Medullary control of lumbar motoneurons during carbachol-induced motor inhibition

The present study examined the effects of stimulation of the medullary nucleus reticularis gigantocellularis (NRGc) on the Ia-monosynaptic reflex and the membrane potential of lumbar motoneurons. Stimulation of the NRGc was carried out in acute decerebrate cats during motor suppression induced by the intrapontine microinjection of carbachol. During carbachol-induced motor suppression, compared with control conditions (prior to the administration of carbachol), NRGc stimulation resulted in a statistically significant reduction in the Ia-monosynaptic reflex. This effect was maximal at an interval of 45 ms following NRGc stimulation. NRGc stimulation also induced, in lumbar motoneurons, a large amplitude (3.17 mV ± 0.36 [S.E.M.]), long duration (54.73 ms ± 3.52 [S.E.M.]) inhibitory postsynaptic potential whose peak coincided with the interval of maximum reflex suppression. These results suggest that carbachol activates pontine neurons that excite cells of the medullary NRGc. We believe that these medullary neurons, in addition to those of the nucleus pontis oralis (NPO)⁷, participate in the modulation of the descending inhibitory pathway that is responsible for the phenomenon of response-reversal^5,15 and generalized atonia during naturally occurring active (i.e. REM) sleep.