Relationships between electrical activity in medullary vasomotor areas and reflex changes in peripheral vascular resistance were studied in dogs in which the hindquarters were autoperfused. Electrical stimulation through bipolar electrodes indicated a diffuse distribution of vasoactive points. Points yielding vasoconstrictor responses were found primarily in the rostral half of the medulla oblongata, while vasodilator points were located in the caudal and ventral portions of the medulla. Extensive crossing of vasomotor pathways was shown by the fact that stimulation of a single medullary point resulted in vascular resistance changes in both the ipsilateral and contralateral limbs. Stimulation of vasodilator points resulted in noncholinergic active vasodilatation in the hindlimb. The dilatation was considered to be active because it exceeded the sustained dilatation produced by lumbar sympathectomy and was reduced in magnitude by the antihistamine, ripelennamine. Discharge recorded through the electrodes from the same points which, when stimulated, gave vasomotor responses in the limb was larger in the vasoconstrictor point than in the vasodilator point. This difference may represent a central reflection of tonic sympathetic vasoconstrictor discharge. Reflex vasodilatation produced by intravenous norepinephrine was accompanied by a small but significant increase in activity recorded from vasodilator points. Other reflex stimuli, which included asphyxia and elevation and depression of carotid sinus pressure, elicited changes in electrical activity from the vasoconstrictor and vasodilator areas which, although small in magnitude, followed directions which were predicted on the basis of the existence of separate vasodilator and vasoconstrictor neurone pools. It was concluded that neurones exist in the medulla which probably play a role in exciting vasodilator pathways involved in reflex vasodilatation.
- © 1966 American Heart Association, Inc.