carotid sinus

The carotid sinus is located at the bifurcation of the common carotid artery, primarily at the base of the internal carotid artery. It contains stretch-sensitive baroreceptors (mechanoreceptors) that respond to arterial wall distension caused by changes in blood pressure. These baroreceptors are innervated by the glossopharyngeal nerve (CN IX) via the sinus nerve of Hering, which carries afferent signals to the nucleus tractus solitarius (NTS) in the medulla.

Physiology: When blood pressure increases, the carotid sinus stretches, increasing baroreceptor firing rate. This signals the medulla to increase parasympathetic (vagal) output and decrease sympathetic output, resulting in decreased heart rate, decreased contractility, and vasodilation—lowering blood pressure back toward baseline (negative feedback loop). Conversely, decreased blood pressure reduces baroreceptor firing, leading to increased sympathetic tone and decreased parasympathetic tone, raising heart rate and vascular resistance.

Clinical significance: 1. Carotid sinus massage: Used diagnostically and therapeutically to terminate supraventricular tachycardias (SVT) by stimulating vagal tone, slowing AV node conduction. Contraindicated in patients with carotid bruits or history of stroke/TIA due to risk of dislodging atherosclerotic plaque. 2. Carotid sinus hypersensitivity/syndrome: An exaggerated response to carotid sinus stimulation (e.g., tight collar, head turning) causing bradycardia, hypotension, and syncope—important cause of syncope in elderly patients. 3. Carotid sinus baroreceptor dysfunction: Seen in conditions like diabetic autonomic neuropathy, leading to orthostatic hypotension. 4. Anatomically distinct from carotid body, which contains chemoreceptors sensitive to PaO2, PaCO2, and pH changes (important in hypoxia/hypercapnia response), innervated also by CN IX.

Distinguishing carotid sinus (baroreceptor, pressure-sensing) from carotid body (chemoreceptor, chemical-sensing) is a classic board question point.