Non-Linear Behaviour of Spindle Receptors

Abstract

Non-linear characteristics of primary and secondary spindle receptors were studied in decerebrate cats with and without intact ventral roots. Inputs consisted of slow ramp stretches and releases applied to the soleus muscle. Highly reproducible departures from a smooth change in discharge rate occurred during constant velocity stretching. When the stretch was more rapid, the departures were earlier and resembled initial bursts. Responses of each receptor to a family of ramps of constant duration and variable amplitude were used to construct “linearity plots”. These plots demonstrated a transition from a region of high sensitivity to a region of low sensitivity that was evident during the stretch (or release), during adaptation, and in the steady state. Transition in sensitivity was associated with a particular change in length (100–200 μm). The high sensitivity to small changes in length reset after a receptor had adapted to a new length. All of these non-linearities could result from friction in the poles of intrafusal fibres. Non-linear behaviour was most conspicuous for de-efferented primary endings and least for secondary endings that received spontaneous gamma input. Stretch reflexes studied simultaneously reflected in their properties some of the non-linear features of spindle receptors, but the stretch reflex appears not to be a simple additive result of feedback from primary and secondary endings.