6.14 - Control of Heart Function Flashcards
What can the main anatomical components of the heart be broadly categorised as?
- muscle cells (cardio-myocytes) - can contract and relax in response to electrical stimuli, essential for pumping blood around the body
- specialised electrical cells - cells that create spontaneous currents and those that transmit currents exist within the heart, essential for supplying blood to the heart - most prominent in controlling heart function
- vessels - major BVs transport blood in/out of heart, whilst coronary BVs supply blood to the heart
What is the sinoatrial node and where is it?
- pacemaker of the heart: 60-100 bpm
- junction of crista terminalis - upper wall of right atrium & opening of superior vena cava
What is the atrioventricular node and where is it?
- has pacemaker activity: slow calcium-mediated action potential
- triangle of Koch at base of right atrium
What are the tracts of the heart?
- internodal tracts - specialised myocytes which connect the SAN and AVN
- Bundle of His & bundle branches - AVN –> Bundle of His –> branches at intraventricular septum –> Purkinje fibres –> apex
- Purkinje fibres - specialised conducting fibres
What are the phases of nodal cell action potential?
- nodal AP only has 3 phases (4, 0, 3)
- phase 4 - pre-potential - Na+ influx through a ‘funny’ channel, nodal cells do not have resting membrane potentials
- phase 0 - upstroke - due to Ca2+ influx (and Na+ influx)
- phase 3 - repolarisation - due to K+ efflux
Why do different parts of the heart have different action potential shapes?
Caused by different ion currents flowing and different ion channel expression in cell membrane
What is the difference in length of action potential between cardiac myocytes and nerves?
Cardiac AP much longer - 200-300ms vs 2-3ms
Why is cardiac action potential so long?
- duration of AP controls duration of contraction of heart
- long, slow contraction is required to produce an effective pump
What are the phases of a cardiac myocyte action potential?
- 5 phases - 0,1,2,3,4
- phase 0 - upstroke (Na+ influx)
- phase 1 - early repolarisation (K+ efflux)
- phase 2 - plateau (Ca2+ influx)
- phase 3 - repolarisation (K+ efflux)
- phase 4 - resting membrane potential
What is the absolute refractory period? (ARP)
Time during which no action potential can be initiated regardless of stimulus intensity
What is relative refractory period? (RRP)
Period after ARP where an AP can be elicited but only with larger stimulus strength
Which organ systems are involved in exogenous regulation of heart function?
- brain/CNS - can effect immediate changes through nerve activity or slower changes through hormonal activity
- kidneys - heart and kidneys share a bi-directional regulatory relationship usually through indirect mechanisms
- blood vessels - by regulating the amount of blood that goes to and from the heart
What part of the CNS controls the heart?
Autonomic nervous system - cardioregulatory centre and vasomotor centres in medulla
How does the parasympathetic NS control heart rate?
- ‘rest and digest’
- decreases heart rate - decreases slope of phase 4 of SAN cell
- communicates through vagus nerve to heart
How does the sympathetic NS control heart rate?
- ‘fight or flight’
- increases heart rate (chronotropy) - increases slope of phase 4 of SAN cell and decrease in time
- increases force of contraction (inotropy) - increases Ca2+ dynamics
- communicates through sympathetic nerves to heart
What do parasympathetic nerves consist of and where do they come from?
- arise from cranial and sacral part of spinal cord
- pre-ganglionic fibres release ACh as NT (nAChR)
- post-ganglionic fibres also release ACh (muscarinic AChR)
- PNS important for controlling heart rate
What type of receptor on the SA nodal cell receives the post-ganglionic fibre (parasympathetic NS)?
- 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 where do they come from?
- arise from thoracic and lumbar vertebra
- pre-ganglionic fibres use ACh as NT (nAChR)
- post-ganglionic fibres use NA as NT
- synapse onto paravertebral ganglia (sympathetic chain)
- SNS important for controlling the circulation
What type of receptor on the SA nodal cell receives the post-ganglionic fibre (sympathetic NS)?
