Under axonal transport refers to the transport of substances in the axon of a nerve cell . A distinction is made between slow axonal transport, which only runs in one direction, from the cell body ( soma ) to the peripheral end of the axon, and fast axonal transport, which takes place in both directions.
Tasks of the transport system in the nerve cell
Almost the entire synthesis of a nerve cell takes place in the soma . The source cell organelle of the synthesis products is usually the endoplasmic reticulum . The synthesis products built up there are channeled into the axonal transport mechanism in the Golgi apparatus . The Golgi apparatus is conveniently located at the origin of the microtubules , at the origin of the axon.
Slow axonal transport
The slow axonal transport affects proteins synthesized in the cell body , including the structural proteins of the cytoskeleton such as tubulin and actin . Closer investigations showed that it can in turn be divided into two components with slightly different speeds. The slower component SCa ( slow component a ) moves in axons of vertebrates only 0.2 to 1 mm per day, with the proteins involved staying close together for weeks. It is very likely that this is a shift in intact microtubules and neurofilaments and numerous proteins associated with them (microtubule-associated proteins). The slightly faster component SCb consists of actin and numerous other proteins, including the enzymes involved in glycolysis and other processes involved in intermediate metabolism . It is unclear here whether this is also a displacement of entire actin microfilaments or smaller aggregates.
Fast axonal transport
Fast axonal transport mainly involves vesicles that are moved along the microtubules by motor proteins ( kinesin , dynein ) . Speeds of 25 to 40 centimeters per day can be achieved. The transport can take place both downstream in the direction of the synapse and in the opposite direction from the synapse in the direction of the soma.
Membrane material and substances specific for secretion (such as neurotransmitters ) are transported from the cell body to the synapse (anterograde, downstream) . This happens via granules or vesicles that are attached to the motor protein kinesin .
In the opposite direction (retrograde, upstream) the speed is somewhat slower; Here end products of the metabolism are transported back to the soma, as well as membrane material intended for the breakdown and remodeling and the nerve growth factor that is necessary for the survival of the nerve cell. The retrograde transport occurs via vesicles attached to the motor protein dynein . However, foreign substances can also be transported. Herpes simplex and polioviruses get into the brain through retrograde transport . The tetanus toxin is also transported from the external wound via axons to the inhibiting Renshaw cells of the spinal cord, where it suppresses the release of transmitters, which leads to constant contraction of the skeletal muscles, the tetanus.
- George J. Siegel, Basic Neurochemistry , 1999 ( Internet ).