Snapshot A 20-year-old woman presents to the emergency room for palpitations. She has previously had 1 or 2 episodes of this in the past few months. She reports that coughing or performing the Valsalva maneuver resolves the palpitations. However, this time the maneuver had no effect. Her pulse is 184/min, blood pressure is 100/60 mmHg, and respirations are 20/min with O2 saturation of 99% on room air. The physician presses on her neck and her pulse returns to 80/min. (Carotid massage) Introduction Mean arterial pressure is in part maintained by baroreceptors and chemoreceptors both function as part of an afferent system These peripheral receptors are located at the aortic arch transmits via vagus nerve (cranial nerve [CN] X) to the solitary nucleus of the medulla carotid sinus where common carotids bifurcate transmits via carotid sinus nerve and glossopharyngeal nerve (CN IX) to the solitary nucleus of the medulla Baroreceptors Baroreceptors and mechanoreceptors they respond to ↑ or ↓ in pressure or stretch the strongest stimulus is a rapid change in arterial pressure a change in pressure or stretch causes a change in membrane potential this triggers action potentials in the afferent nerves that travel to the brain stem sensitivity of baroreceptors can be changed by chronic diseases Carotid sinus responds to ↑ or ↓ in arterial pressure Aortic arch responds to primarily ↑ in arterial pressure Baroreceptor reflex a neutrally-mediated reflex that attempts to keep the arterial pressure constant via the sympathetic and parasympathetic nervous systems response to increased arterial pressure ↑ blood pressure is sensed by baroreceptors ↑ pressure = ↑ stretch ↑ firing rate of carotid sinus nerve (which connects to the glossopharyngeal nerve) and afferent vagus nerve solitary nucleus of the medulla receives this information ↑ efferent parasympathetic outflow to the sinoatrial node ↓ heart rate ↓ efferent sympathetic outflow ↓ cardiac contractility and ↓ heart rate → ↓ cardiac output (CO) ↓ vasoconstriction → ↓ total peripheral resistance (TPR) recall that arterial pressure = CO x TPR once the arterial pressure reaches the homeostatic pressure, baroreceptor activity will return to baseline level response to hemorrhage acute ↓ blood pressure is sensed by baroreceptors ↓ pressure = ↓ stretch ↓ firing of afferent nerves CN IX and CN X solitary nucleus of the medulla receives this information ↓ parasympathetic outflow ↑ heart rate ↑ sympathetic outflow ↑ cardiac contractility ↑ heart rate ↑ vasoconstriction ↑ arterial pressure valsalva maneuver ↑ intrathoracic pressure → ↓ venous return to heart → ↓ CO ↓ arterial pressure baroreceptor reflex kicks in to ↑ arterial pressure (as described above) carotid massage ↑ pressure on carotid artery → ↑ stretch → ↑ firing of baroreceptors ↑ atrioventricular node refractory period ↓ heart rate ↓ sympathetic tone in vasculature carotid sinus hypersensitivity refers to increased sensitivity of these receptors to blood pressure in the carotid arteries can cause carotid sinus syncope Chemoreceptors Chemoreceptors sense levels of oxygen, carbon dioxide, and pH Peripheral chemoreceptors carotid bodies in the carotid sinus and aortic bodies along the aortic arch sensitive to ↓ partial pressure of oxygen (PO2) (< 60 mmHg) ↓ pH ↑ partial pressure of CO2 (PCO2) chemoreceptors are more sensitive to changes in PO2 if ↑ PCO2 or ↓ pH response to ↓ arterial PO2 ↑ firing of afferent nerves ↑ sympathetic outflow ↑ vasoconstriction ↑ parasympathetic outflow ↓ heart rate (transient) ↑ ventilation Central chemoreceptors located in the medulla sensitive to ↑ or ↓ in PCO2 or pH reponse to hyperventilation ↑ respiratory rate eliminates CO2 from the body → ↓ in PCO2 → ↑ vasoconstriction in the brain → ↓ cerebral blood volume → ↓ intracranial pressure response to brain ischemia ↓ pH and ↑ PCO2 immediately ↑ sympathetic outflow ↑ vasoconstriction → ↑ TPR blood flow shunted to the brain to maintain perfusion Cushing Reaction Cushing reaction a triad of hypertension, bradycardia, and respiratory depression combination of both cerebral chemoreceptors to maintain cerebral blood flow and baroreceptors to induce reflex bradycardia ↑ Intracranial pressure constricts vessels → cerebral ischemia ↓ pH and ↑ PCO2 → ↑ firing of chemoreceptors ↑ Sympathetic outflow → ↑ TPR → ↑ arterial pressure hypertension → ↑ stretch → ↑ firing of baroreceptors reflex bradycardia