Control of Heart function Flashcards
How can the main anatomical components of the heart be broadly categorised?
Muscle cells - cardio myocytes : pump blood by electrically stimulated contraction
Specialised electrical cells : spontaneous currents created. To regulate contraction of cardio myocytes
Vessels : transporting blood, coronary vessels for the heart
What is the pacemaker of the heart?
Sinoatrial (SA) node
Pacemaker of the heart: 60-100 bpm
Junction of crista terminalis; upper wall of right atrium & opening of superior vena cava
What does the Atrioventricular node do?
Atrioventricular (AV) node
Has pacemaker activity: slow calcium mediated action potential
Triangle of Koch at base of right atrium
What two tracts are found in the heart?
The bundle of his and branches
purkinye fibres
What do the bundle of his do?
Specialised myocytes. AV node: His bundle
branches at intraventricular septum –> apex
What do prukinye fibres do?
Specialised conducting fibres
How many phases does nodal cell action potential have?
Phase 0 - upstroke
Phase 4 - pre-potential
Phase 3 - repolaristion
- no phases 1 or 2
What is the upstroke in nodal cells due to?
Calcium influx
Repolarisation due to k Efflux
- Nodal cells do not have a resting membrane potential - only a pre-potential due to Na+ influx through a ‘funny’ channel
Why do different parts of the heart have different action potential graph shapes?
Caused by different ion currents flowing and different ion channel expression in cell membrane
Characteristics of cardiac muscle action potential?
Compared to nerves, cardiac AP is long (200-300 ms vs. 2-3 ms)
Duration of AP controls duration of contraction of heart
Long, slow contraction is required to produce an effective pump
AP has 5 phases numbered 0-4
Absolute refractory period (ARP) = time during which no AP can be initiated regardless of stimulus intensity
Relative refractory period (RRP) = period after ARP where an AP can be elicited but only with larger stimulus strength
What are the 5 phases of cardiac muscle action potential?
0 - Upstroke 1 - Early repolarisaion 2 - Plateu ( maintains depolarisation 3 - repolarisation 4 - resting membrane potential
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How is the maximum rate the heart can beat at limited?
By refractory periods
- relative and absolute
What ion changes take place in a ventricular cell during action potential?
Depolarisation due to sodium influx
Plataeu due to Calcium influx
repolarisation due to potassium efflux
Why does the CNS regulate the heart?
can effect immediate changes through nerve activity or slower changes through hormonal activity
What components of the cns regulate heart?
autonomic :
- cardio regulatory centre
- vasomotor centres in medulla
Wht does the parsympathethic do to heart rate?
‘Rest & digest’
decrease heart rate (HR) – decreases the slope of phase 4
Affects pre-potential of action potential in nodal cell
What does the sympathetic do to heart rate?
‘fight or flight’ Increases HR (chronotropy) – increases the slope of phase 4 and decreases the time
Increase force of contraction (inotropy) – increases Ca2+ dynamics
What NT do parasympaththic pre ganglionic fibres use
Acetylcholine to Muscarinic receptor
What NT do parasympaththic post ganglionic fibres use?
Acetylcholine
What NT do sympaththic pre gangalionic?
acetylcholine
What NT do sympaththic post ganglionic fibres ause?
Noradrenalie
where is the vasomotor centre located?
located bilaterally in reticular substance of medulla & lower third of pons
What is the VMC composed of?
Composed of:
Vasoconstrictor (pressor) area
Vasodilator (depressor) area
Cardio-regulatory inhibitory area
what does the vasomotor do?
Transmits impulses distally through spinal cord to almost all blood vessels
Lateral portions of Vasomotor controls heart activity by influencing heart rate and contractility
Medial portion of VMC transmits signals via vagus nerve to heart that tend to decrease heart
What can effect the VMC?
Many higher centers of the brain such as the hypothalamus can exert powerful excitatory or inhibitory effects on the VMC.
what do the sympathetic nerves to the kidneys do?
increase activity
decrease glomerular filtration –> decrease Na+ excretion
How is blood volume detected?
Detected by venous volume receptors
How is blood pressure controlled by the kidney?
Increased renin secretion –> increase angiotensin-II production: vasoconstriction & increases blood pressure
Blood pressure :
Detected by arterial baroreceptors
what do the sypathetic nerves from the cns innervate ( kidneys )
innervate afferent & efferent arterioles of the glomerulus (& nephron tubule cells)
What do the afferent aterioles do?
Primary site of sympathetic activity
a1-adrenoceptor –> vasoconstriction
decreasing in glomerular filtration rate –> decrease Na+ filtered
Juxtaglomerular cells are the site of synthesis, storage & release of renin
alpha1-adrenoceptor –> renin secretion
What two circuits do the blood vessels split into?
Cardiopulomnary circuit : volume sensors. large pulmonary vessels
Aterial circuit : Pressure sensors . Aortic arch, carotid sinus & afferent arterioles of kidneys
Where do the volume sensros in large pulomnary veins send signals to?
Glossopharyngela and vagus
What happens when the cardiopulmonary circuit is decreased in filling?
decreased baroreceptor firing –> increase sympathetic nerve (SNS) activity
Distention –> increase baroreceptor firing –> decrease SNS activity
Where do the pressure sensors to?
glossopharyngeal & vagus nerves
What happens during a decrease in pressure ina rterial circuit?
Decrease in pressure –> decrease baroreceptor firing –> increase SNS activity
Increase in pressure –> increase baroreceptor firing –> decrease SNS activity
What is venous volume distrubtuon affected by?
Venous volume distribution affected by peripheral venous tone, gravity, skeletal muscle pump & breathing
What is the central venous pressure?
Central venous pressure (mean pressure in the right atrium) determines amount of blood flowing back to heart.
In veins, constriction ( increases/decreases ) compliance and ( increases/decreases ) venous return
- reduces
- increases
What is smooth muscle for in the terms of blood flow?
Intrinsic to the smooth muscle (or closely associated)
Important for reflex local blood flow regulation within an organ
What chemicals in the endothelium affect blood flow? ( 4 )
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 non- endothelium derived mediators? affecting blood flow
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.
