Origin and conduction of cardiac impulse Flashcards
Where does excitation of the heart originate from?
Pacemaker cells in the SA node
What is autorhythmicity?
The heart can beat without an external stimuli
What is sinus rhythm?
A heart driven by the SA node (normal)
What potential do pacemaker cells exhibit?
Spotaneous Pacemaker potential
What generates an action potential?
The spontaneous pacemaker potential takes the membrane potential to threshold which genereates the action potential
What is pacemaker potential? and what is it due to?
- slow depolarisation
- Due to:
decrease in K+ efflux
Na+ and K+ influx
transient Ca+ influx
What occurs during the rising phase of action potential?
Ca++ influx - depolarisation
What causes the falling phase of action potential?
- inactivation of L type Ca++ channels (depolarisation)
2. activation of K+ channels resulting in K+ efflux
Route taken for excitation to spread to ventricles?
SA node -> AV node -> Bundle of His -> Purkinje Fibres
How does excitation spread through cell to cell conduction?
Gap junctions
Why is conduction delayed in the AV node?
To allow atrial systole to preceed ventricular systole
What is special about the AV node?
Only point of electrical contact between the atria and ventricles
Ventricular Myocyte Action potential: Phase 0?
fast Na+ influx - depolarisation
Ventricular Myocyte Action potential: Phase 1?
closure of Na+ channels and transient K+ efflux
Ventricular Myocyte Action potential: Phase 2?
Ca+ through L-type Ca++ channels
Ventricular Myocyte Action potential: Phase 3?
closure of Ca++ channels - repolarisation
Ventricular Myocyte Action potential: Phase 4?
resting membrane potential
What is normal resting heart rate?
60-100BPM
What is a bradycardic heart rate?
under 60 BPM
What is a tachycardic heart rate?
Over 100 BPM
What does the vagus nerve do to the SA node under resting conditions?
Exerts a continuous parasympathetic influence. Vagal tone dominates under resting conditions and slows the intrinsic heart rate
What is a negative chronotropic effect on the heart and what causes it?
- slowing of the heart rate
2. stimulation of parasympathetic nerves
What is a positive chronotropic effect on the heart and what causes it?
- increase heart rate
2. stimulation of sympathetic nerves
What is a sympathetic neurotransmitter and what is its effect on the pacemaker potential slope?
- noradrenaline - acting through b1-adrenoreceptors
2. increase
What is a parasympathetic neurotransmitter and what is its effect on the pacemaker potential slope?
- acetyl-choline - acting through muscarinic M2 receptors
2. decrease
