Axon

The axon (rarely the axon; . AltGr ὁ ἄξων ho axon, axis '), also called axon or axis cylinder, is an often long tube-like nerve cell process, an axon which runs in a sheath of glial cells and called together with the sheath as a nerve fiber will. Lateral branches of the axon are called the collateral and how the terminal axon can aufzweigen in several Endästchen.

Most neurons have a single axon. There are also neurons, which do not have an axon, for example, various amacrine cells of retina.

• 2.1 Axonal Transport

• 4.1 axotomy and degeneration
• 4.2 regeneration
• 4.3 Demyelinating Diseases

Anatomy of the axon

The axon is divided into the following sections:

• Origin cone / axon hillock

The axon begins at the pyramidal axon hillock, which connects directly to the perikaryon; this area is free of Nissl substance.

• Initial segment

The short initial segment of the axon is always without shell. Since the excitation threshold of the plasmalemma of the initial segment is extremely low, here takes the propagation of excitation its output when an action potential is initiated.

• Main course route

The main flow-out of the axon may have branches that are referred to as collaterals.

• Terminal arborization

At the end of the axon is sometimes branched tree-like, called telodendron. By this Endbäumchen can a nerve cell with several other neurons or effectors are connected. The Telodendrien end in a variety of end portions, the presynaptic terminals (also called Axonterminale, end bulbs or boutons ), each representing the presynaptic part of the synapse.

There are axons with a length of less than one millimeter; but they can be longer than a meter - so when people lying in the spinal cord motor neurons that innervate the foot muscles. Their diameter is between 0.05 and 20 microns and remains relatively constant over the entire length.

The biomembrane that surrounds the axon of the nerve cell, called axolemma. The unit of Axon and the adjacent shell structures of glial cells ( basal lamina and increased in the peripheral nerves ) are called nerve fiber. The cytoplasm is called axoplasm.

In the cytoplasm of an axon to find mitochondria and vesicles; Apart from a few exceptions, there are no ribosomes in the axon still rough endoplasmic reticulum. Maintaining and function of axons are therefore dependent on the protein synthesis in the cell body. When it comes to die -cutting, see section axotomy.

As the dendrites contain axons and neurofilaments neurotubules. An axonal microtubule differs from dendritic firstly by the associated proteins and on the other by its orientation. All axonal microtubules are oriented with their plus end ( the growing end ) to the axon towards. In a dendritic the plus - end, both in the extension end and are located in the cell body.

Neural development

The axon growth starts directly with the aggregation. Both growing axons and dendrites have a growth cone with finger- like extensions ( filopodia ). These spurs " grope " for directions.

• Chemo affinity hypothesis

This hypothesis is based on chemothrophen factors that are emitted by the target cells. The phenomenon was first detected in the optic nerve ( optic nerve ) of the frog.

• Fasciculation

This is based on signals that are emitted from axons and ensures that renewable axons have an affinity for the same route.

Myelination

We distinguish myelinated and unmyelinated axons. The myelin sheath of myelinated axons in the central nervous system (CNS) of the oligodendrocytes and in the peripheral nervous system (PNS ) formed by the Schwann cells. It enables saltatory conduction of the action potential, which requires significantly less energy, a thinner axon allows ( place and material savings) and faster than that of continuous routing.

Became famous for the study of Riesenaxone of the octopus by Alan Lloyd Hodgkin, Sir John Carew Eccles and Andrew Fielding Huxley ( 1963 received the Nobel Prize for their work) in the 1960s. Such axon is typically 100 - to 1000 - times thicker than that of mammals and reached a diameter of up to 1 mm. This enormous diameter only allows squid rapid conduction, as they have in contrast to vertebrates no myelinated axons. Due to the larger Axonquerschnitt the longitudinal resistance ( internal resistance ) of the axon is low, so that the electrotonic current flow from excited to unexcited can be done faster fiber areas.

Depending on the line speed and thickness of different types of nerve fibers were classified ( See Division after line speed according to Erlanger / Gasser and nerve conduction velocity ).

Tasks

The axon passes away electrical nerve impulses from the cell body ( perikaryon or soma). The distribution of nerve cell to nerve cell or to the effector organ is, however, usually not electrically but chemically. At the end button neurotransmitters are released as chemical messengers that bind to a receptor, also affect the membrane permeability to certain ions and thus cause a voltage change in the associated membrane region of the downstream cell.

