I understand that the parasympathetic system is generally inhibitory and its primary function is to decrease HR, decrease the force of contraction and cause vasoconstriction. The sympathetic system, on the other hand, does the opposite and is generally acceleratory with its primary function to increase HR (fight-or-flight) and cause vasodilation. Where I get confused is when baroreceptors and BP are involved. I have read that an increased BP will result in the stimulation of the PARASYMPATHETIC system and, therefore, cause VASODILATION in order to decrease BP. I get the vasodilation part but wouldn’t that be coming from the SYMPATHETIC system rather than the Parasympathetic system?

The sympathetic system, through the release of the neurotransmitter norepinephrine, causes a tonic sympathetic vasoconstriction of arteries and veins and thus accelerates heart rate (HR). This tonic constriction of blood vessels is referred to as vasomotor or vascular tone.

Mean arterial pressure is regulated by a basic scheme of a feedback control system consisting of pressure sensors and effector mechanisms that can alter the blood pressure (BP). If BP becomes altered, for instance during hemorrhage, a sensor (in this case a baroreceptor) senses the reduction in pressure and activates an effector mechanism to return BP to normal. This response is termed negative feedback control.

The primary reflex that helps to control BP on a beat-to-beat basis is the baroreceptor reflex. Activation of the baroreceptors (by increases in pressure or increased stretch of the vessel walls) results in a decrease in sympathetic activity. This decrease in sympathetic activity reduces the vasomotor tone causing dilation of the blood vessels and slows down the HR – both these actions help to lower BP back to normal.

Furthermore, in addition to decreasing sympathetic activity, activation of the baroreceptors increases parasympathetic nerve activity to the heart which helps to slow HR. If BP decreases, the level of baroreceptor activation decreases which results in less inhibition of sympathetic activity and less activation of parasympathetic activity. This results in increased vasomotor tone and HR which help to elevate blood pressure back to normal.

Finally, the sympathetic and parasympathetic nervous systems act reciprocally to control BP – so that an increase in one system is accompanied by a decrease in the other system.

Reference 1: Raff H. Physiology Secrets. Questions and Answers Reveal the Secrets to Understanding Physiology. 2003. 2nd Edition. Hanley & Belfus Inc Medical Publishers. II Organ System Physiology. Chapter 4: Cardiovascular Physiology. Pg 84 – 85.