Development of sensory‐motor synapses in the spinal cord of the frog.

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Abstract
The development and specificity of monosynaptic sensory-motor synapses were studied in the brachial spinal cord of bullfrog tadpoles. Intracellular and extracellular recordings were made from motoneurons innervating several different muscles of the forelimb. Excitatory synaptic potentials (EPSP) were elicited by stimulation of various peripheral muscle nerves. Sensory and motor axons in the triceps brachii muscle nerves were electrically excitable at stage XIII, the earliest stage studied. Their conduction velocities were 0.2-0.4 m/s. These velocities increased during subsequent development so that by stage XXII they were .apprx. 5 m/s. Before stage XVII, synaptic potentials evoked in motoneurons by stimulation of the triceps sensory fibers had a long central latency and fatigued easily. These potentials were probably mediated polysynaptically. At stage XVII, the 1st short-latency triceps synaptic potentials appeared. They had central latencies of < 3 ms and represented the direct, monosynaptic input from muscle sensory cells on to motoneurons. During subsequent development the percentage of triceps motoneurons innervated by triceps sensory fibers increased, while the number of long-latency polysynaptic inputs decreased. Both the electrical and chemical components, characteristic of these monosynaptic EPSP in adult frogs, were prominent from the time the EPSP first appeared. The pattern of innervation of brachial motoneurons by triceps sensory afferents was specific from the beginning. Triceps sensory fibers innervated most triceps motoneurons but very few subscapular or pectoralis motoneurons, just as in adult frogs. At no time were there appreciable numbers of aberrant connections. The developmental time course of several different classes of sensory-motor connections was similar. Thus the synaptic specificity of this system cannot be explained by a differential timing of synaptogenesis.