Axon

synonym

Axial cylinder, neurite

General

The term axon refers to the tubular extension of a nerve cell, which transmits impulses from the nerve cell body into the distance.

Inside the axon there is a fluid, the axoplasm, which contains the cell contents (cytoplasm) corresponds to other cells. Cell organelles such as mitochondria or vesicles are located here, ribosomes do not traditionally occur here.

Illustration of a nerve cell

Nerve cell -
Neuron

1. Dendrites
2. Synapse
   (axodendritic)
3. Cell nucleus -
   Nucleolus
4. Cell bodies -
   Nucleus
5. Axon mounds
6. Myelin sheath
7. Ranvier lace-up
8. Swan cells
9. Axon terminals
10. Synapse
    (axoaxonal)
    A - multipolar neuron
    B - pseudounipolar neuron
    C - bipolar neuron
    a - Soma
    b - axon
    c - synapses

You can find an overview of all Dr-Gumpert images at: medical illustrations

The membrane around the axon is called the axolemm and the structure consisting of these two components is called Nerve fiber. Most cells only own a Axon, but there are exceptions with multiple axons and even cells that have no axon at all (such as the Amacrine Cells of the Retina).

Depending on Nerve cell can the length of an axon from under one millimeter to over one meter (for example those nerves that are from Spinal cord pull up to the muscles of the feet). The diameter of an axon is usually about 0.05 to 20 µm.

construction

An axon takes his origin directly below the Nerve cell body (Soma). There is the so-called Axon moundsthat is always uncovered. This initial segment is followed by the main route, which is either free or from mark- or. Myelin sheaths is surrounded (see below).

Usually they are axons unbranchedIn some of them, however, there are branches in its course, which are called collaterals. At the end of the axon there is typically a tree-like one Branch. This creates a multitude of button-like extensions (Telodendrons), either in direct contact with another Nerve cell stand or have a connection to a muscle or gland cell that they target electrical Transmit impulse.

Education and regeneration

It starts with humans growth of axons already in the embryonic period. This is necessary for proper growth Growth factor NGFwhich is produced by the future target structures of the axon.
The growth cone receives this chemical signal, prompting the axon to move in the appropriate direction extended. If the axon does not succeed in reaching its target structure, it will eventually go through programmed cell death (Apoptosis).
In contrast to this early development phase, if an axon is severed, it is mature CNS no regeneration possible, a scar from glial cells.

in the PNS (peripheral nervous system) on the other hand, depending on the type of injury, is to a certain extent one regeneration possible, whereby the newly forming axon with a maximum speed of about 2 to 3 mm per day grows. So the process can take some time. Sometimes, however, especially with extensive axon damage, is also here no cure possible.

Classification

Axons can be based on various Factors organize. On the one hand, one differentiates:

• myelinated from
• not myelinated Axons.

A myelin layer is made up of specialized Cells that practically wrap around and become the axon insulation contribute, whereby an excitation can be passed on more quickly.

This myelination is not found in all nerve fibers, but mainly in those that have one high line speed need.

in the Central nervous system (CNS, so brain and Spinal cord) are the names of the cells that make up the myelin sheath, Oligodendrocytes,
in the Peripheral nervous system (PNS), however, they are called Schwann cells.

Another way to classify axons or nerve fibers is based on them Line speed. The spectrum ranges from

• C fibers with a line speed of less than 2 m / s up to
• A? Fibersthat have a line speed of up to 120 m / s to reach.

Depending on whether the nerve fibers receive information from the CNS path or to the Leading the CNS, one differentiates between efferents and afferents Fibers.

In addition, one makes the difference whether nerve fibers for conscious, somatic nervous system or for unconscious, visceral nervous system and whether they are for Movements (motorized) or Sensations (sensitive) are responsible.

tasks

An axon met two important ones Tasks:

• First, it is there for the things that have arisen in the nerve cell body electrical impulses on to the next nerve cell or to the target structure (muscle or gland cell) conduct.
• In addition, some substances pass along certain structures through the axon transported. This as axonal transport designated process can take place in both directions.

Diseases

There are several diseases that can affect axons. The most important here is the multiple sclerosis (MS).
In this disease it happens because of autoimmune Processes of unknown cause to a Demyelination (i.e. a loss of myelin sheaths). This decreases the Line speed the affected nerve fiber is considerable and the structures controlled by this nerve fiber are in their function limited or delayed.
The disease typically occurs in flares and is still there today not curable.