Effects of spinal recurrent inhibition on motoneuron short-term synchronization

Spinal recurrent inhibition linking skeleto- motoneurons (α-MNs) via Renshaw cells (RCs) has been variously proposed to increase or decrease tendencies toward synchronous discharges between α-MNs. This controversy is not easy to settle experimentally in animal or human paradigms because RCs receive, in addition to excitatory input from α-MNs, many other modulating influences which may change their mode of operation. Computer simulations help to artificially isolate the recurrent inhibitory circuit and thus to study its effects on α-MN synchronization under conditions not achievable in natural experiments. We present here such a study which was designed to specifically test the following hypothesis. Since many α-MNs excite any particular Renshaw cell, which in turn inhibits many α-MNs, this convergence- divergence pattern establishes a random network whose random discharge patterns inject uncorrelated noise into α-MNs, and this noise counteracts any synchronization potentially arising from other sources, e.g., common inputs (Adam et al. in Biol Cybern 29:229-235, 1978). We investigated the short-term synchronization of α-MNs with two types of excitatory input signals to α-MNs (random and sinusoidally modulated random patterns). The main results showed that, while recurrent inhibitory inputs to different α-MNs were indeed different, recurrent inhibition (1) exerted rather small effects on the modulation of α-MN discharge, (2) tended to increase the short-term synchronization of α-MN discharge, and (3) did not generate secondary peaks in α-MN-α-MN cross-correlograms associated with α-MN rhythmicity.