S2: CVS Reflexes

Question 1

What are the 2 types of reflexes?

Answer 1

1. Pressor Response

2. Depressor Response

Question 2

Describe the Pressor Response

Answer 2

Excitatory inputs e.g. arterial chemoreceptors, muscle metaboreceptors (work). They switch on reflexes to increase CO/TPR/BP.

For example: Chemoreceptors sense low O2 levels which switches on reflexes increasing BP which increases blood flow to the lungs increasing O2 saturation in blood

Question 3

What sensory receptors detect pressor response?

Answer 3

Arterial chemoreceptors
Muscle metaboreceptors (work)

Question 4

What sensory receptors detect depressor response?

Answer 4

Inhibitory inputs e.g. Arterial Baroreceptors

Switch off reflexes to decrease CO/TPR/BP

Question 5

What is the role of arterial baroreceptor?

Answer 5

To sense blood pressure.
They are found in the walls of the carotid arteries and aorta which allows the specific and indirect sensing of blood flow to brain and heart.
These sensors detect arterial wall stretch called baroreceptors.

They are a type of mechanoreceptors.

Question 6

How can blood flow indirectly be measured?

Answer 6

The body has no blood flow sensors so the body instead measures blood pressure (CO = BP/ TPR)
Decrease in BP therefore reflects a difference in CO/TPR which ultimately tells us about changes in blood flow.

Question 7

How do baroreceptors respond to changes in pressure?

What happens to them during hypertension?

Answer 7

Blood vessel stretched is sensed by baroreceptor endings and an AP is sent to the brain.

In the face of continued high or low pressure, the threshold for baroreceptor changes. During an increase in pressure, the baroreceptors start of firing rapidly but they slow down an ‘adaptation’ period as constant high BP decreases their sensitivity.

E.g. In hypertension where baroreceptors are less activated as threshold becomes much greater à high BP maintained

Consequently, we don’t regulate our BP as well

Question 8

What is the effect of increased BP on baroreflex (Loading)?

Answer 8

e.g. Stress

Increased BP stimulates carotid sinus nerve which activates baroreceptors initiated the depressor.

This reduces HR and SV which decreases blood pressure.

In addition, sympathetic NS is switched off so loss of sympathetic vagal tone causes vasodilation of arterioles which decreases TPR consequently BP.

Question 9

What is the effect of decreased BP on baroreflex (unloading)?

Answer 9

e.g. Haemorrhage, standing up

Decreasing BP causes the baroreflex to:

• Increase sympathetic activity and decrease in vagus activity
• Increase HR and force of contraction which increases cardiac output
• Arteriolar constriction = Increases TPR
• Venous constriction = Increases CVP, SV, CO

Pressor reflex: maintains blood pressure/blood flow to vital organs

Question 10

What happens with severe decrease in BP e.g. haemorrhage?

Answer 10

• Adrenaline secretion
• Vasopressin (ADH) secretion due to decrease in blood volume sensed by the heart
• Stimulation of RAAS-Ang II production
• Vasoconstriction of arterioles decreases capillary pressure so absorption of intersisital fluid increases blood volume.
• Aldosterone secretion causes Na+/H2O reabsorption in kidneys so there is an increase in blood volume

Drop in blood pressure is very serious so the body needs to adapt very quickly to maintain blood to and from the heart

Question 11

What are the 3 types of cardiac receptors?

Answer 11

1. Nociceptors –> sympathetic afferents that detect products of pain
2. Veno-atrial mechanoreceptors –> tell us how much right atrium is filling
3. Ventricular mechanoreceptors –> detect stretch

Question 12

Describe the cardiac receptor: Nociceptive sympathetic afferents

What reflex?

Answer 12

Chemosensitive ventricular afferent fibres (C-fibres that lie with sympathetic nerves). It is stimulated by:

• K+
• H+ (from lactate)
• Bradykinin (during ischaemia and they are products of pain and inflammatory mediators). They mediate pain/symptoms of angina and myocardial infarction.

Fibres converge onto same neurones in the spinal cord as somatic afferents (these nerves stimulate other body parts) - basis of referred pain.

~ Acute pressor reflex - Increase sympathetic system: Pale (vasoconstriction), sweaty (Ach –> Musc), Tachycardia (increase HR)

Question 13

Describe the cardiac receptor: Veno-atrial mechanoreceptors

What reflex?

Answer 13

Stimulated by increase in cardiac filling/CVP
- In RA

~ Initial pressor reflex - increase sympathetic system - tachycardia. The bainbridge effect occurs.

