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Lecture 29

Physiology 3120 Lecture Notes - Lecture 29: Purkinje Fibers, Atrioventricular Node, Heart Valve

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Tom Stavraky

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Lecture 29 Conducting System of the Heart
- SA node is located at the posterior wall of the right atrium (top of the right atrium)
- AP is generated at the SA node and it travels to the left atrium so it contracts at the same
time as the right atrium
- Interatrial myocardial band rapidly propagates the AP to the top of the left atrium
- AP from the atria enters the atrial contractile cells
- Atrial contractile cells are joined by intercalated disks that contain gap junctions
- Gap junctions allow the AP to travel from contractile cell to contractile cell
- AP starts at the top of both atria and propagates downwards
o Direction that the AP propagates is the direction that the cells contract in
o Cells contract from the top down
o = pushes blood through the AV valve and into the ventricles
- The way the AP propagates is the way the muscle contracts
- DO NOT want AP to propagate directly into the ventricular contractile cells from the atria
o If it did, the ventricles would contract from the top down
o = must stop the AP because want the ventricles to contract from the bottom up
- Atrioventricular ring is a bundle of fibrous CT that electrically isolates the atria from the
ventricles so that the AP cannot directly propagate into the ventricles
o Stops AP from going directly from the atria to the ventricles
- AP is transmitted from the SA node into the AV node through the intermodal pathway
- AV node SLOWS DOWN the AP to ensure the atria contract before the ventricles contract
o If they contracted at the same time, the heart would not be an efficient pump
- From the AV node, AP enters the Bundle of His
- Bundle of His is a rapid conducting system that propagates the AP to the apex of the heart
o Divides into two branches (left and right bundle branch) which travel down the
interventricular septum
- AP is now at the apex of the heart but it is not at the ventricular contractile cells yet
- From bundle of His, the AP enters the purkinje fibers
- Purkinje fibers rapidly conduct the AP to the ventricular contractile cells
- AP travels from the apex and travels upwards through the gap junctions of the ventricular
contractile cells
- Ventricles contract from the bottom up and push blood up and out through the valves
- Conducting system of the heart is highly coordinated or else the heart will not function as a
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- AP propagation and conduction velocity of the AP differs in different parts of the heart
- Conduction velocity is distinct from generation of the AP SA node generates the AP the
fastest but AP propagation through the SA node is fairly slow
- AP enters the atrial muscle and is conducted through gap junctions from cell to cell
- The AV node slows down the AP to ensure atria finish contracting before the ventricles do
- Bundle of his transmits the AP down to the apex of the heart very quickly
- Purkinje fibers VERY RAPIDLY transmit the AP to the ventricular contractile cells
- Ventricular muscle cells propagate the AP cell to cell through gap junctions
- AP is slow in some places and fast in other places
- SA node is the natural pacemaker (intrinsic rate is 100 times/min)
- Without input, the intrinsic (natural) rate rate at which it spontaneously generates APs
is 100 times/min
- Resting heart rate is 70 bpm (there is influence that influences the intrinsic rate at rest)
o PSYN keeps the heart rate at a lower rate than it wants to be
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