Cardiac circulation control Flashcards
What is a cardiac reflex
◦ “Reflex loops between the heart and central nervous system” which regulate heart rate and peripheral vascular resistance to maintain physiologic homeostasis
List the cardiac reflexes
4B’s 2C’s and a ROD
Bainbridge
Baroreceptor
Bezold Jarish
Barcroft Edholm
Chemoreceptor
Cushing
Respiratory sinus arrhythmia
Oculocardiac
Diving
What is the most important cardiac reflex
Baroreceptor
What are the 5 domains you need to consider for every reflex and every endocrine system
Sensor
Afferent
Processor
Efferent
Effector
Barorecepotr reflex
◦ Sensors: mechanoreceptors detect pressure (carotid sinus and aortic arch)
◦ Afferent: vagus and glossopharyngeal nerves
◦ Processor: nucleus of the solitary tract and nucleus ambiguus (vasomotor centre)in medulla oblongata
◦ Efferent: vagus nerve and sympathetic chain
◦ Effect: increased HR and BP (via SVR + increased stroke volume/cardiac output) in response to a fall in BP
Bainbridge reflex
◦ Afferent: vagus (atrial stretch) - increase in central venous pressure triggers low pressure mechanoreceptors in great veins and RA
◦ Processor: nucleus of the solitary tract and the caudal ventral medulla
◦ Efferent: vagus nerve and sympathetic chain
◦ Effect: increased RA pressure produces an increased heart rate;
Chemoreceptor reflex
◦ Afferent: carotid / aortic chemoreceptors (low PaO2 and/or high PaCO2) - in the context o response to MAP this will only occur under extreme hypotension
◦ Processor: nucleus of the solitary tract and nucleus ambiguus
◦ Efferent: vagus nerve and sympathetic chain
◦ Effect: bradycardia and hypertension in response to hypoxia
‣ (also secondary tachycardia from Bainbridge and Hering-Breuer reflexes)
Cushings reflex
◦ Afferent: mechanosensors in the rostral medulla?
◦ Processor: rostral ventrolateral medulla
◦ Efferent: sympathetic fibres to the heart and peripheral smooth muscle
◦ Effect: hypertension and baroreflex-mediated bradycardia
Bezold Jarish reflex
◦ Afferent: vagus (mechanical/chemical sttimuli to the cardiac chambers - ventricular) C fibres
◦ Processor: nucleus of the solitary tract
◦ Efferent: vagus nerve and sympathetic chain
◦ Effect: hypotension, coronary artery dilation and bradycardia
Oculocardiac reflex
◦ Afferent: trigeminal nerve (pressure to the globe of the eye)
◦ Processor: sensory nucleus of CN V; nucleus of the solitary tract
◦ Efferent: vagus nerve and sympathetic chain
◦ Effect: vagal bradycardia, systemic vasoconstriction, cerebral vasodilation
Diving reflex
◦ Afferent: trigeminal nerve (cold temperature; pressure of immersion)
‣ Processor: sensory nucleus of CN V; nucleus of the solitary tract
◦ Efferent: vagus nerve and sympathetic chain
◦ Effect: vagal bradycardia, systemic vasoconstriction, cerebral vasodilation
Barcroft Edholm reflex
◦ Afferent: emotional distress, hypovolaemia
◦ Processor: unknown
◦ Efferent: vagus nerve and sympathetic chain
◦ Effect: bradycardia, systemic vasodilation, hypotension
Respiratory sinus arrhtyhmia
◦ Afferent: central respiratory pacemaker
◦ Processor: nucleus ambiguus
◦ Efferent: vagus nerve, via the cardiac ganglion
◦ Effect: cyclical increase of heart rate during inspiration
What type of receptor is a baroreceptor? How are they activated?
- Baroreceptors are mechanoreceptors which respond to stretch stimuli.
