This example says that for each structure in the cardiac conduction system, identify in which region of the heart wall it is found by writing the location in the space provided. Then answer the question below. Note: Some structures may span multiple regions. Alright. We have 5 heart wall regions here: the right atrium, the left atrium, the right ventricle, the left ventricle, and the septum. We have 6 structures here, all of which we've discussed previously, so we really just have to remember where they are.
Alright. We'll start with the sinoatrial node, also called the SA node. Do you remember where that is located? Well, there's a little bit of a clue in the name there, right? Sinoatrial. So we know it's in the atria somewhere, and specifically, it's going to be in the right atrium. It's near the top of the right atrium, right under the entrance of the vena cava in that right atrium wall. Remember the sinoatrial node or the SA node initiates the action potential, which then spreads out through the heart and causes the heart to contract.
Next, we have the atrioventricular node, also called the AV node. Where is that AV node located? Remember, the atrioventricular node starts the action potential, which then spreads down the septum causing the ventricles to contract. But, it's actually still located in the right atrium. It's in the right atrial wall, sort of right at the base of the atrial wall there, kind of sitting on top of the septum a little bit.
As we continue, then we have the AV bundle, also sometimes called the bundle of His. Where is that AV bundle? We mentioned that the action potential goes down the septum through that AV bundle. So, this AV bundle is going to be in the septum.
Next up, we have the right bundle branch and the left bundle branch. Where are those right and left bundle branches located? Well, they have the names right and left in them, but these are still in the septum. Remember that the AV bundle goes down, splits into the right and left branch, and those right and left branches continue all the way down the septum to the apex of the heart.
Finally, we have the subendocardial conducting network, also sometimes called the Purkinje fibers. Where are those Purkinje fibers located? From the apex of the heart, those Purkinje fibers spread out, and they conduct that signal through the ventricular walls. So, we're going to say that these are in both the left and right ventricles.
The question here asks: In order to contract the contractile cells, the heart must be stimulated by an action potential, but the conducting fibers of the heart do not directly connect to all contractile cells. What do you think allows them to spread that action potential from one to another? Well, that's going to be the gap junctions. Remember, these cardiac cells are branching muscle cells that are attached to each other and have gap junctions. So when an action potential stimulates one, it spreads from one to another like a wave through this cardiac muscle.
Note that this is very different from skeletal muscle, where each individual muscle fiber or muscle cell needs to be stimulated, and the action potential does not spread from cell to cell. In cardiac muscle, you just need to stimulate some or a subset of this cardiac muscle, and the action potential then spreads like a wave from cell to cell because of those gap junctions.
With that, we've finished our example. We've got more practice problems after this. I'll see you there.