Video Transcript
Starting with an electrical impulse being generated in the sinoatrial node, which of
the following outlines the passage of electrical impulses in the cardiac cycle in
the correct order? SAN, Purkinje fibers, AVN, bundle of His. SAN, AVN, bundle of His, Purkinje fibers. SAN, bundle of His, Purkinje fibers, AVN. SAN, AVN, Purkinje fibers, bundle of His. Or SAN, bundle of His, AVN, Purkinje fibers.
Let’s remove the multiple choice options for now and recap what we know about the
heart and its electrical activity to help us answer this question. This question is all about the cardiac cycle, which is basically just a fancy name
for everything that goes on inside the heart during a single heartbeat, from when
blood enters the heart through the veins to when it leaves the heart through the
arteries.
There are four chambers inside the heart: the right atrium, the left atrium, the
right ventricle, and the left ventricle. Remember, when we’re looking at an image of the heart, the left side of the image is
the right side of the heart and the right side of the image is the left side of the
heart.
You may recall that the heart is myogenic. This means that all the electrical signals that control the rhythm and speed of the
heartbeat are generated within the cardiac muscle itself. This begins at the sinoatrial node, or SAN for short. The SAN is located in the wall of the right atrium and is known as the pacemaker
because it controls the rate at which the heart beats.
Once the atria have filled with blood, an electrical impulse is generated by the SAN,
which spreads across the walls of the atria as a wave of electrical activity. This causes the atria to contract, forcing the blood out of them and into the
ventricles.
The atrioventricular node, or AVN, is found between the atria and the ventricles. The AVN receives electrical impulses from the SAN and passes them on to the bundle of
His. This introduces a delay which allows time for the ventricles to fully fill with blood
before they contract. The bundle of His is a collection of specialized muscle fibers located along the
septum separating the left and right sides of the heart. It conducts electrical impulses from the atria down to the base of the
ventricles.
Here, the bundle of His splits into two branches of muscle fibers called Purkinje
fibers, which carry the electrical impulses up the outer walls of the
ventricles. This causes the ventricles to contract, pumping the blood up and away through the
arteries and out of the heart.
Therefore, the correct order for the passage of electrical impulses in the cardiac
cycle is SAN, AVN, bundle of His, Purkinje fibers.
