Control Of Heart Function Flashcards
2 nodes of the heart
Sinoatrial node:pacemaker 60-100bpm. Found in junction if crista terminallims
Atrioventriculat node:Has pacemaker activity as it slows calcium mediated action potential. Found at triangle of koch
What tract connects SAN and AVN
Internodal tracts which are specialized myocytes
Which fibres have the fastest conduction velocity
Purkinje fibres
3 phases of sinoatrial node excitation
Phase 4 pre potential: Na+ influx through funny channels. Nodal cells have no resting membrane potentials. Increase in mp
Phase 0 upstroke: calcium influx increases mp
Phase 3 repolarisation occurs due to k+ efflux
Why do different parts of the heart have different action potential shapes
As different ion currents flow and different ion channel expressions in cell membrane
Difference in length of AP between cardiac myocytes and nerves
Cardiac ap is longer 200-300ms vs 2-3ms
Longer as duration of AP controls duration of contraction and long slow contraction is needed for effective pump
Ventricular cell action potential
5 phases
Phase 0 - Upstroke (-90 mV up to ~20-30 mV) → Na+ influx
Phase 1 - Early re-polarisation → Decrease in Na+ permeability & K+ efflux
Phase 2 - Plateau (maintains cell at a level of depolarisation at value of 0mV) → Due to Ca2+ influx
Phase 3 - Re-polarisation → Happens at ~270ms after stark of upstroke → K+ efflux
Phase 4 - RMP - Na+/K+ ATPase pump
What two parts of CNS control heart
Autonomic NS- the cardioregulatory centre and vasomotor centres in medulla
Parasympathetic communicates through vagus nerve causing a decrease in heart rate by decreasing phase 4 slope of SA nodal cell thus decreases pressure
Symlathiect cimmunjcates via sympathetic nerves causing increase in heart rate (chronograph) thus increase in slop of phase 4. Increases force of contraction (inotropy) as increases ca2+ thus increase bp
What do parasympathetic nerves consist of, and which part of the spinal cord do they arise from?
- Arise from cranial and sacral part of spinal cord
- They have pre-ganglionic fibres that release ACh as NT
- Post-ganglionic fibres also release ACh
- Important for controlling heart rate
What type of receptor on the SA nodal cell receives the post-ganglionic fibre? For pns
M2 muscarinic receptor → g-coupled receptor
Coupled with Gi protein which inhibits adenylyl cyclase which prevents conversion of ATP to protein kinase A
What do sympathetic nerves consist of, and which part of the spinal cord do they arise from?
- Arise from thoracolumbar part
- Pre-ganglionic fibres use ACh
- Post ganglionic fibres use NA
- Synapse onto paravertebral ganglia
- Important for controlling circulation
What type of receptor on the SA nodal cell receives the post-ganglionic fibre? SNS
beta-1-receptor
Stimulate adenylyl cyclase and increase levels of protein kinase A through second messenger pathway
Where is vasomotor centre located
Bilaterally in reticular substance of medulla & lower third of pons
What is VMC composed of
- Vasoconstrictor area
- Vasodilator area
- Cardio-regulatory inhibitory area
What does VMC do
Transmits impulses distally through spinal cord to almost all blood vessels
Many higher centres of brain such as hypothalamus can exert powerful excitatory or inhibitory effects on the VMC
- What do the lateral VMC portions do?Lateral portions of VMC controls heart activity by influencing heart rate and contractility
- What does the medial VMC portion do?Medial portion of VMC transmits signals via vagus nerve to heart that tend to decrease HR
What does sympathetic innervation to kidney do?
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Decrease glomerular filtration → reduced Na+ excretion
- This causes increase in blood volume (also done through aldosterone)
- Change in blood volume is detected by venous volume receptors
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Increase in renin secretion which increases angiotensin II production
- Causes vasoconstriction and increase in blood pressure which is detected by arterial baroreceptors
- Also causes aldosterone release which impacts blood volume
What part of the kidney do sympathetic nerves innervate?
Afferent and efferent arterioles of glomerulus and nephron tubule cells
What happens at afferent arterioles due to sympathetic activity (2)?
- Primary site of sympathetic activity
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Activation of alpha-1-adrenoreceptors by noradrenaline (NA) causes vasoconstriction
- This reduces glomerular filtration rate which decreases Na+ filtered
- Juxtaglomerular cells are site of synthesis, storage and release of renin
- Activation of beta-1-adrenoceptor in JGA cells causes renin secretion which increases blood volume (and increases blood pressure)
- What does the cardiopulmonary circuit consist of and what do they sense?
- Large pulmonary vessels
- Are volume sensors (also atria and right ventricle)- send signals through glossopharyngeal and vagus nerves
Explain how the cardiopulmonary circuit responds to a decrease and increase in volume?
- Decrease in filling (due to less blood returning to heart) → decreased baroreceptor firing → increased SNS activity
- Distention (more filling of heart) → increased baroreceptor firing → decreased SNS activity
What does the arterial circuit consist of (3)?
Aortic arch, carotid sinus and afferent arterioles of kidneys
Explain how the arterial circuit responds to a decrease and increase in pressure?
- Pressure sensors- send signals through glossopharyngeal and vagus nerves
- Decrease in pressure (detected by baroreceptors in carotid sinus & aortic arch) → decreased baroreceptor firing → increased SNS activity (received by beta-1-receptor of SAN on heart, which increases blood pressure)
- ## Increase in pressure → increased baroreceptor firing → decreased SNS activity (which decreased blood pressure)
What are the 2 circulations of the blood?
- Pulmonary and systemic
- Right heart → lungs → left heart → body
What is venous volume distribution affected by
Veins and venules contain 61% of blood
- Peripheral venous tone
- Gravity
- Skeletal muscle pump
- Breathing
What is central venous pressure and what does it determine?
Mean pressure in the right atrium
Determines amount of blood flowing back to heart, which in turn determines stroke volume (Starling’s Law)
How does constriction of veins affect compliance and venous return?
Reduces compliance and increases venous return
- What does constriction in arteries determine?
- Blood flow to downstream organs
- Mean arterial blood pressure
- The pattern of blood flow to organs
What are some local mechanisms which regulate local blood flow in an organ? (4)
Are intrinsic to smooth muscle and are endothelium-derived mediators:
- Nitric oxide (NO): potent vasodilator, which diffuses into vascular smooth muscle cells.
- Prostacylin: vasodilator that also has antiplatelet & anticoagulant effects
- Thromboxane A2 (TXA2): vasoconstrictor that is also heavily synthesised in platelets
- Endothelins (ET): vasoconstrictors generated from nucleus of endothelial cells
What are some systemic mechanisms which regulate blood flow? (5)
These include autonomic NS and circulating hormones and non-endothelium-derived mediators:
- Kinins: bind to receptors on endothelial cells & stimulate NO synthesis – vasodilator effects
- Atrial natriuretic peptide (ANP): secreted from the atria in response to stretch – vasodilator effects to reduce BP
- Vasopressin (ADH): secreted from pituitary gland. Binds to V1 receptors on smooth muscle to cause vasoconstriction
- Noradrenaline/Adrenaline: secreted from adrenal gland (& SNS); causes vasoconstriction
- Angiotensin II: potent vasoconstrictor from the renin-angiotensin-aldosterone axis. Also stimulates ADH secretion.
