The primary integrating center for the baroreceptor reflexes is a diffuse network of highly interconnected neurons called the medullary cardiovascular center, located in the brainstem medulla oblongata. The neurons in this center receive input from the various baroreceptors. This input determines the action potential frequency from the center along neural pathways that terminate upon the cell bodies and dendrites of the vagus (parasympathetic) neurons to the heart and the sympathetic neurons to the heart, arterioles, and veins. When the arterial baroreceptors increase their rate of discharge, the result is a decrease in sympathetic outflow to the heart, arterioles, and veins, and an increase in parasympathetic outflow to the heart. A decrease in baroreceptor firing rate results in just the opposite pattern.
As parts of the baroreceptor reflexes, angiotensin II generation and vasopressin secretion are also altered so as to help restore blood pressure. Thus, decreased arterial pressure elicits increased plasma concentrations of both these hormones, which raise arterial pressure by constricting arterioles. For simplicity, however, we focus in the rest of this chapter mainly on the sympathetic nervous system when discussing reflex control of arterioles. The roles of angiotensin II and vasopressin will be described further in Chapter 14 in the context of their effects on salt and water balance via the kidneys.