Reflex control of the CVS

Question 1

What do Baroreceptors detect?

Answer 1

Changes in Pressure

Question 2

What do Chemoreceptors detect?

Answer 2

Carbon dioxide and oxygen

Question 3

What do Muscle metaboreceptors detect?

Answer 3

Products of metabolism

Question 4

Where are the main messages coordinated?

Give an example of the pathways coordinated

Answer 4

These messages are coordinated centrally, one of the main places this happens is in the medulla, where integrates different pathways. These pathways include vagal (heart rate, contractility, peripheral resistance)

Question 5

What are excitatory inputs and give some examples?

Answer 5

Excitatory inputs eg. arterial chemoreceptors, muscle metaboreceptors (work). Stimulation of reflexes - increase cardiac output, TPR, and blood pressure PRESSOR response

Question 6

What are inhibitory inputs and give some examples

Answer 6

Inhibitory inputs eg. arterial baroreceptors, cardiac-pulmonary receptors. Stimulation of reflexes - Decrease cardiac output, TPR, and blood pressure
DEPRESSOR response

Question 7

What is the purpose of arterial baroreceptors?

Answer 7

Vital to maintain blood flow to brain and myocardium

Question 8

Why do we need arterial baroreceptors?

Where are they found?

Answer 8

Because there are no blood flow sensors, the body monitors blood pressure in the carotid and coronary arteries

Question 9

How does monitoring blood pressure tell us about blood flow?

Answer 9

CO= Pa /TPR

Question 10

What does a decrease in Pa reflect?

Answer 10

A decrease in Pa reflects a decrease in either CO or TPR which compromises blood flow to brain and heart

Question 11

What do baroreceptors also detect?

Answer 11

Arterial wall stretch

Question 12

How often do the baroreceptors turn on?

Answer 12

They don’t fire at much at rest, however, when pressure increases, they go fast and gradually slow down, allowing them to adapt to a new normal. They respond to sudden changes in pressure.

When there is a decrease in pressure, they slow down.

Question 13

What happens when there is a long term increase in BP

Answer 13

So, if we have slow or long lasting changes in blood pressure, for example high blood pressure over long term, they re-normalise at that level. They prevent acute changes in blood pressure. Over long periods of time, they will adapt.

Question 14

What happens when we reduce blood pressure?

Answer 14

Pulse pressure falls

Vasodilation - decreased TPR and BP

Decreased sympathetic nerve activity

Increased Vagus nerve activity

Question 15

What happens when we have a decrease in blood pressure (5)

How does the body respond to a decrease in blood pressure?

Answer 15

• Increased sympathetic activity & decreased Vagus activity.
• Increased HR and force of contraction so increased CO.
• Arteriole constriction gives increased TPR.
• Venous constriction increases CVP and so by Starlings law increases SV & CO.
• This all maintains blood pressure therefore blood flow to vital organs.

Question 16

How else is blood volume raised in relation to the kidney?

Answer 16

Also, adrenaline secretion, vasopressin (ADH) secretion & stimulation of RAAS (ie Angiotensin II increases Na+/H2O absorption in kidneys raising blood volume).

Question 17

What are veno-atrial mechanoreceptors stimulated by and what do they do?

What do they detected and cause?

Answer 17

1. Stimulated by an increase in cardiac filling/ CVP. Increased in sympathetic activity, tachycardia
2. Detects venous pressure, so if we get more blood in the venous system, we get a sudden increase in heart rate

Question 18

What do ventricular mechanoreceptors do?

Answer 18

If the heart stretches to much, we get volume overload, it sends signals to the medulla, we get mild vasodilation, just to reduce preload. Prevents from to much stretch from venous or arteriole system.

Question 19

What are Nociceptive sympathetic afferents?

Answer 19

These are pain receptors, they send signals to the brain and spinal cord. It increases sympathetic activity, causing tachycardia

Question 20

Where are the arterial chemoreceptors found?

Answer 20

Carotid and aortic bodies

Question 21

What are the arterial chemoreceptors stimulated by?

Answer 21

Low oxygen (hypoxia), high carbon dioxide (hypercapnia), H+ and K+

Question 22

What is the blood flow through the arterial chemoreceptors?

Answer 22

They are well supplied with blood flow around 20 ml/ml/g

Question 23

What is the function of the arterial chemoreceptors

Answer 23

Regulate ventilation and also drive cardiac reflexes during asphyxia (low oxygen and high carbon dioxide), shock (systematic hypotension) and haemorrhage

Question 24

Describe the pressor response

Answer 24

1. Increased sympathetic activity
2. Tachycardia and increased selective arterial/ venous constriction
3. Increased cardiac output and blood pressure - especially preservation of cerebral blood flow

Question 25

Give an example of a muscle metaboreceptor

Answer 25

Sensory fibres in Group IV motor fibres located in skeletal muscle
• Activated via metabolites: K+, lactate, adenosine.

Question 26

How is the muscle metaboreceptor Important during isometric exercise?

Answer 26

• Continually contracted muscle but joint angle and muscle length do not change
• eg. weight lifting / handgrip.
• Higher BP drives blood into the contracted muscle to maintain perfusion.

These muscles undergo metabolic hyperaemia allowing blood flow to the contracted tissue.

Question 27

What is the Central role of the nucleus tractus solitarius (NTS)?

Answer 27

1. Signal from baroreceptor afferent fibres enter nucleus tractus solitarius (NTS).
2. This then sends information out to the caudal ventrolateral medulla (CVLM).
3. The CVLM sends inhibitory information to the rostral ventolateral medulla (RVLM).
4. This results in inhibition of sympathetic efferent nerves to heart and vessels.
5. Less sympathetic efferent signals result in reduction in HR, less vasoconstriction, lower BP etc.
   • The situation is reversed when ‘unloading’ baroreceptors in which case efferent sympathetic activity increases increasing HR, vasoconstriction and BP.
   • Spinal injury can ablate this so hypotension is a possibility when unloading.

Question 28

Describe the experimental link between CVLM and RVLM (6)

Answer 28

1. Intravenous phenylephrine is injected (α1 agonist increases TPR and BP).
2. BP rises and loads baroreceptors (carotid & aortic).
3. Signal from baroreceptor to NTS then to CVLM.
4. CVLM signal to inhibit RVLM signals.
5. Sympathetic activity to heart and vessels decreases.
6. Lower sympathetic gives vasodilation and lowers BP.

Question 29

Describe a second experimental link between the CVLM and the RVLM

Answer 29

Electrical stimulation of the CVLM lowers BP coupled RVLM activity

Question 30

Describe the pathway that leads to a slow heart rate, for example following strong emotional news, so you don’t get enough blood to the brain and faint.

Answer 30

Loading of the baroreceptors also stimulates the vagus nerve which activates the NTS.

The signal from the NTS stimulates the nucleus ambiguous (vagal nuclei)

Vagal parasympathetic impulses are sent to the heart and these have a depressor effect

Question 31

What is Sinus tachycardia?

Answer 31

Inhibitory input from the inspiratory centre. Each inhalation switches off nucleus ambiguous and heart rate increases slightly

Question 32

Describe Limbic stimulation

Answer 32

Causes a decrease in heart rate and repressor effect on AVN and SAN and lead to fainting and reduced cerebral blood flow due to sudden drop in cardiac output