Rectangular fluctuations in potential of the afferent nerve terminal during depolarization in the frog muscle spindle.

Rectangular fluctuations in potential are observed during application of depolarizing currents from 1.5 to 6.3 nA across the air-gap on which the myelinated segment just outside the capsule in isolated muscle spindles is laid. The rapid decay in the potential is often triggered by after-hyperpolarization following afferent discharges. The fluctuations are not affected by TTX or in Na+-free solution but are reversibly blocked by 100 microM quinine. The results suggest that the fluctuations may be due to [Ca2+]i-activated Pk.