Learning Objectives
Map the afferent pathways of the aortic arch and carotid sinus to the medulla. Distinguish between peripheral and central chemoreceptor triggers. Analyze the baroreceptor reflex arc in response to hypotension and carotid hypersensitivity. Explain the pathophysiology of the Cushing reflex triad.
Neuroanatomy of Baroreceptors
Baroreceptors are stretch receptors located in high-pressure areas that transmit signals to the Nucleus Tractus Solitarius (NTS) in the medulla.
- Carotid Sinus: Located at the bifurcation of the common carotid arteries. Transmits via the Glossopharyngeal nerve (CN IX).
- Aortic Arch: Transmits via the Vagus nerve (CN X).
- Central Integration: Both pathways terminate in the medulla to regulate sympathetic and parasympathetic output.
Chemoreceptors: Peripheral vs. Central
These receptors monitor blood chemistry to drive respiratory and cardiovascular adjustments.
| Type | Location | Primary Stimuli |
|---|---|---|
| Peripheral | Carotid and Aortic Bodies | ↓ PO2 (< 60 mmHg), ↑ PCO2, ↓ pH |
| Central | Medulla | ↓ pH and ↑ PCO2 of brain interstitial fluid (H+ cannot cross BBB) |
Note: Central chemoreceptors do not respond to PO2. In COPD, they become desensitized, making respiration dependent on peripheral O2 sensing.
The Baroreceptor Reflex in Action
- Response to Hypotension: ↓ Arterial pressure → ↓ stretch → ↓ afferent firing → ↑ sympathetic efferent firing → Vasoconstriction, ↑ HR, ↑ contractility.
- Carotid Sinus Hypersensitivity: Triggered by shaving or tight collars → ↑ pressure → ↑ afferent firing → ↑ AV node refractory period → ↓ HR and ↓ CO. Can cause syncope.
The Cushing Reflex
A high-yield triad indicating severely increased intracranial pressure (ICP):
- Hypertension: Brain ischemia drives a massive sympathetic surge to maintain perfusion.
- Bradycardia: Reflex response to the high systemic blood pressure.
- Respiratory Depression: Brainstem compression.
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