Fast inward and outward current channels in a non-spiking neurone.

Although the crustacean coxal receptors and no-spiking, indirect pharmacological and electrophysiological evidence suggests that fast sodium channels may be present in their membrane. The properties of these channels are not known, but it has been suggested that they might be "incompletely differentiated", perhaps lacking "appropriate gating mechanisms", and/or "too sparsely distributed". The former hypothesis is not supported by the results of voltage-clamping experiments done on dendritic segments isolated from these mechanoreceptors. Instead, the results reported here provide direct evidence for a voltage-dependent fast inward current, sensitive to tetrodotoxin (TTX) and requiring external sodium (but not calcium). This current is shunted by a transient fast outward current, also voltage dependent, and it is suggested that this shunting may account, at least in part, for the non-spiking behaviour of the coxal receptors.