Characteristics of the acetylcholine-operated channel at twitch and slow fibre neuromuscular junctions of the garter snake.

1. Synaptic responses to acetylcholine were recorded from garter snake (sp. Thamnophis) neuromuscular junctions with the voltage clamp. 2. In the following respects the responses were identical in twitch and slow fibres: exponential miniature end-plate current decay, reversal potential (approximately -5 mV), permeant ionic species (Na+, K+, Ca2+, but not Cl-), two component miniature end-plate current decay in the presence of procaine, insensitivity to tetrodotoxin, alpha-bungarotoxin and leiurus toxin. 3. The responses differed in several important ways: miniature end-plate current decay rate was only half as sensitive to voltage at slow fibre end-plates as at twitch; while all twitch fibre current fluctuation spectra were single component, about 60% of the slow fibre spectra were not. The latter could be fitted with two Lorentzian components. In all cases the mean end-plate current was proportional to total induced noise variance. 4. Two mechanisms which might account for the response differences were identified and tested in a fashion independent of specific molecular kinetics. The different responses do not arise from separate ion-selective channels for the permeant ions at slow fibre end-plates and seem unlikely to derive from separate populations of synaptic and extrasynaptic channels.