Various patterns of the mandibular movement were induced by repetitive electrical stimulation of the cerebral cortex and subcortical structures in the rabbit. Substantia nigra(SN) was important on rnotor function related to the execution of jaw movements, but the role of SN has been controversial. This experiment was performed to clarify the modulatory effect of SN on mastication.
The experimental animals were anesthetized with urethane and tracheal cannula was inserted. Wire electrodes were inserted into both masseter muscles and anterior belly of digastric muscles for recording of EMGs. The head was placed to stereotaxic apparatus and bipolar electrode was fixed to the right cortical jaw motor area and SN, and threshold intensity was determined. In some experiments, SN was destructed by lesion generator or the brain was decerbrated. The jaw movement was evoked by electrical stimulation of cortex, SN or both simultaneously and recorded by using photo - sensitive detector. All patterns of jaw movements and EMGs activity were recorded with magnetic tape recorder and compared before and after destruction of the SN or decerebration.
The cortical stimulation produced prominent vertical and lateral movement. But the cortically-induced mastication was decreased by destruction of SN. The SN stimulation in normal rabbit mainly produced vertical movement. In decerebrated animal, however, vertical and lateral movements evoked by SN stimulation were markedly changed and masseter and digastric EMGs were reduced, especially, masseter EMGs showed tonic activity. Simultaneous stimulation of cerebral cortex and SN with subthreshold intensity could induce masticatory movements. At simultaneous stimulation of cerebral cortex and SN with subthreshold and threshold intensity, the pattern of jaw movement depended upon higher intensity.
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