Dynamic behaviour of alpha-motoneurons subjected to recurrent inhibition and reflex feedback via muscle spindles.

The dynamic transfer characteristics of mammalian spinal skeleto-motoneurons are determined by intrinsic properties and various sorts of feedback. Here, recurrent inhibition via Renshaw cells and reflex feedback via muscle units and muscle spindle (in particular Ia) afferents, in the cat, are considered. The dynamic properties of the motor axon-Renshaw cell and the motor unit-spindle afferent subsystems were experimentally determined by stimulating motor axons with pseudo-random patterns of electrical pulses at two mean rates (low: 9.5-13 pulses/s; high: 20-23 pulses/s) and recording discharges of the two output elements. Spectral analysis yielded frequency responses to which transfer functions were fitted. These transfer functions in conjunction with those previously derived for alpha-motoneurons were used to study the stability and input-output characteristics of motoneurons with regard to two issues: stability and input-output relations of the combined (recurrent plus reflex) system as compared with each subsystem alone, with (i) each feedback path consisting of a single loop at some moderate level of force production, and (ii) each pathway consisting of two loops related to two motoneuron subpopulations active at a higher level of recruitment. It is shown that Renshaw cells have frequency characteristics well suited to contribute to the stabilization of the reflex loop. They can do so at low gains of recurrent inhibition.