Contribution of Different Size Motoneurons to Renshaw Cell Discharge During Stretch and Vibration Reflexes

Abstract

Publisher Summary The contribution of motoneurons of different size to Renshaw cell discharge can be understood by considering the organization of autogenetic reflex pathways. Two basic mechanisms of autogenetic inhibition tend to reduce excitation of extensor motoneurons elicited by the corresponding Ia pathway. The first is a mechanism of presynaptic inhibition due to primary afferent depolarization (PAD) of both the homonymous and heteronymous Ia afferents. Muscle vibration was particularly effective in producing PAD of the Ia pathway, while static stretch was apparently inoperative in this respect. The second is a mechanism of postsynaptic inhibition due to recurrent excitation of Renshaw cells. It has shown that Renshaw cells are mainly sensitive to dynamic changes in muscle length, as produced by muscle vibration, but less sensitive to static muscle stretch. It has been hypothesized that static stretch and vibration of the gastrocnemius-soleus (GS) muscle activate small and large motoneurons in different proportions in the two experimental conditions, and that axon collaterals of large phasic motoneurons, on the average, make more powerful excitatory connections onto Renshaw cells than do those drginating from small tonic motoneurons.