Organization of electrical coupling between frog lumbar motoneurons

The organization of electrical coupling between lumbar motoneurons in the isolated frog spinal cord was investigated. Intracellular recordings were made from unidentified and identified thigh muscle motoneurons in segment 9. Excitatory postsynaptic potentials (EPSPs) elicited by stimulation of individual muscle nerves with dorsal roots cut (VR-EPSPs) were observed when the cord was perfused with either normal frog Ringer solution or calcium-free Ringer solution containing 2, 2.5, or 5 mM magnesium. The VR-EPSPs recorded following stimulation of the homonymous muscle nerve were greater in amplitude and occurred with a shorter latency than the VR-EPSPs observed following stimulation of muscle nerves to different muscles. In addition, the latter VR-EPSPs had a shorter half-decay time. Electrical connections made by motoneurons innervating synergistic muscles had a similar distribution, as indicated by the linear correlation observed between the amplitudes of VR-EPSPs in the same motoneuron elicited by stimulation of muscle nerves innervating synergistic muscles. No such linear correlation was observed between VR-EPSP amplitudes when muscle nerves innervating antagonistic muscles were stimulated. Small-amplitude, spikelike potentials were often observed on top of the muscle nerve VR-EPSPs. In the case of VR-EPSPs elicited by stimulation of muscle nerves not containing the axon from the recorded motoneuron, the spikelike component accounted for a major part of the response. These spikelike potentials had a rapid time course (half-decay time, 2.5-5.9 ms) and in a few cases were observed to be all or none. These observations are consistent with dendritic action potentials in the recorded motoneuron as well as electrotonic coupling between a small number of neurons close to the recording site in the impaled motoneuron. These results are interpreted to mean that a high degree of specificity exists in the organization of electrical coupling among frog lumbar motoneurons.