RHYTHMIC EXCITATION OF A STRETCH REFLEX, REVEALING (a) HYSTERESIS AND (b) A DIFFERENCE BETWEEN THE RESPONSES TO PULLING AND TO STRETCHING

Abstract

When sinusoidally fluctuating tensions were applied to the soleus muscle of a decerebrate cat, plots of tension against length showed a hysteresis loop, implying damping. Various tensions and frequencies of oscillation were used, with and without simultaneous head-tilting or inhibition of the stretch reflex. The damping was not frequency-dependent (i.e. not viscous or due to velocity feedback). The pattern of the loops for overlapping ranges of tension indicated a "distributed simple friction" mechanism. The way the loops shifted during change of gamma bias indicated that the damping was located in the series-elastic elements. During imposed length-changes the response curve shifted sporadically along the tension axis and sudden twitch-like increases in tension occurred in an unpredictable fashion. In contrast, the responses to tension-changes were more consistent. This difference was most marked when other reflex effects were simultaneously elicited, e.g. by head-tilting. The effects are accounted for by defining the stretch reflex as "a reflex increase in the stiffness (δT/δL) of a muscle in response to increased tension on its spindles". The sequence of changes in the "clasp-knife" response can also be explained in this way.