Neural Regulation Of Cvs

NEURAL REGULATION OF CVS

Q. 1

Vasomotor centre of medulla is associated with?

 A

Acts with the cardiovagal centre to maintain B.P.

 B

Independent of corticohypothalamic inputs

 C Influenced by baroreceptors not chemoreceptors
 D

Essetially silent in sleep

Q. 1

Vasomotor centre of medulla is associated with?

 A

Acts with the cardiovagal centre to maintain B.P.

 B

Independent of corticohypothalamic inputs

 C Influenced by baroreceptors not chemoreceptors
 D

Essetially silent in sleep

Ans. A

Explanation:

Acts with the cardiovagal centre to maintain B.P. (Ref Ganong 23/e p556]

  • Neural regulation of the blood pressure is brought about by 2 different centres located in medulla – Vasomotor centre which control the sympathetic outflow, and Cardiovagal centre which controls the parasympathetic outflow.
  • TheVasomotor centre and the Cardiovagal centre act together to maintain the B.P. (Note that many books do not make the distinction between the vasomotor and cardiovagal centre and collectively use the term vasomotor centre for all the neurons in medulla that control the B.P.

Vasomotor centre

Cardio vagal centre

Vasomotor centre is group of neurons located in Rostral

Cardiovagal centre lies in the nucleus ambiguous.

Ventrolateral Medulla (RVLM) associated with sympathetic

It sends parasympathetic impulses to the heart via the

discharge controlling the cardiovascular system.

vagus(note that blood vessels receive only

The sympathetic discharge from vasomotor centre goes to

sympathetic impulses whereas heart receives both

heart and blood vessels resulting in:

sympathetic and parasympathetic supply).

Increased heart rate (Chronotropic effect)

This results in:

– Increased force of cardiac contraction (Inotropic effect)

– Decreased heart rate

– Increased rate of transmission in the cardiac conductive

– Decrease cardiac output

tissue (Dromotropic effect)

– Decreased B.P.

– Vasoconstriction

(this leads to increased stroke volume and increased BP)

 

  • The vasomotor centre receive inputs from Corticohypothalmic fibres

            Corticohypothalmic .fibres are descending tracts to the vasomotor area from the cerebral cortex (particularly the limbic cortex) that relay in the hypothalamus. These fibers are responsible .for the blood pressure rise and            tachycardia produced by emotions such as sexual excitement and anger.

  • The vasomotor centre is not silent during sleep The vasomotor centre neurons arc tonically active and discharge rhythmically.
  • The vasomotor centre receives inputs from baroreceptors and also from chemoreceptors.

The baroreceptors are stretch receptors in the walls of carotid sinus and aortic arch. The baroreceptors are stimulated by distention of the structures in which they are located, and so they discharge at an increased rate when the pressure in these structures rises. Increased baroreceptor discharge inhibits the tonic discharge of sympathetic nerves and excites the vagal innervation of the heart. These neural changes produce vasodilation, venodilation, a drop in blood pressure. bradycardia, and a decrease in cardiac output.

Chemoreceptors are located in carotid and aortic bodies. These receptors are primarily activated by a reduction in partial pressure of oxygen (Pa02), but they also respond to an increase in the partial pressure of carbon dioxide (PaCO2) and pH. Chemoreceptors exert their main effects on respiration; however, their activation also leads to vasoconstriction. Heart rate changes are variable and depend on various factors, including changes in respiration.

Factors Affecting the Vasomotor centre (RVLM)

Direct stimulation

CO2

Hypoxia

Excitatory inputs

Cortex via hypothalamus

Mesencephalic periaqueductal gray

Brain stein reticular formation

Pain pathways

Somatic afferents (somatosympathetic reflex)

Carotid and aortic chemoreceptors

Inhibitory inputs

Cortex via hypothalamus

Caudal ventrolateral medulla

Caudal medullary raphe nuclei

Lung inflation afferents

Carotid, aortic, and cardiopulmonary baroreceptors

  • Thus we see that the cardiovascular system is under neural influences of medulla, which in turn receive feedback from sensory receptors in the vasculature (eg, baroreceptors). An increase in neural output from the brain stem to sympathetic nerves leads to a decrease in blood vessel diameter (arteriolar constriction) and increases in stroke volume and heart rate, which contribute to a rise in blood pressure. This in turn causes an increase in baroreceptor activity, which signals the vasomotor centre to reduce the neural output to sympathetic nerves.

