The formation of specific synaptic connections between muscle sensory and motor neurons in the absence of coordinated patterns of muscle activity.

The influence of patterned neuronal activity on the formation of specific monosynaptic connections between muscle sensory and motor neurons was studied in the developing spinal cord of the bullfrog. Motor innervation of the forelimb was disrupted in tadpoles by resection of the brachial ventral root before these synaptic connections began to form in the spinal cord. In those frogs accepted for analysis, motor reinnervation of the forelimb was nonspecific and there was no coordinated movement of the limb. Synaptic connections therefore developed in the absence of temporal correlations of activity in muscle spindle afferents and motoneurons. Despite this disruption, afferent fibers supplying the triceps brachii muscles selectively innervated a restricted subpopulation of brachial motoneurons. Those motoneurons that received large synaptic inputs from afferents in one branch of the triceps nerve also received large inputs from afferents in the other triceps branches. Inputs from afferents supplying other muscles were not correlated with those from triceps afferents, suggesting the existence of a property common to all triceps afferents causing them to innervate a common subpopulation of motoneurons. These results show that in the absence of normal patterned sensory activity, sufficient cues still exist to permit the formation of specific sets of synaptic connections, and they argue indirectly for the existence of chemical labels that can determine the pattern of these connections.