Rubrospinal and rubrobulbospinal influences on dynamic and static gamma-motoneurones

In a series of experiments made on alpha-chloralose-anaesthetized cats, the effects evoked both by electrical stimulation of the rubrospinal (RS) and robrobulbospinal (RBSP) paths and by graded electrical stimulation of muscle, skin and joint nerves were investigated in the same sample of intra- and extracellularly recorded lumbar gamma-motoneurones (n = 120). The cells were classified as dynamic or static by stimulation of the RBSP, which is known to influence dynamic gamma-motoneurones selectively. The majority of classified cells, which responded to peripheral stimuli, were also responsive to NR stimulation. Multivariate analysis of the cell response patterns in gastrocnemius et soleus gamma-motoneurones indicated that in general the types of effects evoked via RS and RBSP could not be predited from the effects elicited from the periphery. Yet, at least for some cells, covariation between excitatory RS effects and excitatory responses from the superficial peroneal nerve and between inhibitory RS effects and inhibitory effects from group III muscle afferents was indicated. Hence it is tentatively suggested that there may exist functional subpopulations of gamma-cells with coupling between effects from the RS path and different peripheral afferent systems and that, if the RS path coactivates alpha- and gamma-motoneurones, then movements with simultaneous activation of certain afferent systems may favour alpha-gamma-linkage.