The control of jaw-opener motoneurons during active sleep

Brainstem and spinal cord motoneurons that innervate somatic musculature serving antigravity functions are postsynaptically inhibited during active sleep [3,12,22,23]. However, it has been reported that hypoglossal motoneurons (which do not innervate antigravity muscles) are not postsynaptically inhibited during active sleep, but are disfacilitated [16,17]. In the present report we describe changes, during active sleep, in the excitability and membrane potential of digastric and synergistic motoneurons of the trigeminal motor pool; these neurons do not perform antigravity functions [21]. The experiments described in the present report were performed in chronic cats that were prepared for intracellular recording. The motoneurons hyperpolarized an average of 11 mV (S.D. ± 1.29, n = 8, P < 0.005) during active sleep compared to quiet sleep. Hyperpolarization was accompanied by a reduction in the excitability of the somadendritic regions of the neurons, as indicated by an increase in the delay of propagation of antidromic spikes from the initial segment to the somadendritic portion of the cell. High gain membrane potential recordings from these motoneurons revealed the occurrence of a remarkably large number of hyperpolarizing potentials during active sleep. When K-chloride-filled microelectrodes were utilized and chloride ions were injected intracellularly, the polarity of these potentials was reversed. During phasic episodes of active sleep, there was a clear increase in hyperpolarizing potential activity, a blockade of somadendritic spikes and phasic reductions in the amplitude of the initial segment spikes. Hyperpolarizing potentials occurred in conjunction with ponto-geniculo-occipital waves. These potentials were reversed in polarity when chloride ions were injected into the cells, indicating that motoneurons of the trigeminal motor pool that do not perform antigravity functions are postsynaptically inhibited during active sleep. The similarities between the inhibitory phenomena in these neurons and those observed in jaw-closer and spinal cord motoneurons lead us to conclude that the excitability of trigeminal motoneurons that innervate muscles with reciprocal functions is suppressed by a postsynaptic inhibitory process during active sleep, as are most other somatic motoneurons [3,12,22,23].