S46 Recent insights into the biophysical differences between human sensory and motor axons

Abstract

Axons serve only one role, to transmit an impulse from one to the other with minimal expenditure of energy. In doing so sensory and motor axons discharge at different rates and in different patterns, and the most energy-efficient solution will impose different properties on different axons. It is a theme of many of our studies that motor axons are more likely to block than sensory for the same insult and that, in critically impaired axons, normally innocuous manoeuvres can precipitate conduction block. There are three relevant differences between sensory and motor axons. (i) The resting membrane potential of sensory axons seems to be about 4 mV more depolarised than that for motor axons. (ii) There is a greater persistent Na+ current on sensory axons than motor. This can be adequately explained by the difference in resting membrane potential, such that there is no need to postulate a greater expression of Na+ channels behaving in a persistent manner. (iii) There is greater activity of HCN channels (hyperpolarization-activated cyclic nucleotide-gated channels) on sensory axons than motor, such that activity causes greater hyperpolarization for motor axons than sensory for the same discharge rate. These three differences render sensory axons more excitable than motor, with a lower threshold for activation, as all electromyographers can confirm. Being more excitable, ectopic activity is more easily triggered in them, but the differences also offer some protection against conduction block.