Professor David Burke will present Action Potential Generation and Intermittent Conduction Block in Myelinated Axons.

Intricate mechanisms have evolved such that axons can conduct impulse trains securely, but the fine details differ for sensory and motor axons. It is a theme of this talk 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. We have identified three differences between sensory and motor axons. (i) The resting membrane potential of sensory axons seems to be about 4 mV more depolarised than for motor axons. (ii) There is a greater persistent Na⁺ current on sensory axons than motor, probably due to the difference in resting membrane potential. (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. These differences render sensory axons more excitable than motor, and more prone to ectopic activity. However, they also offer some protection against conduction block. Fluctuation in the ability to conduct can produce a fluctuating deficit if a sufficient number of axons is affected.