~ Long term depressor reflex - Increases the amount of blood going to the kidney so more blood filtered. This increases diuresis which decreases blood volume. There is a feedback loop with changes in ADH, ANP, RAAS which is a reflex that makes less blood fill the heart.

Question 14

What is the Bainbridge effect?

Answer 14

Reflex tachycardia due to rapid infusion of volume into venous system (veno-atrial stretch receptors and pacemaker disension). This is an reflex (initial pressor reflex) that increase HR to get rid of more blood.

Question 15

Describe the cardiac receptor: Ventricular mechanoreceptors

What reflex?

Answer 15

Stimulated by over distension of ventricles.
It is linked to MI –> Poor contractility –> decrease ejection –> CO decreases

~ Depressor reflex - Heart senses and switches off reflex which protects the heart from using too much O2 leading to a reduction of HR. It role is however not well known and it may override veno-atrial reflex.

Question 16

What is the role of the 3 cardiac receptors?

Answer 16

All these receptors stimulated together result in a decrease in HR and BP overall (depressor) . However, in real life, the receptors are usually stimulated in different situations and not at the same time.

Question 17

What is the role of arterial chemoreceptors?

Where are they located?

Answer 17

• Chemoreceptors lie close to baroreceptors
• They monitor blood flow via blood pressure (blood flow to heart and brain) and monitor oxygen in these areas.

Regulate ventilation and drive cardiac reflexes during:
▪ Asphyxia (low O2/high CO2)
▪ Shock (systemic hypotension)
▪ Severe Haemorrhage (when BP below range of baroreflex)

The pressor response will occur:

• Increase sympathetic activity
• Tachycardia, increase selective arterial/venous constriction
• Increase CO/BP
• Increase drive of BF to organs supplying them with oxygen which preserves cerebral BF during low O2 (asphyxia/haemorrhage)

They are located in carotid and aortic bodies.

Question 18

What stimulates arterial chemoreceptors?

Answer 18

They are stimulated by:

• Low O2 (hypoxia)
• High CO2 (hypercapnia)
• H+
• K+
• Very high blood flow

Question 19

Where are muscle metaboreceptors (work receptors) found, what activates them and what is their role?

Answer 19

• Muscle metaboreceptors are found in sensory fibres in group IV motor fibres located in skeletal muscle
• It is activated via metabolites: ATP, K+, Lactate, Adenosine (produced during excersize)

Pressor response: Increase sympathetic, tachycardia, increase arterial/venous constriction, increase CO/BP

It is important during isometric excersize where joint angle and muscle length do not change - static excersize e.g. weight lifting

Muscle metaboreceptors aid maintenance of blood perfusion to contracted muscle. These muscles undergo metabolic hyperaemia allowing blood flow to the contracted tissue.

Question 20

How are sensory receptor signals coordinated in the brain?

Answer 20

There is the central role of the nucleus tractus salitarius (NTS)

Information from baro and chemoreceptors enter brain stem and then NTS (relay centre). Some of this information is taken into vagus and sympathetic system - CVLM and RVLM

Question 21

What is CVLM and RVLM?

Answer 21

CVLM (Caudal Ventro-Lateral Medulla) - Inhibitory Pathway

RVLM (Rostro Ventro-Lateral Medulla) - Excitatory info to spinal cord which then send out info via pre and post ganglionic nerves

Question 22

Describe the central pathways regulating sympathetic outflow for depressor effect

Answer 22

Increase baroreceptors stimulated (more stretch increasing BP)

1. Information sent to nerves excitatory NTS
2. Increase CVLM which is made of neurones that produce inhibitory signals
3. Inhibitory signals to RVLM which switches it off
4. Sympathetic nerves switched off - Depressor reflex

Question 23

Describe the central pathways regulating sympathetic outflow for pressor effect

Answer 23

Increase arterial chemoreceptors/muscle work receptors stimulated

1. Information sent to inhibitory nerves in NTS
2. This inhibits CVLM which inhibits the inhibitory signals
3. RVLM is switched on
4. Sympathetic nerves activated - Pressor response

Question 24

Describe the

central pathways regulating vagal parasympathetic outflow to SA and AV nodes in the heart

Answer 24

1. Stimulation of baroreceptors stimulates NTS
2. Excitatory information to NTS
3. Vagus nerves activated
4. Switches off reflex (sympathetic)
5. Results in the depressor effect

Question 25

Compare ECG of normal to sinus tachycardia (Sinus Arrhythmia)

Answer 25

Less vagal activity - Regular rhythm but higher HR

Question 26

What happens during syncope, vasovagal attack?

Answer 26

It is caused by decrease in cerebral blood flow (reduced oxygen delivery) due to sudden drop in arterial BP - Vagal bradycardia (vagus stimulated)