Baroreceptors

Morphology and localization

Baroreceptors are pointed-type free nerve endings located in the adventitia of blood vessels. They are scattered throughout the vascular bed, but form specific clusters — baroreceptive reflexogenic zones:

• Sinocarotid zone: located in the carotid sinuses (at the bifurcation of the common carotid artery). Receptors here have the highest sensitivity. The vessel wall in this area is thinner, more elastic, and contains fewer muscular elements.
• Aortic zone: located in the aortic arch.
• Mesenteric zone: localized in the mesenteric vessels.

In addition, baroreceptors (stretch receptors) are present in the pulmonary circulation (bifurcation of the pulmonary arteries), in the atrial walls (subendocardially), and in the ventricles (mainly in the epicardium, more abundantly in the left ventricle).

Classification of baroreceptors

Depending on the specifics of the reaction to pressure changes, three main groups of vascular baroreceptors are distinguished:

1. Phasic (rhythmic): respond to phase fluctuations of pressure during the cardiac cycle (systole and diastole). There are separate pools of these receptors tuned to specific pressure ranges, up to 240 mmHg. For example, some receptors of the aortic arch remain "silent" at normotension and activate only when pressure drops below 80 or rises above 120 mmHg.
2. Static: activated exclusively when a constant level of blood pressure is maintained.
3. Vibrational: detect oscillations caused by turbulent blood flow.

Specific types of receptors are present in the heart. For example, the atria contain Type A (firing during systole, increasing sympathetic tone — Bainbridge reflex) and Type B (active during diastole upon blood filling, exciting parasympathetic tone and inhibiting heart activity).

Physiological properties

The main feature of baroreceptors is their differential sensitivity. They respond not only to the absolute value of blood pressure, but also to the rate of its change. During a rapid, sharp pressure surge, impulse generation occurs much more intensively than during a slow rise to the same values.

The dependence of impulse frequency on pressure is non-linear (exponential). The higher the baseline pressure level in the vessel, the more powerfully the receptors will respond to its further increase. For example, on the flatter part of the curve (100–120 mmHg), the increase in impulse frequency is moderate, whereas at high values (130–150 mmHg), a surge in impulse generation is observed.

Another important property is adaptation. If rhythmic receptors are constantly exposed to high pressure, they stop generating accelerated impulses.

Baroreflex mechanism (depressor response)

When pressure increases, the walls of the major vessels stretch. Baroreceptors generate phase bursts of impulses that travel to the brainstem via afferent pathways:

• From the carotid sinus, the signal travels via Hering's nerves as part of the glossopharyngeal nerve (n. glossopharyngeus).
• From the aortic arch — via aortic (depressor) nerves as part of the vagus nerve (n. vagus).

The signal enters the cardiovascular center of the medulla oblongata (nucleus of the solitary tract). This causes a powerful inhibition of sympathetic influence on the heart and blood vessels, and by reciprocal principle — excitation of the parasympathetic nuclei of the vagus nerve. Result: vasodilation (decreased TPR), slowing of the pulse, and a drop in blood pressure to normal levels.

Upon a drop in pressure (hypotension), vessel stretching disappears. Afferent inhibitory impulses cease, sympathetic centers are "disinhibited", causing vasoconstriction, tachycardia, and the release of catecholamines.

Regulatory failure in stress

The normal homeostasis scheme can break down under severe psycho-emotional stress ("conflict situations"). Under these conditions, the cortex and limbic system generate such a powerful descending sympathetic drive that it overcomes or "ignores" the inhibitory depressor impulses from baroreceptors. Despite baroreceptors signaling high pressure, the reflex fails to return it to normal, which becomes the basis for the development of hypertension.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• The role of peripheral and central chemoreceptors in vascular tone
• Bainbridge reflex: significance of atrial Type A receptors
• Electrophysiology of the aortic (depressor) nerve
• Modeling the pathogenesis of neurogenic hypertension

Other topics in "Vascular Bed Hemodynamics"

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