Following the direction of the excitation line is divided into afferent and efferent axons. Based on the nervous system as a whole conduct afferent axons stimulation of the sense organs to the CNS through. We distinguish these afferents in somatic (of the body surface ) and visceral ( from the bowels ). Efferent neurites, however, conduct impulses from the CNS to peripheral effectors (eg muscles or glands); also be differentiated somatic (of motor neurons to skeletal muscle, for example, the foot) and visceral efferent fibers ( smooth muscle and cardiac muscle and glands ).

Axonal transport

In addition to the transmission of electric signals takes place, a mass transport in the axon. We distinguish a slow axonal transport, the runs in only one direction, from the cell body ( soma) to the peripheral end of the axon, and fast axonal transport, which takes place in both directions - both anterograde as well as retrograde, from the terminal axon to the soma.

History

Following the discovery that nerve despite similar appearance are no tendons, muscles and bones connect ( altgr. νεῦρον neuron ' tendon, tendon '), but a related form that pervades the whole body, various models have been developed for their tasks. So mechanistic as that of René Descartes in 1632 in his "Treatise on the people " ( Traité de l' homme, posthumously De homine 1662), after which their fibers produce heat by mechanical power train movements would be able, like a machine. The further developed in the 17th century, light microscopes allowed increasingly finer insights into the structure of the tissue, and the discovery of galvanic currents in the late 18th century made ​​other ideas of its functioning possible.

However, studies using intracellular recordings from single neurons in the nervous system could be carried out only in the 1930s by K. Cole and H. Curtis. Previously peripheral nerves were studied in the considered gebündelteten nerve fibers closer and its course traced. The German anatomist Otto Deiters was in 1860, " the transition of the axis cylinder CONTROLLER nature in a ganglion cell extension " already known; he is credited as the first to have distinguished the single " main cell process " of another " protoplasmic processes ", for which the Swiss anatomist Wilhelm His later the term " dendrite " coined. The Swiss Albert of Kölliker and the German Robert Remak were the first who identified and described the initial segment of the axon.

Disease and injury

Axotomy and degeneration

Under axotomy refers to the transection of an axon. This can happen due to an accident or is part of controlled animal experiments. The Controlled transection of axons led to the identification of two types of neuronal degeneration ( see also Neural plasticity, apoptosis, necrosis).

• Anterograde degeneration

This degeneration of the cut distant (distal) portion of the affected neuron, ie, the terminal axon and many collaterals occurs rapidly, since the distal portion is dependent on the metabolic supply from the soma.

• Retrograde Degeneration

If the severed point is located near the cell body, it can also lead to the degeneration of the proximal ( proximal ) segment. This is slower and manifests itself in two to three days due to degenerative or regenerative changes of the neuron. The course depends crucially on whether the neuron can resume the synaptic contact with a target cell.

In the worst case, even adjacent neurons can degenerate. Depending on the location of the then additionally affected neurons one talks of anterograde or retrograde degeneration transneuraler.

Regeneration

The original capacity of the targeted axon growth during development of the nervous system is lost in the mature human brain. Neuroregeneration found in the CNS that is usually not take place. Dead neurons (mostly astrocytes ) are replaced by glial cells and there are so-called Glianarben.

The neuro- regeneration in the PNS usually begins two to three days after injury of the axon and essentially depends on the type of injury of the neuron from:

• If the myelin sheath remains intact ( for example, after contusion ), the axon can it grow back to the original destination at a speed of about 2-3 mm per day ( complete functional recovery ).
• Are the severed ends still close together, so a regrowth in the myelin sheaths is also possible, but then, not infrequently, at the wrong destination ( difficult functional recovery )
• Are the severed ends far apart and there is a large-scale damage before, so is allermeistens without surgery no functional regeneration possible and even after this, in many cases only a partial.

Demyelinating diseases

Demyelinating diseases ( demyelinating diseases ) cause the axons in the CNS lose some of their myelin sheath and thus myelin sheath sections are destroyed. That is, for example, in multiple sclerosis (MS ), the Baló 's disease, acute disseminated encephalomyelitis (ADEM ) or neuromyelitis optica ( Devic's syndrome) the case.