Location & Afferent Pathways
- Aortic Arch → Transmits signals via the vagus nerve (CN X) to the solitary nucleus of the medulla.
- Responds to increased BP only (not decreases).
- Carotid Sinus (dilated region near carotid bifurcation) → Transmits via the glossopharyngeal nerve (CN IX) to the solitary nucleus of the medulla.
- Responds to both increases and decreases in BP.
Baroreceptor Reflex Responses
- Hypotension Response (↓ BP)
- ↓ Arterial pressure → ↓ Stretch → ↓ Baroreceptor firing
- ↑ Sympathetic outflow (via spinal cord) → Vasoconstriction, ↑ HR, ↑ Contractility → Restores BP
- Important in compensating for hypovolemic shock.
- Carotid Massage Response
- External pressure on carotid sinus → ↑ Stretch → ↑ Baroreceptor firing
- ↑ Vagal output → ↑ AV node refractory period → ↓ HR → ↓ CO
- Used to terminate supraventricular tachycardia (SVT) but may cause presyncope/syncope in susceptible individuals.
- Cushing Reflex (Response to ↑ Intracranial Pressure)
- Triad: Hypertension, bradycardia, respiratory depression
- ↑ ICP → Cerebral ischemia → ↑ Sympathetic outflow (hypertension) → Reflex bradycardia via baroreceptors
Chemoreceptors: Chemical Sensors Regulating Ventilation
Peripheral Chemoreceptors (Carotid & Aortic Bodies)
- Located near carotid bifurcation (carotid body) and aortic arch (aortic body).
- Stimulated by:
- ↑ PCO₂ (hypercapnia)
- ↓ pH (acidosis)
- ↓ PO₂ (<60 mmHg, hypoxia)
- Increase respiratory drive to correct abnormalities.
Central Chemoreceptors (Brainstem Medulla)
- Located in medulla, bathed in CSF.
- Directly respond to ↑ PCO₂ and ↓ pH in brain interstitial fluid.
- Do not respond directly to PO₂ changes (unlike peripheral chemoreceptors).
- Chronic CO₂ retention (e.g., COPD) → Central receptors become less responsive, increasing dependence on peripheral chemoreceptors for respiratory drive.
Summary Table: Baroreceptors vs. Chemoreceptors
| Feature | Baroreceptors | Chemoreceptors |
|---|---|---|
| Function | Detects BP changes (mechanoreceptors) | Detects chemical changes in blood (chemoreceptors) |
| Location | Carotid sinus, aortic arch | Carotid body, aortic body (peripheral); Medulla (central) |
| Stimuli | Arterial stretch (BP changes) | Peripheral: ↓ O₂, ↑ CO₂, ↓ pH Central: ↑ CO₂, ↓ pH in CSF |
| Response | Adjusts HR & BP via autonomic system | Adjusts ventilation via respiratory centers |