An axon is a special nerve process 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 capable of transporting certain molecules in both directions toward the cell soma and also in the opposite direction via a process called axonal mass transfer.
What is the axon?
The axon is the nerve process of a cell, also called a neurite, that transmits nerve impulses from the nerve cell to other nerve cells, or to organs or muscles. The impulses include some kind of command to secrete certain hormones or other substances, and in the case of muscle fibers, they cause contraction or relaxation. Axons can branch toward the end and form so-called telodendrons at the ends, button-like thickenings that play an important role in chemical signal transmission via synapses to the target organ. Each nerve cell usually has only a single axon, which can reach a length of less than 1 mm to more than 1 m as, for example, in axons extending from a nerve cell of one of the spinal plexuses to the muscles of the foot 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 scaffold and electrical insulation of the neurons from each other. According to recent findings, glial cells also perform essential tasks in axonal transport of substances and in storage, transmission, and processing of information in the brain.
Anatomy and structure
The axon originates from a characteristic protrusion of the nerve cell body, the axon hillock. As they progress, axons usually acquire a myelin sheath that serves to provide support and electrical insulation, as well as other important functions. It consists of a lipid-rich biomembrane of glial cells. In the case of central nervous system (CNS) axons, the biomembrane is formed from oligodendrocytes, a specialized type of glial cell, and in the case of the peripheral nervous system (PNS), Schwann’s cells perform this function. Typically, myelinated axons contain approximately 1 µm wide Ranvier cord rings at 0.2 to 2 mm intervals. They represent regular interruptions of the myelin sheath and conductance. Nerve impulses are transmitted at the Ranvier lacing rings via extremely fast Na ion transport. The impulses virtually “jump” from lacing ring to lacing ring. Axons contain a cytoskeleton for mechanical stabilization, which is composed of neurofilaments and neurotubules. The neurotubules also perform additional tasks in the transport of substances within the axon. The cytoplasm contained within the axon, called the axoplasm, contains hardly any ribosomes, which are necessary for protein synthesis, so axons depend on the supply of proteins from the nucleus and thus also on the relatively slow transport of substances within the axon.
Function and tasks
An important function and task of the axon is to transmit nervous impulses from the nucleus of the cell to the dendrites of another (interconnected) neuron or to target organs – usually muscles or glands. While the transmission of signals within the axon is electrical, the signal transmission at the end heads, the telodendrons, occurs chemically via neurotransmitters. The electrical action potential is “translated” into the release of neurotransmitters, which dock onto special receptors of the receptor and in turn cause a retranslation into an electrical action potential. In principle, a distinction is made between efferent and afferent axons. The “classical” axons are efferent transmission directions of the nerve signals, which are transmitted from the nerve cell to other neurons or to target organs. Axons, depending on which nervous system they belong to, may be subject to volition in their signal transmission (somatosensitive, somatomotor) or, in the case of the autonomic nervous system, may transmit unconscious, viscerosensitive signals to control the autonomic body systems. Another function of axons is axonal mass transport. It becomes necessary because axons cannot synthesize the proteins needed to maintain their tasks and functions “on site.” They depend on obtaining the 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. Axons have a slow and a fast axonal mass transport to fulfill this task. Slow solute transport works only in the direction away from the perikaryon toward the end of the axon. Fast solute transport functions in both directions; therefore, to a limited extent, substances can also be transported from axons to the cytoplasm of the neuron.
Diseases
Accidents that result in crushing or severing of axons are associated with partial or total loss of function of nerve conduction. This means, for example, that certain muscle areas are virtually paralyzed and are rapidly broken down by the body. Axons of the CNS lose their regenerative capacity after complete maturation, so that severed axons cannot regrow. Axons of the peripheral nervous system are capable of regeneration to a certain extent. If the myelin sheath is still intact but the nerve pathway 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, neurosurgical intervention can achieve improvements. Relatively common are diseases that lead to degeneration of the axons in the form of demyelination. Most often, as in multiple sclerosis (MS), these are autoimmune processes that lead to gradual demyelination of the axons. Demyelination of axons leads to limitations in nerve conduction velocity and other impairments, gradually resulting in serious effects in motor coordination and general performance impairments.
Typical and common nerve disorders
- Nerve pain
- Nerve inflammation
- Polyneuropathy
- Epilepsy
