Temperature effects on the discharge frequency of primary and secondary endings of isolated cat muscle spindles recorded under a ramp-and-hold stretch.

The effects of changes in temperature on primary and secondary endings of isolated cat muscle spindles were investigated under ramp-and-hold stretches and different degrees of pre-stretch. Temperature-induced alterations of the discharge frequency were compared over a temperature range of 25-35 degrees C. Both primary and secondary endings responded to warming with increasing discharge frequencies when the spindle was pre-stretched by 5-10% of its in situ length. The following differences between the temperature effects on primary and secondary endings were observed: (1) The temperature coefficients (Q(10)) obtained from the discharge frequencies during the dynamic and static phase of a stretch were similar for endings of the same type, but they were larger in primary endings (range of Q(10): 2.3-3.3; mean: 2.9) than in secondary endings (range of Q(10): 1.6-2.2; mean: 2.0); (2) With primary endings, but not with secondary endings, the temperature sensitivity (imp s(-1) degrees C(-1)) was larger during the dynamic phase than during the static phase of a stretch; (3) In primary endings, the fast and slow adaptive components occurring in the discharge frequency during the static phase of a stretch clearly increased with warming while in secondary endings, the slow decay was less affected, and the fast decay showed no change; (4) In relaxed spindles, the excitatory effect of warming was overlaid by a strong inhibitory effect as soon as the temperature exceeded about 30 degrees C, resulting in an abrupt cessation of the background activity in most secondary endings, but not usually in primary endings. In general, warming induced an enhanced stretch sensitivity in both types of ending, and additionally an inhibitory effect that is obvious only in secondary endings of relaxed spindles. The different effects of temperature on the discharge frequency of primary and secondary afferents are assumed to be caused by different properties of their sensory membranes.