Anatomy and Topography
The carotid sinus is located in the anterior neck region at the carotid bifurcation (bifurcatio carotidis), where the common carotid artery branches into the internal carotid artery (a. carotis interna) and external carotid artery (a. carotis externa). Distal to this dilation, the internal carotid artery enters the carotid canal (canalis caroticus) of the temporal bone.
- Skeletotopy: The bifurcation and sinus project at the level of the superior border of the thyroid cartilage (cartilago thyroidea), corresponding to the C4 cervical vertebra.
- Carotid sheath (vagina carotica): Medially lies the trachea and esophagus, laterally lies the internal jugular vein (v. jugularis interna), and posteriorly between them runs the vagus nerve.
- Adjacent structures: Situated on the posterior or medial wall of the bifurcation, adjacent to the sinus, is the carotid body (glomus caroticum), a small chemoreceptor cluster connected to the arterial wall by connective tissue.
Receptor Types
The wall of the carotid sinus contains specialized mechanoreceptors and sensory endings:
- Baroreceptors: Sensitive to stretching caused by changes in blood pressure.
- Osmoreceptors: Responsive to changes in plasma osmolarity.
- Volume receptors: Responsive to changes in circulating blood volume and vascular stretch.
Function and Regulatory Mechanism
Blood pressure fluctuations are normally regulated via baroreceptor reflex mechanisms:
- When arterial blood pressure increases, the firing rate of the baroreceptors increases.
- Afferent depressor signals are sent to the vasomotor center in the medulla oblongata.
- The balance between central excitatory drives and inhibitory afferent input from the baroreceptors maintains normal systemic blood pressure. Conversely, a drop in blood pressure decreases afferent firing.
Innervation
The carotid sinus features a dense periadventitial nerve plexus. Its sensory innervation is supplied by:
- Hering's nerve (sinus nerve), a branch whose fibers travel centrally within the glossopharyngeal nerve (n. glossopharyngeus, cranial nerve IX).
- Branches of the vagus nerve (n. vagus, cranial nerve X).
- Fibers from the sympathetic trunk.
Clinical Significance
- Reflex responses: Mechanical stimulation or hypersensitivity of the carotid sinus can lead to pronounced reflex bradycardia and hypotension (carotid sinus hypersensitivity).
- Blood pressure regulation pathology: Chronic stress increases mechanical load on the baroreceptors of the sinocarotid zone, which can impair central blood pressure regulation and chronically activate the sympathoadrenal system.
- Surgery and imaging: The common carotid artery bifurcation at the C4 level serves as a key surgical landmark for carotid endarterectomy and vascular access. The anatomy of the common, external, and internal carotid arteries, along with the carotid sinus, is clearly visualized using contrast-enhanced MR angiography.