Baroreceptors and Chemoreceptors

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 O
2 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 chemoreceptorsboth function as part of an afferent systemThese peripheral receptors are located at theaortic archtransmits via vagus nerve (cranial nerve [CN] X) to the solitary nucleus of the medullacarotid sinuswhere common carotids bifurcatetransmits via carotid sinus nerve and glossopharyngeal nerve (CN IX) to the solitary nucleus of the medulla

Presentation

Baroreceptors and chemoreceptors are specialized sensory receptors that play crucial roles in maintaining homeostasis within the body. Baroreceptors are primarily responsible for monitoring changes in blood pressure, while chemoreceptors detect alterations in the chemical composition of the blood.

Baroreceptors are found in the walls of blood vessels, particularly in the carotid sinuses and aortic arch. They detect changes in blood pressure and send signals to the brain, specifically to the cardiovascular control center in the medulla oblongata. In response to elevated blood pressure, baroreceptors trigger a decrease in heart rate and dilatation of blood vessels, promoting vasodilation and lowering blood pressure. Conversely, if blood pressure drops, baroreceptors stimulate an increase in heart rate and vasoconstriction to elevate blood pressure.

Baroreceptors

Baroreceptors and mechanoreceptorsthey respond to ↑ or ↓ in pressure or stretchthe strongest stimulus is a rapid change in arterial pressurea change in pressure or stretch causes a change in membrane potentialthis triggers action potentials in the afferent nerves that travel to the brain stemsensitivity of baroreceptors can be changed by chronic diseasesCarotid sinusresponds to ↑ or ↓ in arterial pressure Aortic archresponds to primarily ↑ in arterial pressureBaroreceptor reflex a neutrally-mediated reflex that attempts to keep the arterial pressure constant via the sympathetic and parasympathetic nervous systemsresponse 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 nervesolitary 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 TPRonce the arterial pressure reaches the homeostatic pressure, baroreceptor activity will return to baseline levelresponse to hemorrhage  acute ↓ blood pressure is sensed by baroreceptors↓ pressure = ↓ stretch↓ firing of afferent nerves CN IX and CN Xsolitary nucleus of the medulla receives this information↓ parasympathetic outflow

↑ heart rate↑ sympathetic outflow

↑ cardiac contractility↑ heart rate↑ vasoconstriction↑ arterial pressurevalsalva maneuver↑ intrathoracic pressure → ↓ venous return to heart → ↓ CO↓ arterial pressurebaroreceptor 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 vasculaturecarotid sinus hypersensitivity refers to increased sensitivity of these receptors to blood pressure in the carotid arteriescan cause carotid sinus syncope

Chemoreceptors

Chemoreceptors sense levels of oxygen, carbon dioxide, and pHPeripheral chemoreceptorscarotid bodies in the carotid sinus and aortic bodies along the aortic archsensitive to↓ partial pressure of oxygen (PO
2) (< 60 mmHg)↓ pH↑ partial pressure of CO
2 (PCO
2)chemoreceptors are more sensitive to changes in PO
2 if ↑ PCO
2 or ↓ pHresponse to ↓ arterial PO
2↑ firing of afferent nerves↑ sympathetic outflow

↑ vasoconstriction↑ parasympathetic outflow

↓ heart rate (transient)↑ ventilationCentral chemoreceptorslocated in the medullasensitive to ↑ or ↓ in PCO
2 or pHreponse to hyperventilation ↑ respiratory rate eliminates CO
2 from the body → ↓ in PCO
2 → ↑ vasoconstriction in the brain → ↓ cerebral blood volume → ↓ intracranial pressureresponse to brain ischemia↓ pH and ↑ PCO
2 immediately↑ sympathetic outflow↑ vasoconstriction → ↑ TPRblood flow shunted to the brain to maintain perfusion

Treatment

Treatment related to baroreceptors and chemoreceptors depends on the specific condition or dysfunction being addressed. However, here are a few examples:

1. Baroreceptor stimulation therapy: This involves the use of an implanted device to electrically stimulate the baroreceptors, aiming to lower blood pressure in individuals with hypertension.
2. Medications: Pharmacological interventions may be prescribed to regulate blood pressure, targeting the baroreceptor reflex or other mechanisms involved in blood pressure control.
3. Oxygen therapy: In cases where chemoreceptor dysfunction leads to low oxygen levels, supplemental oxygen therapy may be administered to improve oxygenation and alleviate symptoms.
4. Treatment of underlying conditions: If baroreceptor or chemoreceptor dysfunction is secondary to an underlying condition such as heart disease or respiratory disorders, treating the primary condition can help restore normal receptor function.

It is important to note that treatment plans should be individualized based on a person’s specific medical history, symptoms, and diagnostic findings. Consulting with a healthcare professional is crucial to determine the most appropriate treatment approach for any specific baroreceptor or chemoreceptor-related condition.

Cushing Reaction

Cushing reaction a triad of hypertension, bradycardia, and respiratory depressioncombination of both cerebral chemoreceptors to maintain cerebral blood flow and baroreceptors to induce reflex bradycardia↑ Intracranial pressure constricts vessels → cerebral ischemia↓ pH and ↑ PCO
2 → ↑ firing of chemoreceptors↑ Sympathetic outflow → ↑ TPR → ↑ arterial pressurehypertension → ↑ stretch → ↑ firing of baroreceptorsreflex bradycardia

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