AN ANALYSIS OF IMPULSE DISCHARGES FROM THE SPINDLE RECEPTOR

Abstract

Impulses discharged from single spindle receptors of frog''s sartorius muscle were recorded during stretch at various velocities and their patterns were analyzed on the basis of a visco-elastic model of the muscle spindle. The model comprises a spring (G2) and a dash-pot ([eta]) in parallel, and a spring (Gl) in series with them; Gi represents the elasticity of the sensory portion of the intrafusal muscle fibers, and G2 and n the elasticity and viscosity of the muscular portion, respectively. It is further assumed that the impulse frequency is related linearly to the tension along Gi. The spike frequency was measured as Fj before stretch, as Fs at the time when the muscle attained the final length and as Fj, long thereafter. In good agreement with the prediction from the model, both Fj and Fb were linearly related to the muscle length at the time of measuring them, while Fj measured as (Fs-FD) was merely determined by the stretch velocity. The whole time course of the actual spindle response also conformed to that of the ideal response of the spindle model. The parameters of the model were determined by the relations of Fi or FD versus muscle length and of F$ versus stretch velocity. The actual spindle response differed from the behavior of the model in 2 points 1st, the former showed an initial transient rise of the frequency which might be caused by pressure exerted upon the spindle by the neighboring extrafusal muscle fibers. Second, the decay of the spindle discharge at the cessation of stretch was accelerated with an increase of the stretch velocity, which might be due to thixotropic property of the intrafusal muscle fibers.