Ion channels in human axons

Some puzzling differences between rat and human myelinated nerve fibres emerged from work in this department some years ago. Rat nerve was being used to investigate the origin of the spontaneous activity which seems to underlie paraesthesiae, a common and disturbing symptom in neuropathies. It proved to be surprisingly difficult to induce rat nerve to become spontaneously active, although this is easily done in human nerve by manoeuvres such as ischaemia. A related finding is that rat nerve fibres accommodate more than those of humans to long stimuli. The electrotonic behaviour of nerve fibres differs between the two species in ways which suggest different populations of K + channels (Bostock & Baker, 1988). To try to understand these differences, single-channel and multi-channel patch clamping were applied to human axons, followed by voltage clamping of the node of Ranvier. Much of this work was done in collaboration with groups in Giessen, Hamburg and Munich. The results show that the ion channels in human axons are very similar to those in rat axons, as are the action potential and membrane currents in the node. The species differences are still not understood, but may result from differences in ion channel distribution or density, probably in the paranode or internode rather than the node. This chapter will summarize the information about ion channels in human peripheral myelinated axons which arose from this work, and relate this to descriptions of channels in other species.