Membrane properties of the stretch receptor neurones of crayfish with particular reference to mechanisms of sensory adaptation.

1. Membrane properties of neurones of the two morphologically different types of stretch receptor of crayfish, the slowly adapting (RM(1)) and the rapidly adapting (RM(2)) receptors, were investigated with two microelectrodes inserted into the same neurone.2. The action potential was usually larger in the slowly adapting than in the rapidly adapting neurone. But the distributions of the height were not sharply delimited, and there was an overlap from the two groups of neurone.3. There were no marked differences in the current-voltage relationship between the two types.4. Under voltage clamp, depolarizations evoked a large delayed outward current, which slowly diminished during maintained depolarization (K-inactivation). Under a moderate depolarization, development of the K-permeability increase was very slow.5. When stimulated by intracellularly applied constant currents, the slowly adapting neurone always adapted slowly, and gave rise to long-lasting trains of spikes, whereas the rapidly adapting neurone never produced maintained repetitive discharges.6. The same marked differences in the adaptation behaviour of spike discharge between the two types were also observed when the neurones were stimulated by constant currents applied through external electrodes.7. When the stimulating point was shifted along the axon of the slowly adapting neurones, the ability to produce long-lasting repetitive discharges was found to be confined to the axonal region near the soma, where the diameter was very small, and where impulses were first initiated.8. Possible ionic mechanisms of the adaptation of the spike generating membrane were discussed. The importance of slowly occurring changes in the Na- and K-permeabilities and changes in the electromotive force of the membrane due to electrogenic pump was emphasized.