Activity of renshaw cells during fictive scratch reflex in the cat

Summary The experiments were performed on decerebrate curarized cats with a hindlimb either completely deafferented or partly deefferented. Through tactile stimulation of the pinna, a fictive scratch reflex was evoked and activity in muscle efferents was then observed, similar to that in actual scratching. The duration of the cycle was about 250 ms, with extensor activity during a short period of about 50 ms (S phase) and flexor activity during a much longer one (about 200 ms; L phase). Appearance of rhythmic bursts of discharges was preceded by tonic flexor activity (tonic phase of scratching). Discharges of Renshaw cells were recorded extracellularly during these three phases, in parallel with discharges in the gastrochemius-soleus nerve. During the tonic phase of the scratch reflex, some Renshaw cells with input from flexors decreased their activity while others increased. No change in Renshaw cell activity with input from extensors was then observed. During the rhythmic phases of the scratch reflex a majority of Renshaw cells was phasically active. They usually responded once per cycle, with a burst of 1–30 impulses of 50–100 ms duration, most often occurring at the end of the L phase and during the S phase. Bursts of Renshaw cells with input from flexors and of extensors, respectively, overlapped to a high degree. However, maximal firing of extensor-coupled Renshaw cells occurred somewhat later than that of flexor-coupled cells. Flexor-coupled Renshaw cells discharged mainly at the end of the L phase and during the S phase, i.e. when the flexor moto-neurones terminated their activity. Extensor-coupled Renshaw cells reached maximal activity during the S phase, i.e. when the extensor motoneurones were recruited. After spinal transection at C1 level, Renshaw cells responded with an increased number of spikes but without change in timing of the discharges during the scratch cycle. Most of the contralaterally located Renshaw cells studied were also phasically active during the scratch reflex. The role of motoneurones and spinal interneurones in determining the timing of Renshaw cell activity and the role of the latter in control of posture and rhythmic movements are discussed.