Journal of Neurophysiology, Vol 48, Issue 4 904-913, Copyright © 1982 by APS
Specificity of electrical coupling among neurons innervating forelimb muscles of the adult bullfrog
M. Westerfield and E. Frank
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.
