Baroreceptors are located in the carotid sinus and aortic arch. The afferent nerve fibers from carotid sinus are carried by the glossopharyngeal nerve (cranial nerve IX) and the afferent nerve fibers from carotid sinus are carried by the vagus nerve (cranial nerve X). These afferent fibers end in the nucleus of tractus solitaries in the medulla. Nucleus of tractus solitaries thereafter sends:
Inhibitory fibers to sympathetic preganglionic nucleus located in the lateral horn of spinal nucleus.
Excitatory fibers to dorsal nucleus of vagus nerve (regulates parasympathetic response)
(many interconnections of both excitatory and inhibitory neurons exist. Final net effect is inhibitory)
When the blood pressure increases, the stretch will be felt by these baroreceptors, and they will fire more signals. The rise in baroreceptor firing will cause an increase in nucleus solitarius firing. This will increase the firing of inhibitory-excitatory neurons causing the decrease in sympathetic effect and increase in parasympathetic effect. This will decrease the heart rate and total peripheral resistance.
The reverse phenomenon occurs when blood pressure falls. These reflexes are very rapid. In the patient with sudden onset of supraventricular tachycardia, doing carotid massage and valsalva maneuver will immediately increase baroreceptor firings and will alleviate SVTs.
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