CVS gap fillers

Question 1

Initiation and Conduction of an Impulse During a Heartbeat

Answer 1

1. An action potential is initiated in the SA node. The signals spread through the atria.
2. The impulse is conducted to cells of the AV node.
3. From the AV node, the impulse travels through the atrioventricular bundle, also known as the bundle of His.
4. The signal travels a short distance before it splits into left and right bundle branches, which conduct impulses to the left and right ventricles, respectively.
5. From the bundle branches, impulses travel through an extensive network of branches referred to as Purkinje fibres. From these fibres, impulses travel through the rest of the myocardial cells.

Question 2

Why are impulses generated by the SA node rather than the AV node?

Answer 2

 Action potentials originating in the SA node travel through the AV node on their way to the ventricles, causing the AV node to go into refractory period during which they cannot generate their own action potentials.
 SA node has a higher frequency of action potentials than the AV node (about 70 impulses per minute vs 50 impulses per minute).

Question 3

Electrical Activity in Pacemaker Cells

Answer 3

A pacemaker cell is able to fire action potentials spontaneously because it does not have a steady resting potential. Following this event, the membrane potential returns to about –60 to –70 mV and then begins another round of slow depolarization until another action potential is triggered.