The effects of brain stem transections on the neuronal networks responsible for rhythmical jaw muscle activity in the guinea pig.

The purpose of this study was to determine the critical areas of brain stem necessary for the production of cortically induced rhythmical jaw muscle activity (RJMs) in the anesthetized guinea pig. It was found that longitudinal midline transections of the lower brain stem starting at the obex and extending to the rostral third of the inferior olivary nucleus (IO) were able to abolish rhythmical EMG activity in the jaw-opener muscle (digastric) on the same side (ipsilateral) as the cortical stimulus. Under these conditions, rhythmical activity in the contralateral digastric (DIG) EMG was not affected. Midline transections extending from the rostral superior colliculus to 500 microns rostral to the trigeminal motor nucleus had no effect on cortically evoked bilateral DIG rhythmical EMG activity. Serial transverse hemisections of the left side of the medulla, starting at the obex and extending to the rostral third of the IO, reduced the amplitude of the left DIG EMG without producing significant effects in the cycle characteristics [cycle duration (CD) or burst duration (BD)] of that muscle during stimulation of the contralateral (right) cortex. Hemisections more rostral to the IO completely abolished bilateral rhythmical DIG activity induced by stimulation of the cortex on the side opposite to the transection (right cortex). Under these conditions, stimulation of the cortex on the side ipsilateral to the left hemisection (left cortex) initiated rhythmical EMG activity with normal cycle characteristics in only the contralateral (right) DIG muscle. Transverse hemisections as little as 500 microns rostral to the trigeminal motor nucleus (Mot V), which spared the pyramidal tract, had no effect on RJMs induced by stimulation of either cortex. These data suggest that each ipsilateral cortex initiates activity in neuronal oscillatory networks located exclusively in the contralateral brain stem; bilateral rhythmical DIG activity is produced by neurons in the contralateral brain stem; each side of the lower brain stem is capable of producing rhythmical DIG activity independent of the integrity of the other side; and the location of the neuronal oscillators responsible for RJMs is between the rostral IO and the trigeminal motor nucleus.