Different spike mechanisms in axon and soma of molluscan neurone

STUDIES with a few invertebrate preparations have suggested different ionic mechanisms for spike generation in the soma and axon of the same neurone. Iwasaki and Satow1 found that somatic spikes in the crayfish eyestalk X organ had a mixed Na–Ca dependency, while the extracellular axon spikes disappeared in Na-free or tetrodotoxin (TTX)-containing solutions. Wald2 recorded attenuated axon spikes in hyperpolarised somata of snail ganglion cells and showed that they varied in amplitude with external sodium concentration but, curiously, were not blocked by TTX. In voltage-clamp experiments with the left pleural giant cell in Aplysia, Kado3 found that early inward currents in the axon-hillock region were much more sensitive to TTX than early inward currents in the opposite end of the soma, suggesting a greater Na-dependency of the axon spike.