DECUSSATION OF THE PATHWAYS IN THE CAROTID SINUS CARDIOVASCULAR REFLEX: AN EXAMPLE OF THE PRINCIPLE OF CONVERGENCE

Abstract

The isolated carotid sinus was stimulated by sudden changes in perfusion pressure in dogs anesthetized with nembutal. It is shown in unilaterally vagotomized and completely sympathectomized dogs, in which the complication due to sympathetic inhibition is eliminated, that on distention of the carotid sinus on the vagotomized side, there is a small crossed component to the opposite vagus as indicated by slight cardiac slowing. The sinus vasomotor reflex has a large crossed component; this is demonstrated on hemisympa-thectomized dogs in which stimulation of either sinus produces a marked depressor response. The fact that hemisection of the spinal cord at C2 in a hemisympathectomized dog most markedly affects the depressor response on the same side as the cord hemisection indicates that the major part of the decussation occurs at a level below C2. The principle of facilitation is demonstrated in the sinus vagal response since the bradycardia following bilateral distention is invariably greater than the sum of the reductions in heart rate obtained from the 2 sinuses distended separately. The principle of occlusion holds in the sinus vasomotor reflex since the fall in blood pressure which occurs on bilateral sinus distention is very much less than the sum of the individual vasomotor responses. This occlusion phenomenon occurs in the C. N. S. because it is effected through the mechanism of active inhibition.