axial cyinder, Neurit
The term axon is used to describe the tubular extension of a nerve cell that transmits impulses originating from the nerve cell body to the farthest reaches. Within the axon is a fluid, the axoplasm, which corresponds to the cell content (cytoplasm) of other cells. Here are cell organelles such as mitochondria or vesicles, ribosomes are not classically found here.
The membrane around the axon is called axolem and the structure consisting of these two components is called nerve fibre. Most cells have only one axon, but there are exceptions with several axons and even cells that have no axon at all (such as the amacrine cells of the retina). Depending on the nerve cell, the length of an axon can vary from less than one millimeter to more than one meter (for example, those nerves that run from the spinal cord to the foot muscles). The diameter of an axon is usually about 0.05 to 20 μm.
An axon takes its origin directly below the nerve cell body (soma). There is the so-called axon mound, which is always uncovered. This initial segment is followed by the main segment, which is either exposed or surrounded by medullary or myelin sheaths (see below).
Normally, axons are unbranched, but in some cases there are also branches in its course, which are called collaterals. At the end of the axon there is typically a tree-like branching. This creates a multitude of button-like extensions (telodendrons) that are either in direct contact with another nerve cell or connected to a muscle or glandular cell to which they transmit an electrical impulse.
Education and regeneration
In humans, the growth of axons already begins in the embryonic period. The growth factor NGF, which is produced by the future target structures of the axon, is necessary for proper growth. The growth cone receives this chemical signal, whereupon the axon extends in the appropriate direction.
If the axon fails to reach its target structure, it will eventually perish through programmed cell death (apoptosis). If an axon is severed, in contrast to this early developmental phase, no regeneration is possible in the mature CNS. In the PNS (peripheral nervous system), however, regeneration is possible to a certain extent, depending on the type of injury, whereby the newly forming axon grows at a maximum speed of about 2 to 3 mm per day. The process can therefore take some time. Sometimes, however, especially in the case of extensive axon damage, healing is not possible here either.
Axons can be classified according to various factors. Firstly, a distinction is made between the following: A myelin layer consists of specialised cells that practically wrap around the axon and contribute to its isolation, thereby enabling excitation to be transmitted more quickly. However, this myelination is not found in all nerve fibres, but mainly in those that require a high conduction speed.
In the central nervous system (CNS, i.e. brain and spinal cord), the cells that form the myelin or myelin sheath are called oligodendrocytes, whereas in the peripheral nervous system (PNS) they are called Schwann cells. Another way to classify axons or nerve fibres is by their conduction velocity. Depending on whether the nerve fibres conduct information away from or to the CNS, a distinction is made between efferent and afferent fibres.
In addition, a distinction is made as to whether nerve fibres belong to the conscious, somatic nervous system or the unconscious, visceral nervous system and whether they are responsible for movement (motor) or sensation (sensitive). – myelinated by
- Non-myelinated axons. – C-fibres with a line speed of less than 2 m/s up to
- A? fibers, which achieve a line speed of up to 120 m/s.