Reflex Control of the CVS Flashcards
What is the difference between excitatory and inhibitory inputs
Excitatory inputs -
- Arterial chemoreceptors and muscle metaboreceptors
- Stimulation of reflexes - increased cardiac output , increased total peripheral resistance (TPR) and blood pressure - PRESSOR response
Inhibitory inputs -
- Arterial baroreceptors and cardiopulmonary receptors
- Stimulation of reflexes - decreased cardiac output, decreased total peripheral resistance and blood pressure - DEPRESSOR response
Describe arterial baroreceptors
- Vital to maintain blood flow to brain and myocardium
- There are no ‘blood flow’ sensors, so the body monitors blood pressure in carotid and coronary arteries
- Monitoring BP tells us about blood flow from:
Pa = CO x TPR - Decrease in Pa reflects a decrease in either CO or TPR which compromises blood flow to brain and heart
- Blood pressure sensors in walls of the carotid arteries/aorta inform brain of pressure changes in these key feeder vessels
- Sensors detect arterial wall stretch - Baroreceptors
Describe how baroreceptors respond to an increase or decrease in pressure
- Increase:
Not much firing at rest then as pressure increases fast firing which eventually slows down and becomes constant, but at a higher level than before. ADAPTATION to a new normal - Decrease:
For a decrease in pressure the firing slows down proportionately
In the face of continued high or continued low pressure the threshold for baroreceptor activation can change (eg. long term hypertension - baroreceptors become normalised at the new pressure and less activated)
Describe the effects of increased blood pressure on baroreflex
Increase in BP is termed loading (eg. stress or exercise):
* Pulse pressure falls (decreased stroke volume)
* Vasodilation - decreases TPR & BP
* Decreased sympathetic nerve activity
* Increased Vagus nerve activity
Describe the effect of decreased blood flow on baroreflex
- Decrease in BP is termed unloading (eg. haemorrhage):
- Increased sympathetic activity & decreased Vagus activity
- Increased HR and force of contraction so increased cardiac output
- Arteriole constriction gives increased TPR
- Venous constriction increases central venous pressure and so by Starlings law increases stroke volume & cardiac output
- This all maintains blood pressure therefore blood flow to vital organs
- Also, adrenaline secretion, vasopressin (ADH) secretion & stimulation of RAAS (ie Angiotensin II increaseas Na+/H2O absorption in kidneys raising blood volume)
- Vasoconstriction decreases capillary pressure which increases absorption of interstitial fluid which also increases blood volume
Describe where veno atrial mechanoreceptors are found, when they are stimulated and what they result in
- Present on the superior and inferior vena cava and right atrium - connected to vagus afferents from the heart to the nucleus tractus solitarius in the medulla
- Stimulated by an increase in cardiac filling/central venous pressure when cardiac output is increasing
- Increased diuresis switches off ADH and RAAS which reduces sympathetic activity to the kidney to increase glomerular filtration - secretes atrial natriuretic peptide (increasing Na+ excretion)
Describe where the ventricular mechanoreceptors can be found and what their function is
- Present on the right and left ventricles - connected to vagus afferents that connect the nucleus tractus solitarius
- Stimulated by over distension of ventricles - depressor response
- It is a weak reflex - mild vasodilation, lower blood pressure and preload - protective
Describe nociceptive sympathetic afferents - where they are found, what they are stimulated by and what they cause
- Present on the ventricles towards the apex
- Stimulated by K+, H+ - things that may cause pain
- They send signals to neurons in the spinal cord - same as some somatic afferents - referred pain
- Mediate the pain of angina and myocardial infarction
- Reflex increased sympathetic activity- causes someone to be pale, sweaty, tachycardic for angina and MI symptoms
Explain why the baroreflex is so important
- Normally arterial pressure doesn’t change much it is around 100 mmHg
- When afferent fibres from baroreceptors are removed arterial pressure varies enormously - mean pressure is increased slightly past 100 mmHg
- When afferent fibres from cardiac receptors are also removed arterial pressure still varies but the mean arterial pressure is much higher - close to 150 mmHg
Describe the arterial chemoreceptors
Located in carotid and aortic bodies
- Stimulated by low O2 (hypoxia), high CO2 (hypercapnia), H+ and K+
- They are well supplied with blood flow around 20 ml/g/min
- Regulate ventilation and also drive cardiac reflexes during asphyxia (low O2/high CO2) shock (systemic hypotension) & haemorrhage
- When BP below the range of baroreflex (maximally unloaded) the chemoreceptors are still active and may compensate
Describe the pressor response produced by arterial chemoreceptors
- Increased sympathetic activity
- Tachycardia, increased selective arterial/venous constriction
- Increased cardiac output & blood pressure - especially preservation of cerebral blood flow
Describe what muscle metaboreceptors are and what they are stimulated by, as well as the pressor response they produce
- They are sensory fibres in group IV motor fibres located in the skeletal muscle
- Activated via metabolites - K+ , lactate and adenosine
Pressor response -
- Increased sympathetic activity
- Tachycardia, increase arterial/venous constriction
- Increase cardiac output/blood pressure
Explain why muscle metaboreceptors are important during isometric exercise
- Continually contracted muscle but joint angle and muscle length do not change e.g. in handgrip
- Higher blood pressure drives blood into the contracted muscle to maintain perfusion
- These muscles undergo metabolic hyperaemia allowing blood flow to the contracted tissue
Describe the central role of the nucleus tractus solitarius (NTS)
- Signal from stretched baroreceptor sent via afferent fibres enter Nucleus Tractus Solitarius (NTS)
- This then sends information out to the Caudal Ventrolateral Medulla (CVLM)
- The CVLM sends 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.
State what happens when baroreceptors are stimulated in the heart
- Loading of the baroreceptors also stimulates the vagus nerve which again 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
