Position and velocity sensitivity of muscle spindles in the cat. II. Dynamic fusimotor single-fibre activation of primary endings.

Abstract

AbstractImpulse frequency responses of cat primary muscle spindle endings to ‘triangular’ length changes were recorded during stimulation of single dynamic fusimotor γ‐fibres at a constant rate. Endings in the ankle extensor and flexor muscles were studied. The dynamic fusimotor effects consisted of an unchanged or decreased position sensitivity of the endings and an increased velocity sensitivity to length increment. The effects were quantitatively similar in primary endings of all the muscle studied. In the velocity response, which was subdivided into ‘quick’ and ‘slow’ components, the ‘slow’ velocity response was increased much more by the stimulation than the ‘quick’ velocity response. The velocity sensitivity to length decrement could usually not be exactly determined. In the few cases in which it could be assessed, it seemed to be lower than the velocity sensitivity to length increment. When relatively high rates of stimulation and high velocities of length change were applied to the endings, a velocity dependent ‘saturation’ of the response appeared. In the discussion interest is focused on spindle mechanisms responsible for generating the observed response. It is suggested that the response is derived from intrafusal components with the mechanical and electrical characteristics of amphibian slow muscle fibres.