Two students, Vineet and Kamlesh were asked to demonstrate in dogs the role of sinus nerve in hypovolemic shock. Vineet severed the sinus nerve when the mean blood pressure (MBP) was 85 mm Hg and Kainlesh cut the sinus nerve when the mean blood pressure was 60mm Hg. On cutting the sinus nerve:
|A||Vineet recorded an increase in MBP but Kamlesh recorded a decrease in MBP.|
|B||Vineet recorded a decrease in MBP but Kamlesh recorded an increase in MBP|
|C||Both recorded an increase in MBP|
|D||Both recorded a decrease in MBP|
Arterial Baroreceptor control system:
1. The Baroreceptor are stretch receptors in the wall of the heart and blood vessels. The carotid sinus (small dilation of the internal carotid artery) and Aortic arch receptors monitor the arterial circulation.
Carotid sinus Aortic Arch
Afferents go via-Hering’s nerve Afferent go via vagus nerve
(carotid sinus nerve) via IX CN
Reach medullary cardio-vascular centre Reach Medullary Cardio-vascular centre
2. Normally discharge from the carotid sinus inhibit the medullary cardio-vascular centre and thus inhibit the sympathetic tonic discharge from medullary cardi-vascular centre
3. When BP rises→ stretch the carotid sinus and Aortiarch→Baroreceptor stimulated →↑Baroreceptor discharge →inhibits the medullary cardiovascular centre and excite the vagal discharge → thus ↓sympathetic discharge and ↑ vagal tone → leads to → vasodilation venodilation↓Bp, Brachycardia and ↓COP.
4. When BP falls→ Baroreceptor discharge decreases →inhibition of medullary cardiovascular centredecreses→↑
sympathetic discharge → leads to vasconstriction (↑BP), ↑HR, and ↑COP.
5. When both carotid sinus nerve are sectioned →loss of inhibitory impulse from carotid sinus nerves - allows escape of the medullary cardio vascular centre→↑essympathetic discharge → leads to vasoconstriction ( ↑BP), ↑HR, and ↑COP.
6. Effect of Bilateral carotid occlusion →leads fall pressure in the carotid sinus reduces the stretch on its wall →↓ Baroreceptor discharge →loss of inhibition on medullary cardio-vascular centre→↑ sympathetic discharge →leads to ↑ BP, ↑ HR, and ↑ COP.
7. A mean blood pressure of some 60-70 mm Hg is required in the sinus segment before any reflex inhibition is exercised upon the medullary cardiovascular centre.
8. The carotid sinus nerves and vagal fibers from the aortic arch are commonly called the Buffer nerves.