Localization of central rhythm generator involved in cortically induced rhythmical masticatory jaw-opening movement in the guinea pig

Abstract

The location of the central rhythm generator involved in the cortically induced rhythmical masticatory jaw-opening movement was studied in the ketamine-anesthetized guinea pig. These studies show that a population of neurons is activated by a nonrhythmical input from the cortical masticatory area (CMA) and produces a rhythmical output to the trigeminal motoneurons innervating the jaw-opening muscles. Repetitive stimulation (30 Hz) of the pyramidal tract (PT) rostral to the middle level of the medulla oblongata, in the animal with a precollicular transection as well as with an intact neuraxis, induced a rhythmical reciprocal EMG activity in the anterior digastric and masseter muscles. The rhythmical activity could be monitored by a rhythmical burst in the efferent discharge in the mylohyoid nerve innervating the anterior digastric muscle. Essentially the same pattern was observed when stimulating the PT as that induced by repetitive stimulation of the CMA. The rhythmical efferent burst in the mylohyoid nerve could still be induced after paralyzing the animal. Repetitive PT stimulation in the isolated brain stem after precollicular and bulbospinal transections induced a rhythmical pattern in the anterior digastric EMG and an efferent activity in the mylohyoid nerve. The rhythmical mylohyoid nerve burst could be induced after paralyzing the animal. After section of the medial part of the brain stem at the pontobulbar junction, including the PT, repetitive PT stimulation at the pontine level did not induce any masticatory activity either in the digastric EMG or in the efferent discharge in the mylohyoid nerve, while stimulation at the rostral bulbar level still induced a rhythmicity that was essentially the same pattern as before the section. By testing the effects of total and partial transections of the brain stem in coronal and sagittal planes at various locations, we found that the medial bulbar reticular formation, the lateral pons including the trigeminal motor nucleus and nerve, and the reticulotrigeminal motoneuronal pathways composed the minimum structures that must be left intact to induce a rhythmicity in the anterior digastric EMG and the efferent discharge in the mylohyoid nerve by repetitively stimulating the PT. Repetitive PT stimulation induced a field potential in the medial bulbar reticular formation, which periodically fluctuated in the masticatory rhythm coincident with the rhythmical activity in the digastric EMG. This fluctuation persisted in the same rhythm after paralysis of the animal.(ABSTRACT TRUNCATED AT 400 WORDS)

Overdrinking, swallowing inhibition, and regional brain responses prior to swallowing

“…The location of the pons activation instead corresponds approximately to the position of the facial nuclei and the oral part of the spinal trigeminal nucleus, two components of the central pattern generator (CPG) for mastication (58). The CPG contains neurons that, rather than swallowing, are primarily involved in rhythmic and repetitive orofacial movements involving the jaws and tongue (59). These movements also contribute to the oral preparatory phase of swallowing (51).…”

“…The pharyngeal and esophageal phases of swallowing, in comparison, are automatic, and the pharyngeal phase is irreversible when it has been initiated (53). If the inhibition were to act on the CPG implicated in the pons activation, for example via GABAergic or glycinergic inhibitory premotor neurons (60,61), it has interesting implications for the role of the prefrontal cortex during the preswallow period: does the frontal cortex modulate the CPG directly, as previous evidence implies (49), via projections to the facial (62) or peritrigeminal (59,63,64) regions? Or does it circumvent the influence of the inhibition by controlling the oral preparatory phase independently, via innervation of the orofacial region by the motor cortex (49,50,65,66)?…”

Classification of Masseter Activity Patterns during Chewing in Healthy Young Adults: The Effect of Taste Signals

“…Histologically, the neurons in the rostral nucleus of the tractus solitarius project their axons to the hypoglossal, facial, and likely trigeminal motor nuclei [18]. Physiological studies of the motor control of chewing show that the neural circuits in the reticular formation of the brainstem generate the basic masticatory pattern; these circuits are recognized as the masticatory central pattern generator [19]. Precise studies have provided evidence of the neural structures that control the rhythmic activation of chewing by sensory signals (including taste) from the orofacial region (e.g., [20,21].…”

The Effect of Masseter Activity Patterns during Chewing on Suprahyoid Activity in Subsequent Chewing Cycles

“…The lateral pterygoid and suprahyoid (SH; anterior belly of the digastric, geniohyoid and mylohyoid) muscles constitute the jaw opening muscles (or jaw openers), while the temporalis, masseter (Mass) and medial pterygoid muscles serve as the jaw closing muscles (or jaw closers). Chewing movements, which usually begin with jaw opening and are followed by jaw closing, have been noted among many mammalian species, including humans [1][2][3].…”

“…Two groups of nuclei, the paragigantocellular and gigantocellular reticular nuclei, and the medial bulbar reticular formation, located in the region of the medulla oblongata, are believed to play essential roles in the central pattern generator that controls the basic chewing rhythm [1][2][3]. Neurons of either or both nucleus groups send action potentials, via the parvocellular reticular nucleus, to the jaw opening and closing motoneurons, innervating the muscles responsible for chewing movement [4].…”