ACTIVITY OF INTRAFUSAL MUSCLE FIBRES IN MUSCLE SPINDLES OF XENOPUS LAEVIS.

Abstract

AbstractObservations were made of the mechanical and electrical activity of intrafusal muscle fibres in semi‐isolated muscle spindles from Xenopus laevis. The spindle capsule, which formed a part of the recording system, was studied and was found to be a a laminated, ohmic, structure with a characteristic length of approximately 0.7 mm. Electrical and histological evidence combined with observations of the movements of the intrafusal muscles led to the conclusion that the intrafusal muscle bundle consisted of either two or three muscle fibres and that these muscle fibres could be divided into large and small diameter groups. The large diameter muscle fibres showed a fast contraction in response to single stimuli, and were supplied by large diameter motor nerves. The small diameter muscle fibres showed a very slow contraction in response to single stimuli and were supplied by small diameter motor nerves. Evidence was obtained that the innervation of the large and the small fibres did not overlap. The electrical event in both types of muscle fibre was conducted transcapsularly except on repetitive stimulation, and external current records indicated that the membrane potential change in the small muscle fibres was considerably longer than in the large fibres. Transcapsular conduction failed in the large fibres at a a stimulation rate of approximately 30/sec, and in the small fibres at 5/sec. This conduction failure was associated with a fusion of contraction on the innervated side of the preparation. Evidence was obtained that a region of low safety factor exists in the large intrafusal muscle fibres in the approximate region of the sensory contacts.