Flashcards in Physiology Deck (179)
What is meant by autorhythmicity of the heart?
It is able to generate its own electrical impulses without external stimuli
Where does excitation of the heart normally originate?
What are the specialised cells within the SA node that initiate the heart beat called?
What is meant by sinus rhythm?
Describes the heart's pace being controlled by the SA node
SA node cells have a stable resting membrane potential. True/False?
They exhibit spontaneous pacemaker potential
What is the function of the spontaneous pacemaker potential?
Takes the membrane potential to threshold (depolarisation) to generate an action potential
What gives rise to pacemaker potential?
Decrease in K+ efflux
Slow Na+ influx
What causes the rising phase of the action potential (depolarisation) in SA node cells?
Opening of Ca++ channels, resulting in Ca++ influx
What causes the falling phase of the action potential (repolarisation) in SA node cells?
Opening of K+ channels, resulting in K+ efflux
Summarise the phases of the SA node action potential
Pacemaker potential: decreased K+ efflux, slow Na+ influx
Rising phase: Ca++ influx
Falling phase: K+ efflux
Which junctions allow cell-to-cell spread of excitation?
The AV node is the only point of electrical contact between atria and ventricles. True/False?
AV node cells are large and slow to conduct. True/False?
They are small and slow to conduct
Why is AV nodal delay present?
To allow time for atrial systole to precede ventricular systole
Which fibres enable the excitation to spread to the ventricles?
Bundle of His and Purkinje fibres
What gives rise to the rising phase of the action potential in ventricular contractile cells?
Fast Na+ influx
Describe Phase 0 of the cardiac action potential
Fast Na+ influx causes reversal of the resting membrane potential from -90mV to +30mV
Describe Phase 1 of the cardiac action potential
Closure of Na+ channels + transient K+ efflux causes some repolarisation
What gives rise to the plateau phase (phase 2) of the cardiac action potential?
What gives rise to the falling phase (phase 3) of the cardiac action potential?
Describe Phase 3 of the cardiac action potential
Closure of Ca++ channels and opening of K+ channels allows K+ efflux which causes repolarisation of the membrane potential back to -90mV
Sympathetic stimulation causes increased heart rate. True/False?
What is meant by vagal tone?
Parasympathetic stimulation to the heart dominating in resting conditions
The vagus nerve supplies only the SA node. True/False?
Supplies both SA and AV nodes
What does parasympathetic stimulation do to the AV node?
Increases AV nodal delay
Which neurotransmitter acts on which receptor in parasympathetic control of the heart?
ACh on M2 receptors
Name a competitive inhibitor of ACh that is used in bradycardia
Vagal stimulation causes the slope of the pacemaker potential to increase. True/False?
Slope decreases (increased AV node delay)
What is meant by negative chronotropic effect?
Decreased contraction of the heart due to less frequent action potentials