chemoreceptor

Chemoreceptors are essential components of the autonomic reflex arcs that maintain homeostasis of blood gases and pH. Peripheral chemoreceptors are located in the carotid bodies (at the bifurcation of common carotid arteries, innervated by CN IX/glossopharyngeal) and aortic bodies (aortic arch, innervated by CN X/vagus). These are primarily sensitive to decreased PaO2 (significant activation typically below 60 mmHg), and also respond to increased PaCO2 and decreased pH, transmitting signals to the medullary respiratory centers to increase ventilation. Central chemoreceptors, located near the ventral surface of the medulla, are highly sensitive to changes in CSF pH, which reflects arterial CO2 levels since CO2 crosses the blood-brain barrier and combines with water to form carbonic acid, dissociating into H+ and HCO3-. Central chemoreceptors are the primary drivers of minute-to-minute ventilatory control under normal conditions, responding to even small increases in PaCO2. Clinically, chemoreceptor function is crucial in conditions such as COPD, where chronic CO2 retention can blunt central chemoreceptor sensitivity, causing patients to rely on hypoxic drive (peripheral chemoreceptors) for ventilatory stimulation—this has implications for oxygen therapy in COPD patients, where excessive supplemental O2 can suppress the hypoxic drive and lead to hypoventilation. Chemoreceptors also play a role in cardiovascular reflexes, contributing to the chemoreceptor reflex that increases sympathetic outflow, heart rate, and blood pressure in response to hypoxia. Carotid body tumors (paragangliomas) are rare but important clinical entities to recognize on imaging. Additionally, chemoreceptor trigger zone (CTZ) in the area postrema of the medulla, though distinct from respiratory chemoreceptors, is involved in triggering vomiting in response to circulating toxins, drugs (e.g., opioids, chemotherapy), and metabolic disturbances, and is a key target for antiemetic drugs.