DECORRELATING ACTIONS OF RENSHAW INTERNEURONS ON THE FIRING OF SPINALMOTONEURONS WITHIN A MOTOR NUCLEUS - A SIMULATION STUDY

A simulation of spinal motoneurons and Renshaw cells was constructed t o examine possible functions of recurrent inhibition. Recurrent inhibi tory feedback via Renshaw cells is known to be weak. In our model, con sistent with this, motoneuron firing was only reduced by a few pulses per second. Our initial hypothesis was that Renshaw cells would suppre ss synchronous firings of motoneurons caused by shared, dynamic inputs . Each motoneuron received an identical pattern of noise in its input. Synchrony coefficients were defined as the average motoneuron populat ion firing relative to the activity of selected reference motoneurons; positive coefficients resulted if the motoneuron population was parti cularly active at the same time the reference motoneuron was active. W ith or without recurrent inhibition, the motoneuron pools tended to sh ow little if any synchronization. Recurrent inhibition was expected to reduce the synchrony even further. Instead, it reduced the variance o f the synchrony coefficients, without a comparable effect on the avera ge. This suggests-surprisingly-that both positive and negative correla tions between motoneurons are suppressed by recurrent inhibition. In s hort, recurrent inhibition may operate as a negative feedback mechanis m to decorrelate motoneurons linked by common inputs. A consequence of this decorrelation is the suppression of spectral activity that appar ently arises from correlated motoneuron firings due to common excitato ry drive. Without recurrent inhibition, the power spectrum of the tota l motoneuron pool firings showed a peak at a frequency corresponding t o the largest measured firing rates of motoneurons in the pool. Recurr ent inhibition either reduced or abolished this peak, presumably by mi nimizing the likelihood of correlated firing among pool elements. Rens haw cells may act to diminish physiological tremor, by removing oscill atory components from aggregate motoneuron activity. Recurrent inhibit ion also improved coherence between the aggregate motoneuron output an d the common drive, at frequencies above the frequency of the ''synchr onous'' peak. Sensitivity analyses demonstrated that the spectral effe ct became stronger as the duration of inhibitory synaptic conductance was shortened with either the magnitude or the spatial extent of the i nhibitory conductances increased to maintain constant net inhibition. Overall, Renshaw inhibition appears to be a powerful way to adjust the dynamic behavior of a neuron population with minimal impact on its st atic gain.