“Baro-“ means pressure or stretch, so baroreceptors are special nerve cells or receptors that sense blood pressure, by the way that the walls of the blood vessels stretch. That information is sent from the baroreceptors to the brain to help keep blood pressure balanced.
Alright, baroreceptors are actually groups of nerve endings located within the blood vessel walls. and they can be classified into two types based on their location: the arterial ones and the cardiopulmonary ones. The arterial baroreceptors can be found on the wall of the aortic arch as well as on the wall of the carotid sinus, which is basically a bulge of the internal carotid artery just above its split from the common carotid artery in the neck. In the aortic arch, these nerve endings join up to form the vagus, or tenth (X) cranial nerve, and in the carotid sinus, they form the glossopharyngeal, or ninth (IX) cranial nerve. Both of these cranial nerves travel up towards the brainstem, carrying information about the stretch they sense in the arteries. They synapse at the nucleus tractus solitarius in the medulla oblongata of the brainstem, which then relays the information to the cardiovascular centers. The cardiovascular centers are areas in the lower one-third of the pons and medulla oblongata of the brainstem, responsible for the autonomic or involuntary control of the cardiac and vascular function. They do that by coordinating the sympathetic and parasympathetic branches of the autonomic nervous system. There are two main cardiovascular centers - the first is the vasomotor control center, which controls the diameter of the blood vessels, using the sympathetic nerve fibers to cause vasoconstriction. The second is the cardiac control center, which is further divided into the cardiac accelerator and cardiac decelerator centers. The cardiac accelerator center speeds up the heart rate and increases cardiac contractility through the sympathetic outflow tract, while the cardiac decelerator center slows down the heart rate through the parasympathetic outflow tract. Notice that both the sympathetic and parasympathetic system affect the heart rate, but that only the sympathetic system has an effect on the diameter of the blood vessels and the contractility of the heart muscle. This whole process is known as the baroreceptor reflex, or baroreflex in short, and takes place in seconds to minutes, allowing us to rapidly adjust our blood pressure.