THE CENTRAL PATHWAY FOR THE JAW-JERK

Abstract

With the Horsley-Clarke instrument and cathode ray oscillograph the mesencephalic root of the 5th cranial nerve was located, and the root stimulated or destroyed with faradic or direct current. Animals with mesencephalic roots damaged or destroyed were later decerebrated under ether at the thalamic level and the jaw reflexes studied. The brain of each animal was studied microscopically and the site of the lesion correlated with changes in reflex activity. In none of the animals was there damage to the motor nucleus, chief sensory nucleus or spinal nucleus of the fifth nerve. Lesions were placed on one side only and the reflexes of the normal side were used as a control. Muscular activity was determined by recording action potentials from the masseter muscle. Jaw opening reflexes to faradic stimulation of gums and palate were not depressed by lesions in the mesencephalic root. Jaw closure to tapping the teeth was abolished by lesions in the caudal part of the mesencephalic root. The jaw-jerk, a stretch reflex, was abolished by complete destruction of the homolateral mesencephalic root and was depressed by moderate damage to the root. Undamaged caudal parts of the root and nucleus were adequate to maintain a depressed jaw-jerk. Single faradic stimuli to the intact mesencephalic root evoked a brief jaw closure similar to the jaw-jerk. Since the decrease in response time was about one msec, when the stimulating electrode was moved from the mesencephalic root to the fifth motor nucleus it appears that the collateral fibers from the mesencephalic root to the motor nucleus may make direct connections with the motor cells without interpolation of interneurones.