Also know:

  • The neurons of vasomotor centre secrete excitatory transmitter- glutamate (and not epinephrine)
  • Inflation of the lungs causes vasodilation and a decrease in blood pressure. This response is mediated via vagal afferents from the lungs that inhibit vasomotor discharge.
  • In general, stimuli that increase the heart rate also increase blood pressure, whereas those that decrease the heart rate lower blood pressure. However, there are exceptions, such as the production of hypotension and tachycardia by stimulation of atrial stretch receptors and the production of hypertension and bradycardia by increased intracranial pressure (Cushing reflex).
  • Cushing reflex: Increase in intracranial pressure compromises the blood supply to the vasomotor neurons, and the resulting local hypoxia and hypercapnia increase discharge from the vasomotor centre. This results in rise in systemic arterial pressure (Cushing reflex) which tends to restore the blood flow to the medulla. Over a considerable range, the blood pressure rise is proportional to the increase in intracranial pressure. The rise in blood pressure causes a reflex decrease in heart rate via the arterial baroreceptors. This is why bradycardia rather than tachycardia is characteristically seen in patients with increased intracranial pressure.

Q. 2

Massage of the carotid sinus results in all of the following, EXCEPT:

 A

Increased pressure at the carotid sinus baroreceptors

 B

Decreased firing rate of the carotid sinus fibers

 C

Decreased firing rate of cardiac sympathetic fibers

 D

Increased firing rate of the vagus nerve

Q. 2

Massage of the carotid sinus results in all of the following, EXCEPT:

 A

Increased pressure at the carotid sinus baroreceptors

 B

Decreased firing rate of the carotid sinus fibers

 C

Decreased firing rate of cardiac sympathetic fibers

 D

Increased firing rate of the vagus nerve

Ans. B

Explanation:

Increased baroreceptor pressure causes an increase in the firing rate of the carotid sinus nerve (afferent) that inhibits the medullary vasomotor center. Consequently, both a decrease in sympathetic tone and an increases in vagal discharge (efferent) contribute to the fall in heart rate and arterial blood pressure following carotid sinus massage. When performing this maneuver on your patients or peers, be prepared for cardiac sinus arrest and avoid vigorous massage, which might dislodge an embolus and cause permanent neurological damage.
 
Ref: Quinn J. (2011). Chapter 56. Syncope. In J.E. Tintinalli, J.S. Stapczynski, D.M. Cline, O.J. Ma, R.K. Cydulka, G.D. Meckler (Eds), Tintinalli’s Emergency Medicine

Q. 3

Which of the following statements about vasomotor centre (VMC) is TRUE?

 A

It is essentially silent in sleep

 B

Independent of cortico hypothalamic inputs

 C

Influenced by baroreceptor signals but not by chemo-receptors

 D

Acts along with the cardio vagal centre (CVC) to maintain Blood pressure.

Q. 3

Which of the following statements about vasomotor centre (VMC) is TRUE?

 A

It is essentially silent in sleep

 B

Independent of cortico hypothalamic inputs

 C

Influenced by baroreceptor signals but not by chemo-receptors

 D

Acts along with the cardio vagal centre (CVC) to maintain Blood pressure.

Ans. D

Explanation:

The main control of blood pressure is exerted by a group of neurons in the medulla called as vasomotor center. CVC controls the parasympathetic flow which functionally interacts with VMC in maintaining the blood pressure.

Ref: Review of Medical Physiology By William F Ganong, 23rd Edition, Page 478; Textbook of Medical Physiology By Guyton and Hall, 10th Edition, Page 191

 


Q. 4

Discharge from Baroreceptors causes inhibition of

 A

Caudal Ventrolateral Medulla

 B

Caudal Ventrolateral Medulla

 C

Nucleus ambiguous

 D

Nucleus tractus solitarus

Q. 4

Discharge from Baroreceptors causes inhibition of

 A

Caudal Ventrolateral Medulla

 B

Caudal Ventrolateral Medulla

 C

Nucleus ambiguous

 D

Nucleus tractus solitarus

Ans. B

Explanation:

B i.e. Rostral Ventrolateral Medulla


Q. 5

Baroreceptor stimulation produces:

 A

Decreased heart rate & BP

 B

Increased heart rate & BP

 C

Increased cardiac contractility

 D

All

Q. 5

Baroreceptor stimulation produces:

 A

Decreased heart rate & BP

 B

Increased heart rate & BP

 C

Increased cardiac contractility

 D

All

Ans. A

Explanation:

A i.e. Decreased HR & BP

Baroreceptors are the stretch receptors in the walls of heart & blood vessels eg carotid sinus & aortic arch receptors, receptors in walls of atria at enterance of SVC, IVC & pulmonary veins and receptors in pulmonary circulation (cardio- pulmonary receptors). These are stimulated by distension of the structure in which they are located 1/t vagal innervation of heart and producing vasodilation, venodilation, a drop in BP, bradycardia & decrease cardiac outputQ.


