Electrical threshold of the sensory nerve terminal of the frog muscle spindle: a role of spindle potential for generating afferent impulses.

The threshold at the terminal node in the capsule of frog muscle spindle, where afferent impulses are initiated, was calculated to be 2 nA on average, from data of intracellularly recorded threshold depolarization of the node against antidromic stimulation and of a mean attenuation ratio of the stimulation current from the stimulation site to the recording site. Using a similar procedure, the absolute value of orthodromically generating current at the node during static stretch of the spindle from the in situ length was calculated to be approximately 0.9 nA. It thus is supposed that at the terminal node the afferent impulses may be triggered by abortive spikes of 1.1 nA or more in amplitude, which are generated along non-myelinated filaments, being superimposed on the generating current.