- This strecth deforms mechanically sensitive sodium channels (DEG/ENaC, degenerin/epithelial sodium channels)
- With sufficient stimulus, sodium current increases to the point where the membrane potential reaches the threshold of local voltage-gated sodium channels, and generates a propagating action potential
Where are baroreceptors located
- Arterial baroreceptors (“high pressure baroreceptors”) are located at the junction of the intima and media of the aortic arch and carotid sinuses
◦ stretch sensitive mechanoreceptors
◦ Carotid sinus - small neurovascular structure in the adventitia in the dilated portion of the common carotid artery (carotid bulb) at its bifurcation. Not to be confused with the carotid body which is a PaO2/PaCO2 sesning chemoreceptor at the same location at the bifurcation of the vessels
◦ sinus senses stretch, and body senses breathing
◦ Aortic arch - medio-adventitial junction mainly confined to a saddle shaped area between the brchiocephalic trunk and the origin of the left subclavian
What layer of the vessel are baroreceptors in
- Arterial baroreceptors (“high pressure baroreceptors”) are located at the junction of the intima and media of the aortic arch and carotid sinuses
◦ stretch sensitive mechanoreceptors
◦ Carotid sinus - small neurovascular structure in the adventitia in the dilated portion of the common carotid artery (carotid bulb) at its bifurcation. Not to be confused with the carotid body which is a PaO2/PaCO2 sesning chemoreceptor at the same location at the bifurcation of the vessels
◦ sinus senses stretch, and body senses breathing
◦ Aortic arch - medio-adventitial junction mainly confined to a saddle shaped area between the brchiocephalic trunk and the origin of the left subclavian
Where is the carotid baroreceptor
- Arterial baroreceptors (“high pressure baroreceptors”) are located at the junction of the intima and media of the aortic arch and carotid sinuses
◦ stretch sensitive mechanoreceptors
◦ Carotid sinus - small neurovascular structure in the adventitia in the dilated portion of the common carotid artery (carotid bulb) at its bifurcation. Not to be confused with the carotid body which is a PaO2/PaCO2 sesning chemoreceptor at the same location at the bifurcation of the vessels
◦ sinus senses stretch, and body senses breathing
◦ Aortic arch - medio-adventitial junction mainly confined to a saddle shaped area between the brchiocephalic trunk and the origin of the left subclavian
Where is aortic baroreceptor
- Arterial baroreceptors (“high pressure baroreceptors”) are located at the junction of the intima and media of the aortic arch and carotid sinuses
◦ stretch sensitive mechanoreceptors
◦ Carotid sinus - small neurovascular structure in the adventitia in the dilated portion of the common carotid artery (carotid bulb) at its bifurcation. Not to be confused with the carotid body which is a PaO2/PaCO2 sesning chemoreceptor at the same location at the bifurcation of the vessels
◦ sinus senses stretch, and body senses breathing
◦ Aortic arch - medio-adventitial junction mainly confined to a saddle shaped area between the brchiocephalic trunk and the origin of the left subclavian
How does the baroreceptor receptor respond to changes in BP –> how does it convey the change to the controller
- Increased blood pressure (increased stretch, increased receptor firing rate)
- Decreased blood pressure (decreased receptor firing rate)
How does the carotid baroreceptor get to the central controller
- From the carotid sinus: carotid sinus nerve, a branch of the glossopharyngeal nerve - courses anteromedially to the internal carotid artery and joins the body of the glosspharngeal nerve at the base of the skull where its cell bodies lie in the petrosal ganglion
◦ Carotid sinus receptors are innervated by the sinus nerve of Hering, which is a branch of the glossopharyngeal nerve
How does the aortic baroreceptor get back to the central controller
- From the aortic arch: aortic nerve, a branch of the vagus nerve - cell bodies also in the petrosal ganglion which is in the jugular foramen where the nerves then synapse with NTS
- Both of these nerves travel through the jugular foramen to enter the medulla
◦ Both myelinated (A) and unmyelinated (C) fibres - fast response and baseline slower regulation
What is the processor of the baroreflex?
Nucleus of the solitary tract
* Sensory interneurons in the posteiror medulla (caudal ventrolateral medulla)
* Roles
◦ ANS
◦ Taste information - mediating cough and gag
◦ Middle ear - tympanic branch of CN9
What does the NTS do?
Nucleus of the solitary tract
* Sensory interneurons in the posteiror medulla (caudal ventrolateral medulla)
* Roles
◦ ANS
◦ Taste information - mediating cough and gag
◦ Middle ear - tympanic branch of CN9
What connections does the NTS have?
◦ Excitatory glutamate-mediated neurotransmission to the nucleus ambiguus translates the afferent signal into increased vagal activity
◦ GABA-ergic inhibitory neurons of the caudal ventral medulla translate the afferent signal into the inhibition of the rostral ventrolateral medulla (vasomotor centre - constant tonic output), which coordinates sympathetic tone
◦ Effrent fibres to the hypothalamus help coordinate the humoural response to changes in blood pressure.
