Spontaneous afferent discharges and spontaneous intrafusal contractions in isolated muscle spindles of the frog.

Abstract

Intrafusal recordings from isolated muscle spindles of the frog showed spontaneous afferent discharges and action potentials from intrafusal muscle fibres accompanied by contractions. Spontaneous discharges were also recorded from a spindle in situ on the surface of an extensor muscle with intact nerve and vascular supply. The afferent discharge were probably recorded from intrafusal myelinated nerve branches. Spontaneous intrafusal contractions (0.2–0.4/sec) evoked single and repetitive afferent discharges. Most of the spontaneous afferent discharges (minimum rate: 5–6/sec) occurred independently of preceding action potentials from intrafusal muscle fibres and did not disappear when intrafusal contractions were abolished by d-Tubocurarine or injury. The rate of afferent discharges increased with the initial elongation and with the temperature, whereas the rate of spontaneous intrafusal contractions remained unchanged. The transequatorial conduction rate of the spontaneous discharges from single intrafusal muscle fibres was 0.24–0.38 m/sec (18–20° C) increasing with the fibre diameter. Conduction failed when the amplitude of the action potential was below a critical size. In the pre-paralytical state high concentrations of d-Tubocurarine prolonged the time course of the spontaneous intrafusal contractions and initiated bursts of afferent discharges.