EXPERIMENTS UTILIZING MONOSYNAPTIC EXCITATORY ACTION ON MOTONEURONS FOR TESTING HYPOTHESES RELATING TO SPECIFICITY OF NEURONAL CONNECTIONS

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Abstract
Cross-union of muscle nerves in the hind-limb was performed on fifteen kittens and several months later there was an extensive investigation of over 500 motoneurons by intracellular recording. After cross-union of the nerves to lateral gastrocnemius and plantaris muscles there was a statistically significant increase in the monosynaptic action of the afferent fibers from the in-series synergist of plantaris muscle (flexor digitorum brevis) onto the lateral gastrocnemius motoneurons that had reinnervated plantaris muscle. After cross-union of the nerves to the medial gastrocnemius and the peroneus muscles, there was very high statistical significance for the development of new connections from the afferent fibers of the synergic muscles, lateral gastrocnemius and plantaris onto the peroneus motoneurons that had reinnervated the medial gastrocnemius muscle. Besides the development of such functionally meaningful connections, there were two examples of new connections that appeared to be functionally meaningless. There was statistically significant evidence for regression of some of the central synaptic connections of afferent fibers which as a consequence of their changed peripheral origin had now become functionally inappropriate. These results are fully discussed in relation to the various aspects of the myotypic-specification hypothesis. It is concluded that there is statistical evidence strongly in favour of certain aspects of this hypothesis; but, as measured by the effectiveness of the changed synaptic connections, the results are relatively small.