- snip -
> - If it is a insulator like the insulation on a wire, why are not
> all axons thus insulated? We do not see it in networks like the
> retina, and it appears to be unnecessary for muscle fibres which
> also discharge electrically in the manner of neurons.
Simple reason: there is not enough space. To make the retina function with
maximal precision (resolution) it is packed with rods, cones, glia etc.
The same holds true for other areas such as the visual cortex, where cell
densities are extremely high to ensure max. processing power. Also, a lot
of neurons only project to their neighbour nerons and do not need myelin
to reduce impulse travel speed.
Muscles do not need myelin because their function is not to send impulses.
The 'discharge' you see in muscle cells is a side effect due to ion pumps
working and Ca2+ leaving intracellular compartments.
Christian Holscher, PhD
Trinity College Dublin
Dept. Pharmacol. & Therapeutics