Effects of changes in preoptic temperature on stretch response of muscle spindle endings in the cat's soleus muscle

1. The effects of changing the temperature in the preoptic region on the stretch responses of primary and secondary endings of the muscle spindle in the soleus muscle was investigated in urethane anesthetized cats. The local temperature of the preoptic region was controlled by implanting water perfused thermodes (32–42.5°C).
2. A standard ramp and hold stretch stimulus was repeatedly applied to the soleus muscle during changes of preoptic temperature from normal to hypo-or hyperthermic values. During each stretch the instantaneous firing rate was recorded and the static firing frequency and the dynamic index were electronically determined according to Crowe and Matthews (1964).
3. Of a total of 76 investigated muscle spindle afferents, the stretch responses of 49 (65%) were altered by changing preoptic temperature. The static stretch response was increased in all cases during cooling, whereas during preoptic heating the static stretch response increased in about half of the afferents and decreased in the rest. Increase in the static stretch response during both cooling and heating was usually combined with the appearence of spontaneous discharges at rest.
4. With regard to the static stretch response, primary and secondary muscle spindle endings responded similarly to cooling and heating.
5. Two types of preoptic temperature effects on the stretch responses of primary endings were observed. In the majority of afferents only the static stretch response was augmented during cooling, whereas the peak response was little changed resulting in a decrease of the dynamic index (‘static’ response type). In a minority of primary afferents the effect of preoptic cooling consisted of a greater increase of the peak response than of the static stretch response. i.e., the dynamic index also increased (‘dynamic-static’ response type). In secondary muscle spindle endings only the static type of response to preoptic cooling was observed.
6. It is concluded that changes of preoptic temperature influence the stretch response of muscle spindle afferents mainly by altering the activity of the static fusimotor innervation, its activation by preoptic cooling being the most consistent finding. In a minority of muscle spindles the dynamic fusimotor innervation appears to be additionally activated by preoptic cooling.