Where is the PSNS 1st order neurons coming from?
◦ Excitatory glutamate-mediated neurotransmission to the nucleus ambiguus translates the afferent signal into increased vagal activity
◦ GABA-ergic inhibitory neurons of the caudal ventral medulla translate the afferent signal into the inhibition of the rostral ventrolateral medulla (vasomotor centre - constant tonic output), which coordinates sympathetic tone
◦ Effrent fibres to the hypothalamus help coordinate the humoural response to changes in blood pressure.
Where is the SNS first order neurons coming from?
◦ Excitatory glutamate-mediated neurotransmission to the nucleus ambiguus translates the afferent signal into increased vagal activity
◦ GABA-ergic inhibitory neurons of the caudal ventral medulla translate the afferent signal into the inhibition of the rostral ventrolateral medulla (vasomotor centre - constant tonic output), which coordinates sympathetic tone
◦ Effrent fibres to the hypothalamus help coordinate the humoural response to changes in blood pressure.
What are the efferent nerves to the heart? What supplies the SA node? What supplies the AV node
- Sympathetic fibres to the heart and peripheral resistance vessels
- Vagal efferents to the cardiac ganglion (heart rate)
Effector: Myocardium, SA and AV nodes, vascular smooth muscle - The right vagus does the SA node and the left vagus does the AV node, with enough overlap that the loss of a vagus does not produce total parasympathetic denervation. PSNS more important in HR control
Describe how circulating volume changes SV, CO and HR using a graph
In response to arterial hypotension:
◦ Decreased receptor discharge rate
◦ Thus, decreased vagal and disinhibited sympathetic efferents
◦ Thus, systemic vasoconstriction and tachycardia
* In response to arterial hypertension:
◦ Increased receptor discharge rate
◦ Thus, increased vagal and inhibited sympathetic efferents
◦ Thus, systemic vasodilation and bradycardia
How long does a baroreceptor response take?
0.5 - 1 seconds
What is the first and fastest response of the baroreceptor reflex
Increased or decreased HR due to PSNS or vagal supply - rapid effect of vagal on HR is through inward rectifying potassium channels
cAMP system is slower
When is the baroreceptor response active?
Constantly, constant tonic activty
Immediate and proportionate response to changes
What is the minimum value for change for baroreceptor firing change
Depends on the baseline state - more sensitive at the higher and lower range of pressures and depends on abruptness of change
Which baroreceptor is more sensitive to hypotension?
Carotid
Which baroreceptor is more sensitive to hypertension
Aortic
How is firing of baroreceptors related to change in pressure?
Generally proportional within the normal ranges however has hysteresis
- Assymmetry: the firing rate has a hysteresis, i.e. it increases exponentially with increasing carotid sinus pressure, but also plateaus at very high pressures. the steepest part of the response seems to be around the normal systolic pressure range, i.e. 100-140 mmHg.
Describe how mechanosensors sense stretch
- Stretch activates sodium current which then reaches membrane potential threshold for voltage gated channels –> action potential
◦ The more stretch the more frequently the receptors fire
What is a stronger stimulus - Bainbridge or baroreflex
Baroreflex
Bainbridge reflex
- Stimulus - pressure/stretch (increased CVP) - sensitive to a volume change of 5-10% in either direction
- Sensors - stretch sensitive low pressure mechanoreceptors in atria, great veins and pulmonary arteries (type B receptors)
◦ Type A receptors - activated by changes in atrial wall tension during systole
◦ Type B - diastolic low pressure receptors - Afferent: vagus triggers low pressure mechanoreceptors in great veins and RA
- Processor: nucleus of the solitary tract and the caudal ventral medulla
- Efferent: vagus nerve (cardiac ganglion) and sympathetic chain
- Effect: increased RA pressure produces an increased heart rate;
What happens with increased RA pressure as an isolated phenomena
INcreased Bainbridge reflex firing –> tachycardia
What are the two mechanoreceptor subtypes in the Bainbridge reflex
- Stimulus - pressure/stretch (increased CVP) - sensitive to a volume change of 5-10% in either direction
- Sensors - stretch sensitive low pressure mechanoreceptors in atria, great veins and pulmonary arteries (type B receptors)
◦ Type A receptors - activated by changes in atrial wall tension during systole
◦ Type B - diastolic low pressure receptors - Afferent: vagus triggers low pressure mechanoreceptors in great veins and RA
- Processor: nucleus of the solitary tract and the caudal ventral medulla
- Efferent: vagus nerve (cardiac ganglion) and sympathetic chain
- Effect: increased RA pressure produces an increased heart rate;
How is the chemoreceptor involved in BP control?