- beta1-receptor
- stimulates adenylyl cyclase and increases levels of protein kinase A through second messenger pathway
Where is the vasomotor centre (VMC) located?
Bilaterally in reticular substance of medulla and lower third of pons
What is the vasomotor centre composed of?
- vasoconstrictor (pressor) area
- vasodilator (depressor) area
- cardio-regulatory inhibitory area
What does the vasomotor centre do?
- transmits impulses distally through spinal cord to almost all blood vessels
- many higher centres of the brain such as the hypothalamus can exert powerful excitatory or inhibitory effects on the vasomotor centre
- lateral portions of VMC controls heart activity by influencing heart rate and contractility
- medial portions of VMC transmits signals via vagus nerve to heart that tend to decrease heart rate
Describe the graph showing how heart rate changes due to parasympathetic and sympathetic NS stimulation.
- cut nerves show that the paraNS and symNS are constantly sending out signals to heart - there is basal activity of both nerve types
- e.g. as sympathetic nerves cut, HR decreases showing there was some sympathetic activity before
What does sympathetic innervation to the kidney do?
- increased activity decreases glomerular filtration –> reduced Na+ excretion –> increase in blood volume (also done through aldosterone)
- change in blood volume detected by venous volume receptors
- increase in renin secretion –> increased angiotensin-II production –> vasoconstriction and increased blood pressure (detected by arterial baroreceptors)
- renin secretion also causes aldosterone release which impacts blood volume
What part of the kidney do sympathetic nerves innervate?
Afferent and efferent arterioles of the glomerulus (and nephron tubule cells)
What happens at afferent arterioles of glomerulus due to sympathetic activity?
- primary site of sympathetic activity
- activation of alpha1-adrenoreceptors by NA causes vasoconstriction –> reduced GFR –> reduced Na+ filtered –> increased blood volume
- juxtaglomerular cells are the site of synthesis, storage and release of renin
- beta1-adrenoceptor activation causes renin secretion which increases blood volume
What do large pulmonary vessels in cardiopulmonary circuit do?
- they are volume sensors (also atria and right ventricle) - send signals through glossopharyngeal and vagus nerves
- decrease in filling (due to less blood returning to heart) –> reduced baroreceptor firing –> increased sympathetic NS activity
- distention (more filling) –> increased baroreceptor firing –> decreased sympathetic NS activity
What is part of the arterial circuit?
- aortic arch
- carotid sinus
- afferent arterioles of kidneys
What does the arterial circuit do?
- pressure sensors - send signals through glossopharyngeal and vagus nerves
- decrease in pressure –> reduced baroreceptor firing –> increases sympathetic NS activity
- increase in pressure –> increased baroreceptor firing –> decreased sympathetic NS activity
What are the two circulations of the blood?
- pulmonary and systemic
- right heart –> lungs –> left heart –> body
- pulmonary to lungs, systemic to 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 the heart, which in turn determines stroke volume (using Starling’s Law)
What does constriction in veins do?
- reduces compliance and increases venous return
- increased blood volume / SNS activation of veins / skeletal muscle pump / respiratory movements –> increased venous pressure –> increased venous return –> increased atrial pressure
What does constriction in arteries determine?
- blood flow to downstream organs
- mean arterial blood pressure
- pattern of blood flow to organs
What % of blood are in different components of the circulation?
- pulmonary circulation: 17%
- heart: 9%
- veins and venules: 61%
- arteries: 11%
- arterioles and capillaries: 7%
What are some local mechanisms which regulate blood flow in an organ?
Intrinsic to smooth muscle and are endothelium-derived modulators:
- nitric oxide (NO) - potent vasodilator, which diffuses into vascular smooth muscle cells
- prostacyclin - vasodilator that also has antiplatelet and 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?
These include autonomic NS and circulating hormones and non-endothelium-derived mediators that are extrinsic to smooth muscle:
- kinins - bind to receptors on endothelial cells and 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) and cause vasoconstriction
- angiotensin II - potent vasoconstrictor from the renin-angiotensin-aldosterone axis, also stimulates ADH secretion
