How does the mechanism which controls blood pressure in the brain work?
Blood pressure is sensed in blood vessels by baroreceptors. Baroreceptors are stretch-sensitive nerve fibers located primarily in the aortic arch and carotid sinuses. The baroreceptors send afferent fibers via the glossopharyngeal nerve to the nucleus tractus solitarii in the dorsal medulla in the brainstem. From there, efferent cardiovascular neurons send projections to the medulla and spinal cord. There are also stretch-sensitive receptors in the heart and pulmonary vessels, called cardiopulmonary receptors that use the same nerural connections as the baroreceptors. 

The baroreflex loop results in activation of sympathetic or parasympathetic fibers to the heart, the smooth muscle of the peripheral blood vessels, and other organs such as the kidney to maintain arterial pressure at normal levels.

In a simplified scheme, increased pressure stimulates baroreceptors, which attenuates the sympathetic outflow to the peripheral vessels and the heart, restoring pressure to normal levels. The parasympathetic influence will dominate which is mediated by acetylcholine. Conversely, a decrease in pressure relieves the baroreceptors and increases sympathetic outflow. Sympathetic activation causes a release of noradrenaline that leads to vasoconstriction and increased cardiac output and hence an increased blood pressure (see Fig. 1). 


Fig. 1. Control of blood pressure. Source: Human Physiology (2011). 

Reference
- Cougias et al., Med Sci Monit (2010); 16(1): RA1–RA8