Axon

As Axon a special nerve process is called that transmits nerve impulses from a nerve cell to a target organ such as a gland or muscle or to another nerve cell. In addition, axons are able to transport certain molecules in both directions to the cell soma and also in the opposite direction via the so-called axonal substance transport.

What is the axon?

The axon is the nerve process of a cell, also called Neurite refers to the nerve impulses from the nerve cell to other nerve cells or to organs or muscles. The impulses contain a kind of command to secrete certain hormones or other substances, and in the case of muscle fibers they induce contraction or relaxation.

Axons can branch towards the end and form so-called telodendrons, button-like thickenings that play an important role in the chemical signal transmission via synapses to the target organ. Each nerve cell usually has only a single axon, which can be less than 1 mm in length to over 1 m in length, such as B. in axons that extend from a nerve cell in one of the spinal cords to the muscles of the feet and toes. The nerve tracts have a cross section of only 0.08 µm to 20 µm, so they can be extremely thin.

Most axons are surrounded by a sheath of glial cells (myelination), which serve as a support structure and electrical insulation between the neurons. According to more recent findings, glial cells also take on essential tasks in axonal substance transport and in the storage, transmission and processing of information in the brain.

Anatomy & structure

The axon arises from a characteristic protrusion of the nerve cell body, the axon hill. In the further course, the axons usually receive a myelin sheath, which serves as support and electrical insulation as well as other important functions. It consists of a lipid-rich biomembrane made of glial cells.

In axons of the central nervous system (CNS), the biomembrane is formed from oligodendrocytes, a specialized type of glial cells, and in the case of the peripheral nervous system (PNS), this task is carried out by Schwann’s cells. Typically, the myelinated axons contain about 1 µm wide Ranvier rings at a distance of 0.2 to 2 mm. They represent regular interruptions in the myelin sheath and conductivity. Nerve impulses are transmitted to the Ranvier cord rings via extremely fast Na ion transport. The impulses “jump” from lace ring to lace ring.

Axons contain a cytoskeleton for mechanical stabilization, which is composed of neurofilaments and neurotubules. The neurotubules also take on tasks in transporting substances within the axon. The cytoplasm contained in the axon, called axoplasm, hardly contains the ribosomes necessary for protein synthesis, so that axons rely on the supply of proteins from the cell nucleus and thus also on the relatively slow transport of substances within the axon.

Function & tasks

An important function and task of the axon is the transmission of nerve impulses from the nucleus of the cell to the dendrites of another (connected) neuron or to target organs - mostly muscles or glands. While the transmission of signals within the axon takes place electrically, the signal transmission to the terminal heads, the telodendrons, takes place chemically via neurotransmitters.

The electrical action potential is "translated" into the release of messenger substances, which dock on special receptors of the recipient and in turn cause a back translation into an electrical action potential. In principle, a distinction is made between efferent and afferent axons. The "classic" axons are efferent directions of transmission of nerve signals that are transmitted from the nerve cell to other neurons or to target organs.

Depending on which nervous system they belong to, axons can be subject to the will in their signal transmission (somatosensitive, somatomotor) or, in the case of the autonomic nervous system, transmit unconscious, viscerosensitive signals to control the autonomic body systems. Another function of the axons is axonal mass transport. It becomes necessary because axons cannot synthesize the proteins required to maintain their tasks and functions “on site”. They rely on getting their proteins from the perikaryon, the center of their cell.

This can be a challenge given the sometimes enormous length of an axon of over 1 m. To accomplish this task, axons have slow and fast axonal mass transport. The slow mass transport only works in the direction away from the perikaryon towards the end of the axon. The rapid transport of substances works in both directions, so substances can also be transported to a limited extent from the axons to the cytoplasm of the neuron.

Diseases

Accidents that result in the crushing or severing of axons are associated with partial or total loss of function of nerve conductivity. This means that z. B. certain muscle groups are practically paralyzed and are quickly broken down by the body. After they have fully matured, axons of the CNS lose their ability to regenerate, so that severed axons cannot grow back. Axons of the peripheral nervous system are capable of regeneration to a certain extent.

If the myelin sheath is still intact, but the nerve itself is severed, regrowth is possible at a rate of 2 to 3 mm per day if the regrowing end is not too far from the severed end. In some cases, neurosurgery can bring about improvements. Diseases that lead to degeneration of the axons in the form of demyelination are relatively common.

As in multiple sclerosis (MS), it is usually autoimmune processes that lead to the gradual demyelination of the axons. The demarking of the axons leads to limitations in the nerve conduction speed and to other impairments, so that gradually serious effects in the coordination of movements and general performance losses set in.

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Typical & common nerve diseases

• Nerve pain
• Nerve inflammation
• Polyneuropathy
• epilepsy