Calcium spike induced by electrical stimulation to the sensory nerve terminal of the frog muscle spindle

A calcium spike in the sensory nerve terminal of the frog muscle spindle could be elicited by electrical pulses, which were given across an air-gap on which the first myelinated segment of the parent axon outside the spindle capsule was bridged, after the sodium spike had been blocked by treatment with tetrodotoxin (TTX). The threshold current was 1.1-2.4 nA higher than that for the sodium spike, suggesting that the calcium channels distribute distally to the site of afferent impulse initiation, or that the density of calcium channels on the encoding site may be less than that along portions distal to the site.