An analysis of impulse discharges from the spindle receptor.

Abstract

1. Impulses discharged from single spindle receptors of frog's sartorius muscle were recorded during stretch at various velocities and their patterns wereanalysedon the basis of a visco-elastic model of the muscle spindle.2. The model comprises a spring (G2) and a dash-pot (η) in parallel, and a spring (G1) in series with them ; G1 represents the elasticity of the sensory portionof the intrafusal muscle fibres, and G2 and η the elasticity and vis-cosity of the muscular portion, respectively. It is further assumed that the impulse frequency is related linearly to the tension along G1.3. The spike frequency was measured as Fi before stretch, as Fs at the time when the muscle attained the final length and as Fb long thereafter. In good agreement with the prediction from the model, both Fi and Fb were linearly related to the muscle length at the time of measuring them, while Fd meas-sured as (Fs-Fb) 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.4. The parameters of the model were determined by the relations of Fi or Fb versus muscle length and of Fd versus stretch velocity.5. The actual spindle response differed from the behaviour of the model. in two points: Firstly, the former showed an initial transient rise of the frequency which might be caused by pressure exerted upon the spindle by the neighbouring extrafusal muscle fibres. Secondly, 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 fibres.