Baroreceptors are stretch-sensitive mechanoreceptors, meaning they respond to changes in the tension of vessel walls caused by fluctuations in blood pressure. When arterial pressure increases, the walls of the carotid sinuses and aortic arch stretch, triggering the baroreceptors to increase their firing rate. Conversely, when blood pressure decreases, the vessel walls relax, and the firing rate of baroreceptors decreases. This feedback system allows for rapid adjustments to maintain cardiovascular stability. The carotid sinus receptors operate over a mean arterial pressure range of roughly 60 to 180 mmHg, with their steepest response, and therefore the greatest reflex gain, near the normal resting pressure of about 100 mmHg. Below about 50 to 60 mmHg they fall silent and above about 180 mmHg their firing saturates, so the reflex can no longer buffer pressure at either extreme; the aortic arch receptors have a somewhat higher threshold. Pulsatile pressure is a stronger stimulus than the same mean pressure without a pulse, and the receptors fire most during the systolic upstroke.
Baroreceptors transmit information to the brain through two different types of nerve fibers: A-fibers and C-fibers. A-fibers are large, myelinated, and responsible for fast, dynamic responses to rapid changes in blood pressure, making them essential for second-to-second regulation. C-fibers, which are smaller and unmyelinated, contribute to the tonic, long-term regulation of blood pressure by providing baseline input to the central nervous system; they are more numerous than A-fibers, have a higher pressure threshold, and keep firing at sustained high pressures after the A-fibers have adapted. The A-fibers are crucial for handling immediate demands like standing up from a seated position or responding to exercise, while the C-fibers help maintain overall cardiovascular stability under normal conditions.
