Discharge patterns of Renshaw cells evoked by volleys in ipsilateral cutaneous and high threshold muscle afferents and their relationship to reflexes recorded in ventral roots

The most common type of Renshaw cell response to flexor reflex afferent nerve (FRA) volleys was a biphasic excitation. The first phase peaked during the polysynaptic reflexes recorded simultaneously from ventral roots and was termed the concurrent response. The second phase peaked 'near' or after reflex cessation and was called the tail response. Some Renshaw cells responded to FRA volleys with monophasic discharges. By relating the time courses of the Renshaw cell discharges to the simultaneously recorded polysynaptic reflexes, it was possible to classify monophasic discharges as concurrent or tail responses. The times to peak for concurrent responses were positively correlated with the times to peak of the reflexes, but the times to peak of the tail responses were not. FRA induced Renshaw cell discharges usually grew with increasing stimuli at rates similar to those of polysynaptic reflexes. Because of the weak effects of mecamylamine, it was concluded that the majority of FRA evoked Renshaw cell responses in these experiments were directly elicited by interneuronal rather than motoneuronal inputs. Because of the similarities of concurrent responses to reflexes, polysynaptic pathways common to Renshaw cells and motoneurons were postulated. It was further suggested that at least some tail responses were elicited through pathways largely independent of circuits exciting motoneurons.