Responses of quadriceps motor units to mechanical stimulation of knee joint receptors in the decerebrate cat.

Recordings were obtained from single quadriceps motor units in decerebrate cats. Tonic discharges were induced in these units by stretch applied to the patellar tendon. The activity of these motor units was examined during repetitive mechanical indentation of the posterior aspect of the knee joint capsule. The amplitude of indentation could be graded to regularly elicit a volley of one or more action potentials from afferents travelling in the posterior articular nerve (PAN). Either repetitive indentation at a fixed frequency or random indentation resulted in entrainment of motor unit discharge with a latency of 6.5-17 ms from the onset of the first PAN spike. This effect was achieved in one case by a single joint afferent. Usually two or more afferents were required to produce entrainment. This effect was mediated by joint receptors as its reversibly abolished by application of lignocaine to the joint nerve. The modal reflex latency suggests that group II joint afferents were responsible for this effect. Stimulation of some areas of the joint capsule readily produced motor unit entrainment whereas other areas did not although PAN afferents were also activated at the latter sites, suggesting inhomogeneity in the synaptic actions of joint afferents.