Active and Passive Factors in Muscle Spindle Excitation

Abstract

IT is accepted that the discharge of muscle spindle receptors is reduced or abolished during extrafusal contraction caused by unloading^1,2. Acceleration of spindle receptor discharge which may occur during the early phase of muscle contraction on stimulation of the muscle nerve, ventral roots or supraspinal structures, however, has been considered to result not from contraction of intrafusal muscle fibres (active factor) but from pull of spindles induced by extrafusal contraction (passive factor)^3–5. Stimulation of Deiters nucleus in the cat has been shown to result in simultaneous excitation of muscle spindle receptors and extrafusal muscle fibres in the gastrocnemius muscle⁶. The question is whether this acceleration of spindle discharge is caused by intrafusal contraction or mechanical deformations caused by extrafusal contraction. The experiments reported here concern the dissociation of passive from active factors by the use of a new finding, namely, the much longer binding of gallamine to intrafusal than extrafusal neuromuscular junctions.