Q. 6

Baroreceptor are ‑

 A

Carotid body

 B

Carotid sinus

 C

Aortic body

 D

None

Q. 6

Baroreceptor are ‑

 A

Carotid body

 B

Carotid sinus

 C

Aortic body

 D

None

Ans. B

Explanation:

Ans. is ‘b’ i.e., Carotid sinus

  • Baroreceptors are mechanoreceptors that are located in the adventia of carotid artery and aorta, at specialized locations called sinuses.

1) Carotid sinus is a little bulge at the root of internal carotid artery, located just above the bifurcation of the common carotid artery. It is innervated by the sinus nerve, a branch of glossopharyngeal (IX cranial) nerve.

2) Aortic arch (aortic sinus) also contains mechenoreceptors (stretch receptors) which are similar to carotid sinus receptors. However, their afferent nerve fibers travel in the aortic nerve, a branch of Vagus (X cranial) nerve.

The sinus nerve (from carotid sinus) and aortic nerve/vagal fibers (from aortic sinus) are together called `Sino­aortic nerves’. They, together, are also refered to as ‘Buffer nerves’ because they are the afferents of cardiovascular reflexes that buffer abrupt changes in blood pressure.


Q. 7

Signal from Baroreceptors goes to ‑

 A

Caudal ventrolateral medulla

 B

Rostral ventrolateral medulla

 C

Nucleus of tractus solitarius

 D

None of the above

Q. 7

Signal from Baroreceptors goes to ‑

 A

Caudal ventrolateral medulla

 B

Rostral ventrolateral medulla

 C

Nucleus of tractus solitarius

 D

None of the above

Ans. B

Explanation:

Ans. is ‘b’ i.e., Rostral ventrolateral medulla

Baroreceptors are mechanoreceptors that are located in the adventia of carotid artery and aorta, at specialized locations called sinuses.

1) Carotid sinus is a little bulge at the root of internal carotid artery, located just above the bifurcation of the common carotid artery. It is innervated by the sinus nerve, a branch of glossopharyngeal (IX cranial) nerve.

2) Aortic arch (aortic sinus) also contains mechenoreceptors (stretch receptors) which are similar to carotid sinus receptors. However, their afferent nerve fibers travel in the aortic nerve, a branch of Vagus (X cranial) nerve.

The sinus nerve (from carotid sinus) and aortic nerve/vagal fibers (from aortic sinus) are together called `Sino­aortic nerves’. They, together, are also refered to as ‘Buffer nerves’ because they are the afferents of cardiovascular reflexes that buffer abrupt changes in blood pressure.

Baroreceptors are highly sensitive to any change in mean blood pressure. Sinoaortic nerves (buffer nerves) normally discharge rhythmically, synchronous with the pressure fluctuation during systole and diastole. They respond to BP changes between 70 mm Hg and 150 mm Hg. When BP rises, baroreceptors are stimulated and their afferents (through sinoaortic nerves) stimulate nucleus of tractus solitarus (NTS) which inturn inhibits the pressor area ofVMC, i.e., Rostral ventrolateral medula (RVLM). This results in decreased sympathetic outflow and therefore decreases in vasomotor tone and vasodilation. Vasodilation brings down the BP, thereby helping hemostasis. Activated NTS also stimulates nucleus ambiguous (cardioinhibitory center) of medulla, which increases parasympathetic (vagal) output, through vagus, that decreases heart rate. Reduction in heart rate reduces the cardiac output, which also reduces BP. Baroreceptor stimulation also weekly inhibits respiration.

When BP falls, for instance while changing the posture from lying down to standing, reverse change takes place. When a person stands up, his blood is pooled in the veins of lower limbs by the effect of gravity. Central venous pressure and venous return decrease, which causes a fall in stroke volume. Hence the systolic BP falls. As a result, the discharge rate of baroreceptors decreases leading to a decrease in the inhibitory influence on the pressor area of VMC. Hence vasomotor tone increases, leading to vasoconstriction, and consequently an increase in BP. Simultaneously, the nucleus ambiguous of the vagus is also inhibited, increasing the heart rate and consequently stroke volume and eventually BR Thus fall in BP due to change ofposture is very brief (Transient).


Q. 8

Clamping of the carotid arteries below (proximal) the carotid sinus is likely to produce:

 A

Increase in vasomotor centre activity

 B

Increase in discharge of carotid sinus afferent nerves

 C

Decreased heart rate and blood pressure

 D

Baroreceptor adaptation

Q. 8

Clamping of the carotid arteries below (proximal) the carotid sinus is likely to produce:

 A

Increase in vasomotor centre activity

 B

Increase in discharge of carotid sinus afferent nerves

 C

Decreased heart rate and blood pressure

 D

Baroreceptor adaptation

Ans. A

Explanation:

Ans. a. Increase in vasomotor centre activity

  • Clamping of the carotid arteries proximal to the carotid sinus elevates the blood pressure and heart rate because the procedure lowers the pressure in the carotid sinus (Baroreceptor reflex


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