Do Renshaw cells tell spinal neurones how to interpret muscle spindle signals?

In vertebrates many alpha motoneurone pools are subjected both to recurrent inhibition via Renshaw cells and to proprioceptive feedback via muscle fibres and proprioceptors, particularly spindles. In these cases, the two feedback loops have a common input (alpha motoneurone output) and a common target (alpha motoneurones). This implies that the target alpha motoneurones receive a compound information dispatched by the source alpha motoneurones, but processed in different ways via the two feedback paths. Since the Renshaw cells monitor the input to skeletal muscle, and the spindles (and Golgi tendon organs) monitor certain aspects of muscle output, both feedback paths in conjunction contain information about the mechanical state of skeletal muscle. Based on these interrelationships the following hypothesis is discussed. At a micro-level, muscle spindles might provide information about motor unit contractions to the homonymous alpha motoneurones. This information is filtered and enhanced by recurrent inhibition via Renshaw cells. This is effected by correlation of the signals which are propagated through the two feedback loops after having been initiated by firing of the same alpha motoneurone(s). The effects of the correlation can be strengthened by (a) topographical order in the feedback connections, (b) heterosynaptic modulation, and (c) tendencies towards synchronous discharge between motoneurones. The information about the unfused contractions of a muscle unit (or a small group of them), thus retrieved from the barrage of signals delivered by proprioceptive afferents, could then play a role in shaping the precise discharge pattern of the innervating motoneurone. This in turn may be of importance for mechanisms of optimal force production during muscle fatigue.