The discharge from primary endings of muscle spindles during reflex activation of fusimotor neurones in the lightly anaesthetised and spinal cat

Abstract

Summary The responses of primary endings in muscle spindles of peroneus brevis of the cat to a single ipsilateral skin nerve stimulus were measured as frequencygrams and post-stimulus histograms. Under light pentobarbitone anaesthesia a brief, short latency, large amplitude frequencygram was observed with a stimulus too weak to elicit a reflex contraction. The response is deduced to have been caused by an almost synchronous discharge of several static fusimotor fibres. It is suggested that the background activity was due to asynchronous activity in static fusimotor fibres. After section of the spinal cord, reflex acceleration of the spindle was less pronounced and the frequencygram had a slower time course. The response was increased during extension of the muscle. The possibility that the response in spinal cats resulted from a dynamic fusimotor reflex is considered.