Within the conduction system, you have the Sinoatrial node (SAN) and
Atrioventricular (AV) node.
SA node – body’s natural pacemaker and determines heart rate.
AV node – delays the passage of electrical impulses to ventricles. The AV node
passes signals from SA node to bundle of His (divided into right and left towards
apex of heart). Signals are then passed onto Purkinje fibres, which turn upward and
spread throughout the ventricular myocardium.
Resting Adult Heart Rate = 60-100bpm.
Chronotrophic agents that affect the heart rate include caffeine and NA.
PARASYMPATHETIC EFFECTS
Decreased HR.
Negative chronotropism.
Reduced pacemaker potential of SA node.
Vagus nerve signals ACh release to agonise muscarinic receptors.
SYMPATHETIC EFFECTS
Increased HR.
Positive chronotropism.
Increased pacemaker potential of SA node.
NA release agonises B1-adrenoreceptors.
Bradycardia = (<60bpm)
Tachycardia = (>100bpm)
ATRIAL CONDUCTION
Atrial myocytes are joined by intercalated discs with gap junctions (known as a syncytium).
Action potentials spread easily once generated throughout the syncytium. However, the AP
conduction velocity depends on junction resistance and size/velocity of AP upstroke.
AV node has a slower conduction velocity than the SA node.
AV node lacks fast Na+ channels.
AV node is also a dromotrophic agent, dromotropy – control of AV conduction rate.
Purkinje fibres have a rapid conduction rate whereas, Bundle of His only has
electrical connection.
AV node and SA node have slow Na+
channels.
But everywhere else in the heart – they
have a prominent phase 1 due to fast Na+
channels.
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