Baroreceptors Reflex (Carotid Sinus Reflex)
This reflex results in regulation of blood pressure, especially if it is highly elevated
or severely depressed; however, the reflex is usually elicited in systolic blood pressures
over 150–170 mmHg; the other part of the reflex is not often seen when the
systolic blood pressure is below 50–60 mmHg. However, in patients with underlying
hypertension or atherosclerosis or in the elderly, the reflex thresholds might be
altered, and at times, the reflex would not be seen partially or totally. The main
receptors of this reflex are located in the walls of carotid arteries and aortic arch, and
its sequence is as follows:
• The circumferential and longitudinal stretch receptors are located in carotid
sinus and aortic arch; increased blood pressure triggers these receptors leading to
impulse firing.
• The transport of impulse from carotid sinus is through the ninth cranial nerve and
from the aortic arch through the tenth cranial nerve.
• The impulses from these two locations are sent to the nucleus solitaries in the
medullary cardio-regulatory and vasomotor centers.
• The nucleus solitaries, however, have two different parts: the first part is lateral
and rostral, known as the “pressor center,” and the second part is located at the
central and caudal part which is known as the “depressor” center; in these two
parts, the limbic and hypothalamic inputs are integrated to create the final
response as either of the two following responses:
• Decreased sympathetic tone (mainly through inhibition of sympathetic chain and
sympathetic nerves) leading to hypotension and bradycardia and also decreased
vascular tone, leading to blood vessel dilation (i.e., systemic vasodilation).
• Increased parasympathetic tone (mainly through vagus nerve) leading to
decreased heart rate and decreased myocardial contractility.
• These interactions would bring the blood pressure to normal, hence relieving the
pressure over the baroreceptors.
• If the initial event is decreased blood pressure, the decreased tone on the baroreceptors
would initiate the opposite response (Vasquez et al. 1997; Pilowsky and
Goodchild 2002; Campagna and Carter 2003; Kashihara 2009; de Leeuw et al.
2017; Kuhtz-Buschbeck et al. 2017).
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