Projection of single knee flexor Ia fibers to homonymous and heteronymous motoneurons

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Abstract
Individual EPSP [excitatory post synaptic potential] produced by single semitendinosus (ST) group Ia fibers in homonymous and heteronymous cat motoneurons were examined using the spike-triggered averaging technique. Heteronymous motoneurons included biceps (B) and semimembranosus (SM) montoneurons. The observed EPSP amplitude differences and projection frequency differences may represent a true species specificity. The nerve supplying the semitendinosus muscle has 2 major branches each supplying distinct regions of the muscle. There were no significant differences in the projection of Ia fibers to motoneurons supplying these different regions. Despite the fact that flexor and extensor spinal motoneurons receive widely different descending and segmental inputs, no important differences could be found between the properties of monosynaptic Ia-motoneuron individual EPSP evoked by semitendinosus Ia fibers and triceps surae Ia fibers. Common mechanisms may govern the establishment and function of Ia-motoneuron monosynaptic connections in both flexor and extensor systems.