- Regional hypoxia is one of the metabolic stimuli for local vasodilation - systemic hypoxia causes systemic peripheral vasoconstriction
- Afferent: carotid body glomus (Glossopharyngeal) / aortic body glomus (aortic tract of vagus) chemoreceptors (low PaO2 and/or high PaCO2) - in the context o response to MAP this will only occur under extreme hypotension
- Processor: nucleus of the solitary tract and nucleus ambiguus
- Efferent: vagus nerve and sympathetic chain
◦ To the SA node, AV node and vascular smooth muscle
What is the effector response to chemoreceptor triggering cardiovascularly
- Effect:
◦ Primary effects
‣ Vagal - bradycardia
‣ Symapthetic effects - hypertension (tachycardia often absent)
◦ Secondary effects
‣ Increased preload due to increased ventilation –> Bainbridge reflex increasing HR
‣ Activation of pulmonary stretch receptors and thus activation of the Hering Breuer reflex increasing HR
‣ Due to significant hypertension tachycardia is often not marked due Baroreceptors
Where is the receptor for the cushings reflex
mechanosensors in the rostral medulla?/cerebral meduallary vasomotor centre ischameia
Cushings relfex
- Stimulus - ICP or cerebral ischaemia
- Sensors - mechanosensors in the rostral medulla?/cerebral meduallary vasomotor centre ischameia
- Afferent nerves - fibres from the medullary mechanosensitive areas to sympathetic ganglia, descending inhibitory control from cerebral hemispheres to medullary vasomotor centre
- Processor: rostral ventrolateral medulla
- Efferent: sympathetic fibres to the heart and peripheral smooth muscle
- Effect: hypertension and tachycardia –> baroreflex-mediated bradycardia
Where is the vasomotor centre
Rostral ventrolateral medulla
Bezold Jarish reflex is triggered by
vagus (mechanical/chemical sttimuli to the cardiac chambers - ventricular) C fibres
◦ Responds to ventricular or atrial stretch; as well as chemical ATP/capsacin/snake venoms
◦ Loss of stretch decreases firing rat eof C fibre mediated vagal afferent limb, and increased stretch stimulates the reflex
What processes the Bezold Jarish reflex? What does it result in?
- Processor: nucleus of the solitary tract
- Efferent: vagus nerve and sympathetic chain
- Effect: hypotension, coronary artery dilation and bradycardia
- Its physiological role is implicated in haemorrhage or profound hypovolaemia where the vasoconstriction is greater than would purely be seen from barorecepetors and is thought to be due to decreased reflex
Oculocardiac reflex triggered by?
- Stimulus - pressure to the globe of the eye or traction on eye muscles
- Sensor - mechanosensitive stretch receptors in the facial muscles, especially periorbital muscles, and in the globe of the eye
What does oculocardiac reflex result in?
- Afferent: trigeminal nerve - long and short ciliary nerves
- Processor: sensory nucleus of CN V; nucleus of the solitary tract
- Efferent: vagus nerve and sympathetic chain
- Effect: vagal bradycardia, systemic vasoconstriction, cerebral vasodilation
Diving reflex triggered by?
- Stimulus: trigeminal nerve sensory distribution
◦ Pressure to the globe of the eye, or traction on the eye muscles
◦ Pain in the trigeminal nerve distribution
◦ Temperature (cold)
◦ Chemical stimulus of the anterior ethmoidal nerve (noxious) - Sensors: Pain, temperature, chemical and mechanosensitive stretch receptors in the trigeminal nerve distribution
What is the processor fo the diving reflex?
- Processor:
◦ Nucleus of the solitary tract: vagal response
◦ Rostral medulla: sympathetic response
◦ Ventral medulla: apnoea
Using a cardiac function curve illustrate the impact of exercise
Draw a line representing MAP and HR during a valsalva manouvre
