Brain-stem perturbations during cortically evoked rhythmical jaw movements: effects of activation of brain-stem loci on jaw muscle cycle characteristics

Abstract

The purpose of the present study was to further elucidate with the use of microstimulation techniques the influences of rostral pons and midbrain loci on the neuronal networks responsible for cortically induced rhythmical jaw movement (RJM) activity in the anesthetized guinea pig and to establish if these rostral brain-stem loci are capable of modulating the timing as well as the amplitude of rhythmical digastric (DIG) EMG activity. It was found that repetitive electrical stimulation of widespread areas of the rostral pons and mid-brain produced suppression of ongoing cortically induced rhythmical EMG activity. Prior to complete EMG suppression there was a reduction in amplitude of the DIG EMG and an increase in cycle duration. Repetitive stimulation of these suppressive loci also produced a reduction in the amplitude of the short-latency DIG EMG response produced by short pulse train stimulation of the masticatory cortex. This indicates that part of the suppression of cortically induced rhythmical EMG activity was due to a reduction in excitability of the polysynaptic short-latency pathway from cortex to DIG motoneurons. Short pulse train stimulation of these brain-stem suppressive loci during various phases of the rhythmical DIG cycle produced a phase-dependent increase in duration of the ongoing perturbed cycle. The cycles following the stimulus perturbation did not show any compensatory shortening in their durations suggesting that the stimulus produced a true resetting of the cycle. These data suggest that the brain-stem loci that produce suppression of RJMs evoked by repetitive cortical stimulation can affect the excitability of the central circuits responsible for cycle oscillation and timing as well as the excitability of the polysynaptic short-latency corticotrigeminal pathway to DIG motoneurons.

Induction of rhythmic jaw movements by stimulation of the mesencephalic reticular formation in the guinea pig

“…On comparing the location of their MRF sites inducing this suppression (cf. figure 1 of their paper) with the RJM-inducing region in the MRF found in the present study, one notes that the latter is located directly rostra1 and dorsal to the former at the level of the red nucleus, and exactly corresponds to the area from which Chandler and Tal(1987) did not obtain suppression of the CMA-induced RJM.…”

“…Since the MRFinduced rhythmic burst activity of the anterior digastric motoneurons persists after precollicular decerebration, as well as cerebellectomy in the paralyzed preparation with the spinal cord sectioned at the C2 level, the rhythm can reasonably be assumed to be generated in the brain stem. Chandler and Tal (1987) reported that repetitive electrical stimulation of the MRF suppressed CMA-induced RJMs in guinea pigs. On comparing the location of their MRF sites inducing this suppression (cf.…”

“…Sumi (1971) found, however, that repetitive electrical stimulation of the MRF not only enhanced, but also suppressed the cortically induced RJMs in rabbits. Recently, Chandler and Tal (1987) reported that repetitive microstimulation of widespread areas of the rostra1 pons and midbrain produced suppression of the cortically induced RJMs in guinea pigs. In addition, Tal (1987) demonstrated that the induction of RJMs by stimulation of the ventral MRF was, in fact, due to current spread to the fibers in the cerebral peduncle which carried the impulses from the CMA to the masticatory rhythm generator in the bulbar reticular formation of the guinea pig (Nozaki et al, 1986a).…”

“…The exact route responsible for MRF induction of RJMs remains to be clarified, although it certainly descends outside the pyramidal tract to the bulbar reticular formation. It is possible that it descends within the RJM-suppression areas in the mesencephalic, pontine, and bulbar reticular formation (Chandler and Tal, 1987;Chandler and Goldberg, 1988). The RJMsuppression area reported by Chandler and Tal (1987) may include not only the neurons and/or fibers involved in RJM suppression, but also those involved in RJM induction; since the effects of activated RJM-suppressive component may overwhelm those of activated RJM-inducing component, suppression of the CMA-induced RJMs may be produced as the overall result of simultaneous stimulation of these 2 components.…”

Localization of the central rhythm generator involved in spontaneous consummatory licking in rats: functional ablation and electrical brain stimulation studies.

“…6 and 7; for a review, see ref. 8) but can be modified within certain limits by the accessibility of the spout and the length of tongue travel required to reach the spout (9,10 (12,17,18). The neurons of these nuclei, the solitary nucleus, and the spinal trigeminal complex were identified as brain stem afferents to the hypoglossal nucleus (19).…”