Reflex Control of CVS Flashcards
What receptors do excitatory inputs activate?
Arterial chemoreceptors and muscle metaboreceptors
What type of response do excitatory inputs cause?
- Stimulation of reflexes- increased cardiac output , increased total peripheral resistance (TPR) and blood pressure
- PRESSOR response
What receptors do inhibitory inputs activate?
Arterial baroreceptors and cardiopulmonary receptors
What type of response do inhibitory inputs cause?
- Stimulation of reflexes- decreased cardiac output, decreased total peripheral resistance and blood pressure
- DEPRESSOR response
Describe arterial baroreceptors
- Vital to maintaining blood flow to brain and myocardium
- Detects arterial wall stretch
What are the importance of carotid arteries?
- There are no ‘blood flow’ sensors, so the body monitors blood pressure in carotid and coronary arteries
- Blood pressure sensors in walls of the carotid arteries inform brain of pressure changes
- These sensors are baroreceptors
- Decrease in pressure reflects a decrease in either CO or TPR
Describe how baroreceptors respond to an increase in pressure
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
Describe how baroreceptors respond to an decrease in pressure
For a decrease in pressure the firing slows down proportionately
How do long-term pressure changes affect baroreceptor activation?
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
- 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 PART 1
- 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
- Maintains blood pressure therefore blood flow
Describe the effect of decreased blood flow on baroreflex PART 2
- Adrenaline secretion, vasopressin (ADH) secretion & stimulation of RAAS
- Vasoconstriction decreases capillary pressure which increases absorption of interstitial fluid which also increases blood volume
Describe where venoatrial mechanoreceptors are found
- 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
How are venoatrial mechanoreceptors activated?
By an increase in cardiac filling / central venous pressure when cardiac output is increasing
What does venoatrial mechanoreceptor activation result in?
- Increased diuresis switches off ADH and RAAS which reduces sympathetic activity to the kidney to increase glomerular filtration - secretes atrial natriuretic peptide
- Reflex tachycardia due to the rapid infusion of volume into the venous system
Describe where the ventricular mechanoreceptors can be found
Present on the right and left ventricles - connected to vagus afferents that connect the nucleus tractus solitarius
How are ventricular mechanoreceptors activated?
Stimulated by over distension of ventricles
What does ventricular mechanoreceptor activation result in?
Mild vasodilation , lower blood pressure and preload
Describe where nociceptive sympathetic afferents are found.
Present on the ventricles towards the apex
How are nociceptive sympathetic afferents activated?
Stimulated by K+, H+
What does nociceptive sympathetic afferent activation result in?
- Send signals to neurons in the spinal cord
- Mediate the pain of angina and myocardial infarction
- Increased sympathetic activity- causes someone to be pale and increased sweating
Explain why the baroreflex is so important
Involved in maintenance of arterial pressure
Describe where the arterial chemoreceptors are found
Located in carotid and aortic bodies
What can stimulate arterial chemoreceptors?
Stimulated by low O2 (hypoxia) and high CO2 (hypercapnia)
What is the purpose of arterial chemoreceptors?
Regulate ventilation and also drive cardiac reflexes during asphyxia (low O2/high CO2) shock (systemic hypotension) & haemorrhage
- Can compensate when blood pressure below baroreflex
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
Sensory fibres in group IV motor fibres located in the skeletal muscle
What are muscle metaboreceptors activated by?
Metabolites such as K+, lactate, and adenosine
What is the pressor response caused by muscle metaboreceptor activation?
- Increased sympathetic activity
- Tachycardia
- Increase arterial/venous constriction
- Increase cardiac output/blood pressure
Explain why muscle metaboreceptors are important during isometric exercise
- Higher blood pressure drives blood into the contracted muscle to maintain perfusion
- Muscles undergo metabolic hyperaemia allowing blood flow to the contracted tissue
Describe the central role of the nucleus tractus solitarius (NTS). PART 1
- Signal from stretched baroreceptor sent via afferent fibres enter Nucleus Tractus Solitarius (NTS)
- This sends information out to the Caudal Ventrolateral Medulla (CVLM)
- CVLM sends information to the rostral ventolateral medulla (RVLM)
Describe the central role of the nucleus tractus solitarius (NTS). PART 2
- Causes 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
- 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
Describe what happens in vagal parasympathetic outflow - sinus tachycardia
- Inhibitory input from inspiratory centre
- Each inhalation switches off the nucleus ambiguous
- The inhibitory parasympathetic signal to the vagus decreases and the heart rate increases slightly
Describe what happens in vagal parasympathetic outflow - vasovagal syncope
- Cortex higher centres stimulate the hypothalamus limbic system (emotional centre)
- Stimulates the NTS which in turn stimulates the nucleus ambiguous causing increased activity of the vagal nerve and depressor effect on the AV and SA nodes
- Can lead to fainting (syncope)- causes by decreased cerebral blood flow due to sudden drop in arterial cardiac output and blood pressure
What is syncope?
Fainting
In terms of the link between the CVLM and RVLM, describe the effects of intravenous phenylephrine on the body.
- Intravenous phenylephrine (α1 agonist) increases the TPR and BP
- With the BP rising, the baroreceptors are loaded
- There is a signal from the baroreceptors to the NTS, then to the CVLM
- The CVLM signal inhibits the RVLM signals
- Sympathetic activity to the heart and vessels decreases
- The lower sympathetic signal gives vasodilation and increased BP
What is the effect on arterial blood pressure when afferent fibres from baroreceptors are removed?
Large variation in arterial pressure
What is the effect on arterial blood pressure when afferent fibres from cardiac receptors are removed?
Slight variation in arterial pressure
What part of the brain deals with reflexes?
Medulla
What do the following detect?
BARORECEPTORS
CHEMORECEPTORS
METABORECEPTORS
BARORECEPTORS - Pressure changes
CHEMORECEPTORS - CO2 and oxygen
METABORECEPTORS - products of metabolism
What is the importance of having baroreceptors in the aortic arch?
- EXAMPLE: if there is low pressure in the aortic arch, diastolic pressure is low
- May indicate poor perfusion of coronary circulation
