Corpus: Axon

An axon or neurite is the extension of a nerve cell (neuron) that conducts electrical nerve impulses away from the cell body (soma). The unit consisting of the axon and the adjacent sheath structures (axolemma) is called a nerve fibre.

The axon begins at the so-called axon hillock as a protrusion of the nerve cell body. This section is also called the axon initial segment (AIS). The length of the axon depends on the localisation and function of the nerve cell - it varies from fractions of a millimetre to lengths of over a metre. At its end, the axon is usually branched like a tree and ends in a large number of button-shaped thickened endings (telodendrons). They form connections to other nerve cells (synapses) or transmit the electrical excitation to muscle or gland cells.

...according to sheathing[Bearbeiten]

A distinction is made between medullary and non-medullary nerve fibres, which are surrounded by a myelin sheath.

Medullary nerve fibres[Bearbeiten]

In medullary nerve fibres, the axon is surrounded by the medullary or myelin sheath, a structural element of the neuroglia, which is formed by the oligodendrocytes in the central nervous system (CNS) and by Schwann cells in the peripheral nervous system (PNS). When the axon sinks into its sheath cell, a cell membrane duplication is formed, which is known as a mesaxon.

The myelin sheath serves to insulate the axon from its surroundings and thus improve its conduction. The degree of myelination is determined by the function of the nerve cell and is matched to the axon diameter. It can be differentiated according to the Erlanger and Gasser classification.

The thickness of the myelin layer is modified by the glial cells through the number of windings around the axon. Thicker myelin sheaths can thus have up to one hundred lamellar layers.

The myelin sheath allows electrical signals to be transmitted quickly over longer distances without significant signal attenuation. Gaps in the myelin sheath that appear at certain intervals, known as Ranvier rings, play an important role in this process. They enable saltatory excitation conduction, which makes significantly higher conduction speeds possible.

Marrowless nerve fibres[Bearbeiten]

Medullary nerve fibres in the CNS have no myelin sheath. In the PNS, they are embedded in the cytoplasm of Schwann cells. They are less well insulated from the environment and therefore have a lower conduction velocity.

...according to fibre quality[Bearbeiten]

Nerve fibres can be part of the conscious, somatic nervous system or they serve the unconscious, visceral or vegetative nervous system. They can be motorised, i.e. cause a movement, or sensitive, i.e. perceive a sensation. Accordingly, we speak of somatosensory or viscerosensory or somatomotor or visceromotor.

In addition, a distinction is made between fibre qualities that only occur in the cranial nerves (sensory perceptions, supply of the gill arch muscles) and those that occur in the rest of the peripheral nervous system. The former are referred to as "special", the latter as "general".

Motor fibres[Bearbeiten]

• somatomotor fibres (GSE, general somatic efferent)
• general visceromotor fibres (GVE, general visceral efferent, parasympathetic)
• special visceromotor fibres (SVE, special visceral efferent, branchiomotor)

Sensitive fibres[Bearbeiten]

• general somatosensory fibres (GSA, general somatic afferent)
• special somatosensory fibres (SSA, special somatic afferent, sensory)
• general viscerosensitive fibres (GVA, general visceral afferent)
• special viscerosensitive fibres (SVA, special visceral afferent, sensory)

...according to direction of conduction[Bearbeiten]

• afferent: conducts the impulse to the CNS, synonymous with "sensitive" or "sensory"
• efferent: conducts the impulse away from the CNS, synonymous with "motor"

...according to conduction velocity[Bearbeiten]

Axons can be divided into different classes according to their conduction velocity:

...by afference[Bearbeiten]

Growth[Bearbeiten]

The growth of axons during the embryonic or foetal period is mediated by the nerve growth factor NGF, which is produced by the target structures of the axon. At the tip of the axon there is a so-called growth cone, which receives the NGF signals and is filled with lamellipodia and fillipodia, which ensure the further advancement of the axon.

Nerve cells of axons that do not find a connection to a target structure die through apoptosis. Successfully connecting axons maintain the vitality of the nerve cell via the transcription factor CREB.

Axonal transport[Bearbeiten]

Movement of substances within the sometimes very long axons is made possible by special cellular processes in the axoplasm, which are summarised under the term "axonal transport".

• Podcast image source: © canacrtrk / Pexels

Article on CREB