Renshaw cells and recurrent inhibition: comparison of responses to cyclic inputs

Spinal recurrent inhibition influences the discharge patterns of motoneurons and spinal interneurons. The precise pattern of this influence depends on the static and dynamic characteristics of this feedback system. It is thus of importance to quantify its characteristics as well as possible. We here compare nonlinear features (hysteresis) in Renshaw cells and recurrent inhibition in response to cyclic stimulation of motor axons. In pentobarbitone-anaesthetized or decerebrate cats, intracellular recordings were obtained from 26 hindlimb muscle nerves skeleto-motoneurons and extracellular recordings from nine Renshaw cells. Various hindlimb muscle nerves (dorsal roots cut) or ventral roots (dorsal roots intact) were prepared for electrical stimulation to elicit recurrent inhibition in motoneurons or discharges in Renshaw cells. Stimulus patterns consisted of repetitive pulse trains whose rates varied cyclically between around 10 pulses/s and several tens of pulses/s, at modulation frequencies between 0.1 and 1.0 Hz, in one of two waveforms: triangular or sinusoidal. Recurrent inhibitory potentials in motoneurons and discharge patterns of Renshaw cells were averaged with respect to triggers (cycle-triggers) marking a fixed phase in the stimulation cycle. In another two experiments, motor axons to hindlimb muscles (soleus and medial gastrocnemius) were stimulated with sinusoidal and distorted temporal patterns to show their effects on force production. Most often the cycle-averaged motoneuron membrane potential changed in a temporally asymmetrical way, i.e. it fairly rapidly hyperpolarized early in the stimulus cycle (during increasing rate) and then depolarized more slowly throughout the rest.(ABSTRACT TRUNCATED AT 250 WORDS)