Spatial And Temporal Features Of Recurrent Facilitation Among Motoneurons Innervating Synergistic Muscles Of The Cat



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1. The temporal features and strength of recurrent facilitatory potentials were examined in pairs of lumbosacral motoneurons that were separated by a known distance and were identified by antidromic stimulation of muscle nerves. One motoneuron was stimulated by injecting depolarizing current pulses, and responses were recorded in the second motoneuron. The distance between motoneurons in pairs was also measured to assess the spatial distribution in strength of recurrent facilitation in motor pools. All motoneurons in these pairs innervated muscles that act as hip or ankle extensors. 2. Recurrent facilitatory potentials were found frequently among motoneurons innervating the hindlimb extensor muscles examined. Several categories of recurrent facilitatory responses were identified. One category was composed of facilitation responses that followed an inhibition response. A second category was composed of facilitation responses that were not preceded by a significant inhibition and consisted of a monophasic response. There was a considerable range of latencies in this category. 3. Responses in which recurrent facilitatory potentials were preceded by recurrent inhibitory postsynaptic potentials (RIPSPs) among close motoneuron pairs demonstrated an inverse correlation between the durations of the facilitatory and the inhibitory phases. In addition, the duration of inhibition responses without facilitation was longer on average, than the duration of inhibitory responses that were followed by facilitation. It was suggested that recurrent facilitation may restrict the time course of RIPSPs. 4. In contrast to the topographic distribution of RIPSPs described in the previous report, amplitudes of monophasic facilitations were directly correlated with the distance separating motoneurons in pairs, rather than inversely correlated as was the case for RIPSP amplitudes. The amplitudes of other facilitatory responses were unrelated to the distance separating motoneurons in pairs. 5. Several features of recurrent inhibitory circuits were considered as potential mechanisms for producing the facilitation. Of those considered, only mutual inhibition was sufficient to explain all categories of facilitatory responses. 6. A comparison of the topographic patterns of recurrent inhibition and facilitation suggests that the facilitation acts to sharpen the boundaries of the rostrocaudal zone of recurrent inhibition. If recurrent facilitation predominates in distant motoneurons, the facilitation would produce more homogeneous activity levels along the length of each motor pool. That is, more active motoneurons within motor pools would tend to inhibit local motoneurons, whereas more distant less active motoneurons would be facilitated. 7. Our evidence for mutual inhibition between Renshaw cells driven by homonymous and synergistic motoneurons in combination with previous demonstrations of mutual inhibition in other systems of spinal motoneurons suggests that mutual inhibition is a general feature of spinal interneuronal networks. Moreover, we suggest that the topographic pattern identified in the recurrent Renshaw network in this and the companion report provides a potential model for input-output processing of spinal motoneurons.

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Journal of Neurophysiology







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