Excitatory connections between spinal motoneurons in the adult rat.

OBJECTIVES

Dendro-dendritic and dendro-somatic projections are common between spinal motoneurons. We attempted to clarify whether there are functional connections through these projections.

METHODS

Motoneurons were antidromically stimulated by the muscle nerve and recorded intracellularly to examine the direct interaction between them after the related dorsal roots had been cut.

RESULTS

Excitatory connections, demonstrated by depolarizing potentials in response to muscle nerve stimulation, were found between motoneurons innervating the same muscle or synergistic muscles, but never between motoneurons innervating antagonistic muscles. These potentials were finely graded in response to a series of increasing stimuli and resistant to high frequency (50Hz) stimulation.

CONCLUSIONS

These results indicate that excitatory connections, with certain specificity of spatial and temporal distribution, occur in the spinal motoneurons. It is also suggested that electrical coupling should be involved in these connections and this mechanism should improve the excitability of the motoneurons in the same column.