Reflex control of the CVS Flashcards
What do Baroreceptors detect?
Changes in Pressure
What do Chemoreceptors detect?
Carbon dioxide and oxygen
What do Muscle metaboreceptors detect?
Products of metabolism
Where are the main messages coordinated?
Give an example of the pathways coordinated
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)
What are excitatory inputs and give some examples?
Excitatory inputs eg. arterial chemoreceptors, muscle metaboreceptors (work). Stimulation of reflexes - increase cardiac output, TPR, and blood pressure PRESSOR response
What are inhibitory inputs and give some examples
Inhibitory inputs eg. arterial baroreceptors, cardiac-pulmonary receptors. Stimulation of reflexes - Decrease cardiac output, TPR, and blood pressure
DEPRESSOR response
What is the purpose of arterial baroreceptors?
Vital to maintain blood flow to brain and myocardium
Why do we need arterial baroreceptors?
Where are they found?
Because there are no blood flow sensors, the body monitors blood pressure in the carotid and coronary arteries
How does monitoring blood pressure tell us about blood flow?
CO= Pa /TPR
What does a decrease in Pa reflect?
A decrease in Pa reflects a decrease in either CO or TPR which compromises blood flow to brain and heart
What do baroreceptors also detect?
Arterial wall stretch
How often do the baroreceptors turn on?
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.
What happens when there is a long term increase in BP
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.
What happens when we reduce blood pressure?
Pulse pressure falls
Vasodilation - decreased TPR and BP
Decreased sympathetic nerve activity
Increased Vagus nerve activity
What happens when we have a decrease in blood pressure (5)
How does the body respond to a decrease in blood pressure?
- 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.
How else is blood volume raised in relation to the kidney?
Also, adrenaline secretion, vasopressin (ADH) secretion & stimulation of RAAS (ie Angiotensin II increases Na+/H2O absorption in kidneys raising blood volume).
What are veno-atrial mechanoreceptors stimulated by and what do they do?
What do they detected and cause?
- Stimulated by an increase in cardiac filling/ CVP. Increased in sympathetic activity, tachycardia
- Detects venous pressure, so if we get more blood in the venous system, we get a sudden increase in heart rate
What do ventricular mechanoreceptors do?
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.
What are Nociceptive sympathetic afferents?
These are pain receptors, they send signals to the brain and spinal cord. It increases sympathetic activity, causing tachycardia
Where are the arterial chemoreceptors found?
Carotid and aortic bodies
What are the arterial chemoreceptors stimulated by?
Low oxygen (hypoxia), high carbon dioxide (hypercapnia), H+ and K+
What is the blood flow through the arterial chemoreceptors?
They are well supplied with blood flow around 20 ml/ml/g
What is the function of the arterial chemoreceptors
Regulate ventilation and also drive cardiac reflexes during asphyxia (low oxygen and high carbon dioxide), shock (systematic hypotension) and haemorrhage
Describe the pressor response
- Increased sympathetic activity
- Tachycardia and increased selective arterial/ venous constriction
- Increased cardiac output and blood pressure - especially preservation of cerebral blood flow
Give an example of a muscle metaboreceptor
Sensory fibres in Group IV motor fibres located in skeletal muscle
• Activated via metabolites: K+, lactate, adenosine.
How is the muscle metaboreceptor Important during isometric exercise?
- 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.
What is the Central role of the nucleus tractus solitarius (NTS)?
- Signal from baroreceptor afferent fibres enter nucleus tractus solitarius (NTS).
- This then sends information out to the caudal ventrolateral medulla (CVLM).
- The CVLM sends inhibitory information to the rostral ventolateral medulla (RVLM).
- This results in inhibition of sympathetic efferent nerves to heart and vessels.
- 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.
Describe the experimental link between CVLM and RVLM (6)
- Intravenous phenylephrine is injected (α1 agonist increases TPR and BP).
- BP rises and loads baroreceptors (carotid & aortic).
- Signal from baroreceptor to NTS then to CVLM.
- CVLM signal to inhibit RVLM signals.
- Sympathetic activity to heart and vessels decreases.
- Lower sympathetic gives vasodilation and lowers BP.
Describe a second experimental link between the CVLM and the RVLM
Electrical stimulation of the CVLM lowers BP coupled RVLM activity
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.
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
What is Sinus tachycardia?
Inhibitory input from the inspiratory centre. Each inhalation switches off nucleus ambiguous and heart rate increases slightly
Describe Limbic stimulation
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
