Friday, October 21, 2011

Axonal Transport

The neurotransmitter substances or their synthesizing enzymes are produced in the perikaryon (soma) of the nerve cell and must be transported to the axon terminal. The microtubules of the neuron (neurotubules) play a key role in the axonal transport mechanism. If they are destroyed by applying the mitotic poison colchicine, the intra-axonal transport stops. This rapid transport of material is energy-dependent and takes place in vesicles that are moved along the microtubules by motor proteins. The retrograde transport (in the direction of the cell body and toward the minus end of the microtubules) is mediated by dynein, while the anterograde transport (in the direction of the axon terminal and toward the plus end of the microtubules) is mediated by kinesin.

The transporting vesicles are endowed with several motor proteins, the ATP-binding heads of which interact with the surface of the microtubule in an alternating and reversible fashion. This results in ATP being hydrolyzed, and the released energy is converted into molecular movement that causes the vesicles to roll along the microtubules in the target direction. The velocity of the rapid intra-axonal transport has been calculated at 200–400mm per day. Proteins, viruses, and toxins reach the perikaryon by retrograde transport from the axon terminals. In addition to the rapid intra-axonal transport, there is also a continuous flow of axoplasm which is much slower, namely, 1–5mm per day. It can be demonstrated by ligating a single axon; proximal to the constricted site, the axoplasm is held back and the axon shows swelling. The anterograde and retrograde transport mechanisms are used in neuroanatomy to study connecting tracts.