Axon

Axon


The axon is a thin elongated part of a neuron that extends from the soma to the terminal buttons. It transmits information down its length from the soma towards the terminal bottoms in the form of a action potential. The terminal buttons then synapse with other neurons, gland cells, or muscle cells.

The transitional zone between the neuronal perikaryon and the axon is the cone shaped axon hillock. In larger, multipolar neurons the axon hillock lacks Nissl substance, and it is in this region that cytoskeletal elements, the microtubules and neurofilaments, converge to become parallel as they enter the axon. The microtubules then form a fascicle in the initial segment of the axon, a feature which enables axons to be distinguished from dendrites, in which microtubules are more evenly distributed. The more distal portion of the axon may contain various organelles, but granular endoplasmic reticulum and polyribosomes are usually absent. The internal structure of the axon has recently been visualized as a three dimensional lattice: the longitudinally orientated neurofilaments­ments and microtubules are extensively cross linked to each other and to the plasma membrane by thin filaments. Similar bridges also connect membrane bound organelles with the components of the cytoskeletal and with each other .There is axonal transport of material from the perikaryon towards the periphery ( anterograde transport ) and to a lesser extent in the opposite direction ( retrograde transport ). Since the axon terminal, the most active site of neurotransmission­mission, can be far removed from the perikaryon in which proteins are synthesized, this axonal transport is vital for the proper functioning of the nerve cell. Axonal flow has a slow and a fast component, 0~2 8~0mm and 50 500 mm per day respectively. Fast axoplasmic flow carries organ­elles, including vesicles and mitochondria, and membrane bound substances like proteins and neurotransmitters materials which are essential for synaptic activity. The fast axonal flow is effected by microtubules, but the possibility that some membrane bound proteins are transported by the smooth endoplasmic reticulum Or another tubulo vesicular system cannot be excluded. Slow flow, in contrast, transports high molecular weight and soluble materials which are involved in the growth and maintenance of the axon. The structural basis of slow transport is controversial, but the movement of the cytoplasmic matrix itself may represent the prime force. Mechanisms of anterograde and retrograde transport are similar: both need metabolic energy, have similar ionic requirements and are sensitive to the same drugs. Both anterograde and retrograde transport can be blocked by low temperatures and suspended by colchicine and vinblastine, agents which disrupt microtubules. The factors which determine the direction of axonal transport remain conjectural: alternatives envisage either two, oppositely polar­ised transport systems or a single mechanism in which the direction of movement is determined by the nature of the material or organelle to be trans­ported. Investigation of the molecular mechanisms of axonal transport should contribute to our further understanding of this vital neuronal function.


 
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