In which direction does depolarization move during contraction?

During cardiac muscle contraction, depolarization initiates from the sinoatrial (SA) node, which is located at the top of the right atrium. This electrical impulse then travels through the atria, causing them to contract and push blood into the ventricles. As depolarization continues, it moves through the atrioventricular (AV) node and into the bundle of His, subsequently spreading through the Purkinje fibers that branch throughout the ventricles.

In this context, depolarization effectively moves from the inner portion of the heart (the endocardium) to the outer layers (epicardium) of the ventricular walls, which corresponds to the concept of "inside to outside." This process allows for an effective sequential contraction of the heart muscle, beginning with the inner layer and allowing for coordinated ventricular contraction to efficiently pump blood out of the heart and into the circulation.

The other answer choices do not accurately represent the typical conduction pathway and depolarization direction during contraction. For example, depolarization does not move from bottom to top or strictly top to bottom in a way that would reflect the coordinated contraction of the heart. Instead, it moves in a precise and ordered way, ensuring that blood is effectively pushed out