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Physiology

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Cardiovascular System

Question
2 out of 7
 

True for vasomotor centre is: (AIPG 2009)



A Not influenced by cortical centres

B Stimulated by baroreceptors from arterial organs

C In sleep medulla stops sending signals which explains decrease in B.P

D Functional interaction with cardio vagal centre

Ans. D Functional interaction with cardio vagal centre.

a. The sympathetic nerves that constrict arterioles and veins and increase heart rate and stroke volume discharge in a tonic fashion, and blood pressure is adjusted by variations in the rate of this tonic discharge.

b. Spinal reflex activity affects blood pressure, but the main control of blood pressure is exerted by groups of neurons in the medulla oblongata that are sometimes called collectively the vasomotor area or vasomotor center.

c. As we can see from the table listed below (from Ganong) the VMC can be excited by Cortical signals (example increased HR & BP during sexual excitement and anger) or inhibited.

d. Also the Baroreceptors are inhibitory to VMC. In sleep the fall in BP is due to decreased muscle tone thereby decreasing the venous return.

e. The Cardio Vagal Centre (CVC) inhibits the VMC pressor area. The pattern of discharge from CVC is not tonic.

Vasomotor Control

a. The sympathetic nerves that constrict arterioles and veins and increase heart rate and stroke volume discharge in a tonic fashion, and blood pressure is adjusted by variations in the rate of this tonic discharge.

b. Spinal reflex activity affects blood pressure, but the main control of blood pressure is exerted by groups of neurons in the medulla oblongata that are sometimes called collectively the vasomotor area or vasomotor center.

c. Neurons that mediate increased sympathetic discharge to blood vessels and the heart project directly to sympathetic preganglionic neurons in the intermediolateral gray column (IML) of the spinal cord.

d. On each side, the cell bodies of these neurons are located near the pial surface of the medulla in the rostral ventrolateral medulla (RVLM).

e. Their axons course dorsally and medially and then descend in the lateral column of the spinal cord to the IML.

f. They contain PNMT, but it appears that the excitatory transmitter they secrete is glutamate rather than epinephrine.

g. Impulses reaching the medulla also affect the heart rate via vagal discharge to the heart.

h. The neurons from which the vagal fibers arise are in the dorsal motor nucleus of the vagus and the nucleus ambiguus.

i. When vasoconstrictor discharge is increased, there is increased arteriolar constriction and a rise in blood pressure.

j. Venoconstriction and a decrease in the stores of blood in the venous reservoirs usually accompany these changes, although changes in the capacitance vessels do not always parallel changes in the resistance vessels.

k. Heart rate and stroke volume are increased because of activity in the sympathetic nerves to the heart, and cardiac output is increased.

l. There is usually an associated decrease in the tonic activity of vagal fibers to the heart.

m. Conversely, a decrease in vasomotor discharge causes vasodilation, a fall in blood pressure, and an increase in the storage of blood in the venous reservoirs.