Specificity of electrical coupling among neurons innervating forelimb muscles of the adult bullfrog

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Abstract
1. The specificity of electrical connections among sensory fibers and motoneurons in the bullfrog's spinal cord was studied by recording intracellularly from brachial motoneurons. Synaptic potentials evoked by stimulation of individual muscle nerves were recorded in normal and reduced-calcium solutions and after acute section of dorsal or ventral roots. 2. Homonymous motoneurons are electrically coupled. After the dorsal roots were cut to abolish sensory input, short-latency potentials were almost always evoked anti-dromically in a motoneuron by stimulation of its own muscle nerve but rarely by stimulation of nerves innervating other muscles. These potentials differed from chemically mediated synaptic potentials in this preparation; they had a shorter latency and remained after perfusion with reduced-calcium solutions. This evidence suggests that they are mediated electrically. 3. Some motoneurons that innervate functionally equivalent muscles are electrically coupled. Approximately two-thirds of the motoneurons innervating the internal or external heads of the triceps muscles received coupling potentials on stimulation of the other nerve, although never on stimulation of the heteronymous, medial, triceps nerve. 4. The monosynaptic potentials evoked by muscle sensory afferents in motoneurons often have both electrical and chemical components. The electrical component occurred with short delay and persisted in reduced-calcium solutions. The chemical component occurred 1.5-2.0 ms later, at 14 degrees C, and was abolished by reducing calcium in the bathing solution. Muscle sensory afferents make these mixed synapses on homonymous, heteronymous, and other motoneurons.