Physiological evidence for specificity of synaptic connections between individual sensory and motor neurons in the brachial spinal cord of the bullfrog.

We have studied synaptic connections between individual stretch-sensitive muscle afferents and motoneurons in the brachial spinal cord of bullfrogs. Sensory afferents from a given head of the triceps brachii muscle preferentially innervate motoneurons that project to the same muscle head. This preference is characterized in two ways: each class of sensory axon innervates a greater proportion of corresponding motoneurons than of motoneurons projecting to synergistic or unrelated muscles, and the synaptic potentials in these corresponding motoneurons are motoneurons that project to the same muscle head. This preference is characterized in two ways: each class of sensory axon innervates a greater proportion of corresponding motoneurons than of motoneurons projecting to synergistic or unrelated muscles, and the synaptic potentials in these corresponding motoneurons are of larger amplitude. A novel feature of these experiments is that even the smallest averaged synaptic potential is several times larger than the noise level. These smallest synaptic potentials thus represent the smallest synaptic interaction between a sensory and motor cell, and they could be the physiological correlate of a single sensory bouton on a motoneuronal dendrite.