The effect of cooling on mammalian muscle spindles.

Abstract The effect of gradual cooling on discharge from sensory receptors in the gastrocnemius muscle of the cat has been studied. Either the entire muscle as it lay in an oil pool or the vicinity only of individual sensory organs was cooled 10 to 15° C below normal body temperature as monitored by a thermocouple inserted into the muscle. Slowing, in general, occurred in the discharge of deefferented annulospiral, flower-spray and tendon organ afferents, identified by conduction rates of individual units and characteristic patterns of response. The firing from spindles decreased at all tensions above that needed to sustain a steady discharge, with greater absolute decreases at higher tensions. Relative to control rates, annulospiral endings were probably affected more than flower-spray ones. The discharge from spindles having intact efferent innervation was also slowed. Alterations in central excitability may result from changes in afferent discharge produced by cooling of calf muscles. In decerebrate cats paralyzed with gallamine, monosynaptic reflexes elicited by submaximal stimulation of the severed lateral gastrocnemius nerve were reduced when the medial gastrocnemius muscle of the otherwise denervated leg was cooled. Decerebrate rigidity in the triceps surae muscle also decreased with cooling.