BRAIN STEM CONTROL OF POSTURE AND LOCOMOTION

Within the brain stem of the cat, two locomotor centers have already been identified: one is the subthalamic locomotor region (SLR) and the other mesencephalic locomotor region (MLR) . Anatomically, the SLR and the MLR corresponds to the lateral hypothalamic area and the nucleus cuneiformis, respectively. The SLR presumably controls'volitional'aspects of locomotion while the MLR controls'automatic'aspects of it such as interlimb coordination. In a precollicular-postmammillary decerebrate cat, repetitive microstimulation of the MLR evoked locomotion on a moving treadmill.In addition to these locomotor centers, midpontine neuronal structures which subserve'setting'and'resetting'of postural muscle tone have recently been identified in the same preparation. Repetitive microstimulation of the dorsal part and the ventral part of the midpontine tegmentum along the midline evoked long-lasting suppression and augmentation of postural muscle tone, respectively. Paired stimulation of the MLR with the dorsal part changed the pattern of MLR evoked locomotion from well coordinated four-legged locomotion to hindlimb stepping, and from stepping to total suppression of locomotor movements with a substantial decrease in postural muscle tone. In contrast, paired stimulation of the MLR with the ventral part changed the locomotor pattern from total suppression to hindlimb stepping, from stepping to locomotion, and even from locomotion to a gallop, by increasing the level of postural muscle tone in a graded manner. Stimulation of the ventral part alone at a higher stimulus intensity evoked spastic locomotor movements associated with a substantial increase in extensor muscle tone.During spontaneous locomotion in an intact, unrestrained cat, stimulation of the dorsal part of the midpontine tegmentum evoked a series of postural changes. Within a few seconds from the beginning of stimulation, the cat ceased to walk but maintained a standing posture with a locomotor figure. With continuation of this stimulation, it squatted and then lay down on the floor in a sequential manner. The cat continued to keep its last posture for several minutes even after termination of the stimulation. Stimulation of the ventral part of the midpontine tegmentum evoked almost an opposite series of postural changes. Within a few seconds from the beginning of stimulation, the cat changed from a lying to a squatting posture, and then started to walk during continuation of this stimulation. The cat was alway