins through the axoplasm. Anterograde transport proceeds toward the terminals, while retrograde transport returns materials toward the cell body for processing, degradation, or recycling. These movements occur along oriented microtubules. (ncbi.nlm.nih.gov)
Kinesin-family motors carry many anterograde cargoes, including mitochondria and precursors of synaptic vesicles. Cytoplasmic dynein drives much of the retrograde movement. These motor proteins use ATP to power transport. Directed movement is essential because diffusion alone cannot efficiently supply widely separated neuronal compartments. (ncbi.nlm.nih.gov)
Although the cell body supplies much axonal material, some axons also synthesize proteins locally. Experiments on sensory and retinal axons have shown that local protein synthesis and degradation contribute to the formation of new growth cones after injury. This capacity differs among neuronal types and developmental stages. (pubmed.ncbi.nlm.nih.gov)
Terminals and synaptic transmission
At a chemical synapse, an arriving action potential opens voltage-gated calcium channels in the presynaptic terminal. Calcium entry triggers synaptic vesicles to fuse with the membrane and release a neurotransmitter into the synaptic cleft. The transmitter acts on receptors of the receiving cell, converting the axonal electrical signal into a chemical signal and then into a response in the target. Vesicles can subsequently be recycled locally. (ncbi.nlm.nih.gov)
Development and regeneration
A growing axon advances through a growth cone, a motile structure at its tip that probes surrounding tissue. Growth cones combine the machinery for extension with mechanisms that respond to directional cues. On reaching a target region, axons establish synaptic connections; signals from target tissues also influence neuronal survival and the development of those connections. (ncbi.nlm.nih.gov)
After an axon is severed, the disconnected distal portion undergoes Wallerian degeneration. Peripheral axons can regenerate when the neuron activates an appropriate growth program and the surrounding tissue supports extension. Schwann cells and other cells help clear debris and guide regrowth. Regeneration in the adult central nervous system is generally much more limited, reflecting both neuronal growth capacity and an environment containing inhibitors of axon extension. Regrowth must also restore appropriate target connections to recover function. (ncbi.nlm.nih.gov)