Axonal Transport and Nerve Regeneration
After nerve section, clearly the retrograde transfer of these chemical signals stops, and it is the lack of the normal signal that causes the processes that follow nerve transection: chromatolysis, and protein synthesis.
Synthesis of structural proteins and transport of the essential materials for the formation of the growth cone and the extension of axons is an essential part of the process of nerve regeneration.
In the process some neurones undergo apoptosis - programmed cell death, so the number of neurones innervating a muscle is usually reduced; this is one factor in leading to the large motor units after regeneration of motoneurones.
When a regenerating axon again establishes contact with the post-synaptic cell (say skeletal muscle) retrograde chemical signals passed along the axon confirm that the synapses are again functional.
Tetanus: the Infection.
The pathology of the disease tetanus, caused by the release of tetanospasmin (also called tetanus toxin) by the anaerobic bacterium Clostridium tetani within the body depends upon the retrograde transport of tetanospasmin into the nervous system.
The toxin is transported retrogradely along motoneurone axons to the motoneurone cell body.
It then crosses synapses into interneurones that synapse on motoneurones and block the release of inhibitory transmitters, such as GABA and Glycine. This lack of inhbitory influence on motoneurones causes them to fire action potentials continuously and generate the characteristic severe muscle spasms that can be fatal.
The muscles first affected are those with short nerves, so before the generalised convulsions occur, muscles close to their motoneurone cell bodies give the early clinical signs of the disease.
Risus sardonicus is an early sign in which the face has a fixed smile and the patient has difficulty opening his mouth (lockjaw). These are due to overactivity in the facial (VII) and trigeminal (V) nerves, related to the loss of inhibitory influences of Glycine and GABA on V and VII nerve motoneurones.