How does the ANS know to increase or decrease the BP?

on 21.1.09 with 0 comments

Baroreceptors located in the carotid artery can pick up stretch stimuli and convey through afferent fibers this stimulus to the cardiovascular center in the medulla. The cardiac center transmits the excitation to other cells within the center, which are excitatory fibers. The cells that receive this excitatory stimulus (located in different places) are sympathetic, they transmit a negative signal through GABA. But, if they go to parasympathetic fibers, the signals are excitatory and still send a positive signal. This is how the same receptor stimulus can send two different effects on the two sides of the spinal cord.

If BP is increased, baroreceptor activity also increases, which leads to a decrease in sympathetic activity (due to the intermediate negative feedback) and increased parasympathetic activity (vice versa if BP is decrease).

If you have high blood pressure, all of these reflex mechanisms result in different things. First, the sympathetic NS must be reduced so that the total peripheral resistance is reduced (because the arterioles are relaxed). The cardiac output is reduced if the sympathetic NS is decreased. Also, there is regulation in renin—it is released from the kidney. Renin cleaves two amino acids from angiotensin I, which forms the active form, angiotensin. Angiotensin does two things:

1. regulates aldosterone secretion in the adrenal cortex;

2. stimulates the smooth muscle (angio= vessels, tensin= tense) so total peripheral resistance will be reduced.

Finally, from the posterior pituitary the CNS releases vasopressin, which increases the BP.

Category: Pharmacology Notes



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