Profound hypotension and abolition of the vasomotor component of the cerebral ischemic response produced by restricted lesions of medulla oblongata in rabbit. Relationship to the so-called tonic vasomotor center

The authors studied the effect of bilateral electrolytic lesions in the medulla oblongata on the cerebral ischemic reflex in 24 anesthetized rabbits. In 15 animals lesions were placed in areas from which the differentiated vasomotor component of the response was elicited by electrical stimulation. In four rabbits (group A) the lesions entirely abolished the vasomotor, but not cardiac responses to cerebral ischemia, and resulted in an irreversible fall, to 30-40 mm Hg, of the arterial pressure (AP). These lesions destroyed large portions of the nucleus reticularis parvocellularis, the dorsal part of the nucleus reticularis gigantocellularis, and the ventromedial portion of the medial vestibular nuclei at the level of the inferior olive (3 mm rostral to the obex). In four other rabbits (group B) the pressor response was reduced to 25-67% of control with a fall of AP not as marked as that in group A. These lesions were within the same areas or very close to those of group A but smaller. In the remaining seven rabbits (group C) the lesions did not alter the ischemic response or AP; they were either restricted to the nucleus reticularis gigantocellularis alone, were smaller than in group B, or if large, were located several millimeters more rostral to those of group A. In nine control rabbits lesions placed elsewhere in the medulla failed to alter the ischemic response or resting AP. The authors conclude that the vasomotor, but not the cardiac or respiratory, components of the cerebral ischemic response depend upon a restricted portion of the bulbar reticular formation. Moreover, the integrity of this region is essential for maintenance of normal resting levels of AP and, hence, appears to function as the so-called tonic vasomotor center of the brainstem.