6.14: Control of heart function Flashcards
Function and location of Sinoatrial node
Pacemaker of the heart 60-100bpm
Junction of crista terminalis; upper wall of right atrium and opening of superior vena cava
Function and location of Atrioventricular node
Has pacemaker activity : slow calcium mediated action potential
Triangle of Koch at base of right atrium
What are the main tracts in the heart
Bundle of His and bundle branches - specialised myofibres
Purkinje fibres - conducting fibres
Nodal cell action potential (AP)
Upstroke - due to calcium influx
Repolarisation- due to K+ efflux
Nodal cells do not have a resting potential, only pre-potential due to Na+ influx through a funny channel
3 phases of nodal cell action potential
Phase 0 - upstroke
Phase 4 - pre-potential
Phase 3 - repolarisation
Why do different parts of the heart have different action potential shapes
Different ion currents flowing and different ion channel expression in cell membrane
What is cardiac AP like compared to nerves
Long 200-300ms vs 2-3ms
Duration of an action potential controls
Duration of heart contraction
What type of contraction is required to produce an effective pump
Long, slow contraction
Phases of cardiac muscle action potential
Phase 0 - upstroke
Phase 1- early repolarisation
Phase 2- plateau
Phase 3- repolarisation
Phase 4- resting membrane potential
Absolute refractory period is the time during which
No AP can be initiated
Relative refractory period is the period after
ARP
Where an AP can be elicited but only with larger stimulus strength
What part of the CNS controls the heart
Autonomic nervous system - cardio-regulatory centre and vasomotor centre in medulla
Effects of the parasympathetic nervous system on heart control
Rest and digest
Decreases heart rate - decr. Slope of phase 4
Effects of sympathetic nervous system on heart control
Fight or flight
Increases heart rate (chronotropy) - increases the slope of phase 4
Increases force of contraction (initropy) increases Ca2+ dynamics
Where do parasympathetic neurones arise from
Cranial part of spinal cord
Sacral part of spinal cord
Where do sympathetic neurones arise from
Thoracic vertebrae
Lumbar vertebrae
What neurotransmitter do Parasympathetic pre- and post ganglionic fibres use
ACh
-important for heart rate control
What neurotransmitter do sympathetic pre and post ganglionic neurones use
Pre : ACh
Post : NA
Important for control of circulation
Where vasomotor centre located
Bilaterally in reticular substance of medulla and lower third of pons
What is the vasomotor centre composed of
Vasoconstrictor area (pressor)
Vasodilator area (depressor)
Cardio-regulatory inhibitory area
What does the vasomotor centre transmit impulses through and to
Distally through spinal cord to almost all blood vessels
What can exert powerful excitatory or inhibitory effects on the VMC
Higher centres of brain e.g hypothalamus
What do the lateral portions of VMC control, how
Heart activity by influencing heart rate and contractility
What does the medial portion of the VMS transmit signals via and to
Vagus nerve to heart
Decrease heart rate
What nerves are involved in the renal system
Sympathetic nerves
How is an increase in renal activity brought about
Decrease in glomerular filtration
Decrease in Na+ excretion
Increase in blood volume (aldosterone)
What is kidney blood volume detected by
Venous volume receptor
How is a decrease in renal activity brought about
Increased renin secretion
Increased angiotensin-II production
Vasoconstriction
Increased blood pressure
What receptors is blood pressure in the kidneys detected by
Arterial baroreceptors
Where do sympathetic nerve fibres innervate the kidneys
Innervate afferent and efferent arterioles of glomerulus (and nephron tube cells)
How does the sympathetic nervous system bring about a response in the kidneys
Afferent arterioles are the primary site of sympathetic activity
a1-adrenoreceptor induces vasoconstriction
Decrease in glomerular filtration rate
Less Na+ filtered
Juxtaglomerular cells are site of synthesis, storage and release of renin
b1-adrenoreceptor leads to renin secretion
The cardiopulmonary circuit consists of
Large pulmonary vessels
How is the cardiopulmonary circuit regulated
Volume sensors (atria, right ventricle) send signals through glossopharyngeal and vagus nerves
What does a Decrease in filling of the heart causes (cardiopulmonary circuit)
Decreased baroreceptor firing
Increased sympathetic nervous activity
Distension of cardiopulmonary circuit leads to
Increased baroreceptor firing
Decreased SNS activity
The arterial circuit consists of
Aortic arch
Carotid sinus
Afferent arterioles of kidneys
What do the Pressure sensors in the arterial circuit send signals through
Send signals through glossopharyngeal and vagus nerves
What does a decrease in pressure in the arterial circuit lead to
Decreased baroreceptor firing
Increased SNS activity
What does an increase in pressure of the arterial circuit lead to
Increased baroreceptor firing
Decreases SNS activity
Two circulations of cardiopulmonary system
Systemic
Pulmonary
What is venous volume distribution affected by
Peripheral venous tone
Gravity
Skeletal muscle pump
Breathing
Central venous pressure determines the
Amount of blood flowing back to the heart
The amount of blood flowing back to the heart determines
Stroke volume ( using starlings law of the heart)
In veins, constriction reduces and increases
Reduces compliance
Increases venous return
In arterioles, constriction determines
Blood flow to downstream organs
Mean arterial blood pressure
The pattern of blood flow to organs
Why are local mechanisms essential in regulating blood flow
(Intrinsic to smooth muscle)
Important for reflex local blood flow regulation within an organ
4 endothelium derived mediators include :
Nitric oxide
Prostacyclin
Thromboxane A2
Endothelins
How does Nitric oxide act as a local blood flow mediator
Potent vasodilator
Diffuses into vascular smooth muscle cells
How does prostacyclin act as a local blood flow mediator
Vasodilator
Has antiplatelet and anticoagulant effects
How does TXA2 act as a local blood flow mediator
Vasoconstrictor
Heavily synthesised in platelets
How do endothelins act as a local blood flow mediator
Vasoconstrictors
Generated from nucleus of endothelial cells
Characteristics of Systemic mechanisms in regulating blood flow
Extrinsic to smooth muscle
Include autonomic nervous system and circulating hormones
5 non-endothelium derived mediators
Kinins
Atrial natriuretic peptide
Vasopressin (ADH)
Noradrenaline / Adrenaline
Angiotensin II
How do kinins act as a systemic blood flow mediator
Bind to receptors on endothelial cells and stimulate NO synthesis
Vasodilator effects
How does ANP act as a systemic blood flow mediator
Secreted from atria in response to stretch
Vasodilator effects to reduce BP
How does ADH act as a systemic blood flow mediator
Secreted from pituitary gland
Binds to V1 receptors on smooth muscle to cause vasoconstriction
How do noradrenaline / adrenaline act as a systemic blood flow mediator
Secreted from adrenal gland
Vasoconstriction
How does angiotensin II act as a systemic blood flow mediator
Potent vasoconstrictor from renin-angiotensin-aldosterone axis
Stimulates ADH secretion
