Topographic factors in distribution of homonymous group Ia-afferent input to cat medial gastrocnemius motoneurons

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Abstract
Experiments were performed to determine whether the topographic relationships between muscle spindles and their surrounding extrafusal fibers are preserved in the pattern of homonymous, monosynaptic connections from Ia-afferents to motoneurons. The medial gastrocnemius (MG) muscle of adult cats was chosen as a model system because previous studies have shown that its muscle nerve divides into several branches, each of which innervates a distinct muscle compartment and that the Ia-afferent fibers innervating muscle spindles within a compartment are found in the same nerve branch. Intracellular recordings were made from MG motoneurons, determining which intramuscular compartment they innervated, and then compare the synaptic input they received from Ia-afferents innervating the same compartment with that which they received from Ia-afferents innervating different compartments. Apparently the homonymous Ia-afferents input is topographically weighted within the Mg motor nucleus such that afferents innervating a given intramuscular component exert relatively greater synaptic effects in motoneurons that project to the same compartment than in other homonymous motoneurons. The extent of topographic weighting within the homonymous motoneuron pool is compared with weighting across synergist motoneuron pools, and alternative models of topographic weighting are proposed and evaluated.