Reflexes Flashcards
How do excitatory inputs lead to pressor response?
arterial chemoreceptors + muscle metaboreceptors (Work) switch on reflexes to increase CO/TPR/BP
How do inhibitory inputs lead to depressor response?
arterial baroreceptors switch off reflexes to decrease CO/TPR/BP
How does the body monitor blood flow w/o blood flow sensors?
Monitoring arterial BP shows blood flow Blood flow (CO) = BP / TPR BP = CO x TPR CO = blood flow (both have units of volume/time)
Define baroreceptors?
sensors detect arterial wall stretch
Sig of BP sensors?
allow specific sensing of blood flow to brain and heart
Location of arterial baroreceptors?
Carotid sinus at base of internal carotid
Aorta
Location of arterial chemoreceptors?
Carotid body
Aortic bodies
How do baroreceptors respond to changes in pressure?
Dynamic (speed of onset)
Sensitivity (relative change to amount of pressure)
Adaptation (change over time)
How do baroreceptors respond to hypertension?
- baroreceptors are less activated
- threshold becomes greater
- don’t regulate BP as well
Describe effect of increased BP on baroreflex (loading)?
- baroreceptors activated
- stimulate carotid sinus nerve
- BP falls (depressor reflex)
- bradycardia
- pulse p falls so lower SV
- decreased symp + increased para via vagus
- vasodilation -> decreased TPR -> BP
Describe effect of decreased BP on baroreflex (unloading)?
- increased symp + decreased vagus
- increased HR + contractility -> increased CO
- arteriolar constriction
- increased TPR
- venous constriction
- increased CVP -> SV -> CO (Starling’s law)
- pressor reflex maintains BF + BP to vital organs
Describe effect of severe drop in BP on baroreflex?
- secretion of ADH + A
- stimulation of RAAS :
- Ang II production
- vasoconstriction
- decreased capillary p
- absorption of interstitial fluid
- increased blood volume
- Ang II production
- aldosterone secretion
- Na/H2O reabsorption in kidneys
- increased blood volume
Diff types of cardiac receptors?
- Nociceptive sympathetic afferents
- Veno-atrial mechanoreceptors
- Ventricular mechanoreceptors
What are nociceptive sympathetic afferents?
- chemosensitive ventricular afferent C fibres
- stimulated by K+, H+ (lactate), bradykinin during ischaemia
- mediate pain/symptoms of angina + MI
What’s the basis of referred pain?
C fibres converge onto same neurones in spinal cord as somatic afferents
Which type of reflex is nociceptive sympathetic afferents?
Acute pressor reflex - increases symp: pale, sweaty, tachycardia
What are veno-atrial mechanoreceptors?
stimulated by increase in filling/CVP
What’s the Bainbridge effect?
reflex tachycardia due to rapid infusion of volume into venous system (veno-atrial mechanoreceptors + pacemaker distension)
Which type of reflex is veno-atrial mechanoreceptors?
Initial pressor reflex - increases symp: tachycardia
Longer term depressor reflex - increased diuresis decreases blood volume, feedback loop via changes in ADH, ANP, RAAS
What are ventricular mechanoreceptors
stimulated by over distension of ventricles
Which type of reflex is ventricular mechanoreceptors?
Depressor reflex - protective for heart, overrides veno-atrial reflex?
What are arterial chemoreceptors stimulated by?
low O2 (hypoxia), High CO2 (hypercapnia), H+ , K+
Blood flow of arterial chemoreceptors?
v high = 20 ml g-1 min-1
Role of arterial chemoreceptors?
-Regulate ventilation
-Drive cardiac reflexes during:
Asphyxia (low O2/high CO2)
Shock (systemic hypotension)
Haemorrhage (when BP below range of baroreflex)
-Pressor reflex
Describe pressor response of arterial chemoreceptors?
- increased symp
- tachycardia
- increased selective arterial/venous constriction
- increased BP/CO
- preservation of cerebral blood flow during low O2 (asphyxia/haemorrhage)
What are muscle metaboreceptors?
work receptors
Where are muscle metaboreceptors?
sensory fibres in Group IV motor fibres in skeletal muscle
How are muscle metaboreceptors activated?
via metabolites; ATP, K+, lactate, adenosine
Role of muscle metaboreceptors?
- Pressor response
- During isometric exercise
Describe pressor response of muscle metaboreceptors?
- increased symp
- tachycardia
- increased arterial/venous constriction
- increased BP/CO
Why are muscle metaboreceptors important in isometric exercise?
- joint angle and muscle length don’t change when weight lifting / handgrip
- maintains blood perfusion to contracted muscle
- muscle undergoes metabolic hyperaemia
- blood flow to contracted tissue
Effect of local anaesthetic (LA) in muscle?
- metaboreceptor afferent fibres blocked
- prevent pressor response
Role of cerebral cortex?
-Controls hypothalamus, limbic system etc
-Tells body we’re about to do exercise to increase
HR + CO
Role of hypothalamus?
Trigger areas in medulla:
Rostral Ventral Lateral Medulla (RVLM)
Caudal Ventero Lateral Medulla (CVLM)
Nucleus Tractus Solitarius (NTS).
Describe effect of increase baroreceptors
- baroreceptors detect increase in BP
- afferents to excitatory NTS
- increase CVLM
- inhibition of RVLM
- inhibitory pathway acts as ‘thermostat’
- switch off sympathetic nerves - DEPRESSOR
- decrease BP
Describe effect of increase arterial chemoreceptor/muscle work receptors
- inhibitory NTS
- inhibit CVLM
- switch on RVLM and sympathetic nerves - PRESSOR
Effect of destroying spine (spinal transection)?
- sympathetic fibres in spinal cord to CVS destroyed/lost
- so only vagal system remains of baroreflex
- acute hypotension
What do vagal parasympathetic fibres innervate?
SAN + AVN (vagal outflow to pacemaker),
Describe the baroreflex
-aortic baroreceptors stimulated by increase in BP
-afferents to nucleus tractus solitarius
-nucleus tractus solitarius stimulates nucleus ambiguus
-vagal parasympathetics from nucleus ambiguus to SAN
-decrease HR (bradycardia) + CO
-decrease BP
The is
also under
What controls nucleus ambiguus?
- Controlled directly from higher centres : limbic system (one of the reasons someone can experience a vaso-vagal reflex)
- Receives communication from inspiratory centre (what is controlling breathing is also linked to the vagal control of heart)
Describe effect of inspiration on inhibitory control
-nucleus ambiguus receives communication from inspiratory centre
-every inspiration inhibits nucleus
ambiguus
-switch off vagal activity to SAN
-sinus arrhythmia (normal)
-sinus tachycardia when you breathe in.
-seen in the ECG
Describe effect of expiration of inhibitory control
- expiration
- inhibitory pathway is turned
- allows vagal activity on heart
- HR slows
- HR changes due to breathing
- sinus arrhythmia (normal rhythm change in course of breathing)
What’s limbic system?
emotional centre
Describe how fainting/syncope/vaso-vagal attack occurs?
-sudden drop in arterial BP via:
Vagal bradycardia
Sympathetic activity – further decrease in HR due to vasodilation
Peripheral vasodilatation (decerased TPR)
-decreased cerebral BF
-(reduced oxygen delivery)
What happens if you denervate baroreceptors?
spread around mean BP
What happens if you denervate baroreceptors + cardiac receptors?
mean BP rises, less variation stable BP
What happens if you remove CVS reflexes?
less controlled + higher BP because reflexes stabilise BP
