The effects of muscle stretch and vibration on fusimotor activity in the lightly anaesthetised cat

Abstract

Responses from primary and secondary endings of muscle spindles were recorded in the medial gastrocnemius muscle of the cat under light pentobarbitone anaesthesia and intact ventral roots. The mean, root mean square deviation and distribution of frequencies of action potentials were measured for each spindle. High frequency bursts of action potentials, observed in the bacjground firing of primary endings of spindles could be inhibited by vibration of a synergist muscle. The reflexly evoked increase in firing of spindles in response to a single stimulus applied to skin nerve or filament of dorsal root was measured over a range of muscle lengths. The reflex response to electrical stimulation as well as the background firing of a spindle were reduced by section of dorsal roots containing the afferent of the test muscle. For 6 of the primary endings of spindles tested the evoked reflex increased in amplitude after section of dorsal roots when the muscle was stretched near to its maximum length in the body. Three of the 5 secondary endings showed an increase in the reflex following dorsal root section over a wider range of muscle lengths. It is concluded that autogenetic effects on the reflex activity of fusimotor neurones can be detected but under the experimental conditions employed they are weak and variable.