If we want the cardiac muscle cells to contract, we have to excite them (depolarize them). Where does the signal that starts off this depolarization come from? Also, you learned that cardiac muscle cells can spread their depolarizations from cell to cell through gap junctions... but although this works pretty well, it doesn't spread the signal fast enough for all the cardiac muscle cells of the atria or of the ventricles to contract in near synchrony. So, somehow, we have to send the signal that tells these cardiac muscle cells to contract to all the cells of the heart. The term "cardiac conduction system" refers to the system of electrical signaling that instructs these muscle cells to contract.
As described in the Marieb Interactive Physiology CD on the cardiovascular system, in the chapter on the "cardiac action potential," there are regular cardiac muscle cells, and then there are some special ones. Some of the special ones are called autorhythmic cells, and these fire action potentials automatically at some particular rate-- even without neurons telling them to! Have you ever used a metronome to keep the beat when playing a musical instrument? If so, once you start it, it makes a tick, tick, tick sound at a particular rate, and will keep going and going forever (or until you turn it off). A pendulum on a clock also just keeps going and going and going once it gets started. Well, autorhythmic cells have a particular set of channels on their membranes that allow them to repeatedly generate action potentials at a particular frequency. I have tried to show that to you in the figure above... and in the one below. The action potential (cardiac muscle action potentials are wider than axonal action potentials) just keeps coming and coming... If you could see them streaming by, you'd see something more like this version.
You will see other types of specialized cardiac muscle cells in my descriptions below (the different fibers in the heart)... but let's keep going for now with the autorhythmic cells.
So, where do we find these autorhythmic cells? We find them in the cardiac conduction system. But the ones with the fastest rhythm, that keep all the other cells going more quickly, are found in the superior aspect of the right atrium. They are localized to one area within this upper corner of the right atrium, called the sinoatrial node (S-A node). As these cells fire their action potential, it spreads to the neighboring cardiac muscle cells and then runs throughout the atrial syncytium. In this manner, the atrial systole is triggered.
The next question you should be thinking is, how do the ventricles get triggered? If the S-A node activates the atria, what activates the ventricles? Well, we need another signal for the ventricles. But, this ventricular signal has to be connected with the atrial signal in order to keep the atria and the ventricles coordinated. Right?
Therefore, the S-A node has a bigger job than just triggering the atria-- the S-A node is responsible for setting up the entire pace of the heart. Every time it fires, it begins the entire cardiac cycle! Every time the autorhythmic cells fire, then, the cardiac cycle runs through to completion. Because of this, the S-A node is often called the pacemaker of the heart. This should not be confused with an artificial pacemaker that some people get inserted into their thoracic cavities during an operation.
Now let's get the ventricles going. The ventricles have a node called the atrioventricular node (A-V node), but its autorhythmic cells fire much more slowly. When the S-A node eventually activates the A-V node (indirectly), that leads to activation of the ventricular syncytium. The A-V node is located in the wall between the right atrium and right ventricle.
So, how does the A-V node get activated? The S-A node communicates with the A-V node through something called junctional fibers. These junctional fibers are just specialized cardiac muscle cells; they are long and thin, and can carry the action potential from the S-A node to the A-V node. However, these junctional fibers are designed to carry the action potential rather slowly. Carrying the action potential slowly is different from firing in a slow rhythm... the rhythm is still fast, but the action potential just takes time running along the junctional fibers. Because they are so slow, that sets up a delay between the activation of the S-A node and the activation of the A-V node. Since each node's activity leads to contraction of one of the syncytia, the slow characteristics of the junctional fibers is what causes a delay between atrial systole and ventricular systole.
We have now run through the top half of this figure. To review, the S-A node turns itself on through the action of the autorhythmic cells. That activates the atrial syncytium directly, causing atrial systole. The S-A node activity also activates the junctional fibers, but they take their sweet time getting the activity signal to the A-V node. Then, the A-V node is activated... etc... and eventually the ventricular syncytium is activated to contract, causing ventricular systole.
It is now time to fill in all the "..." between activation of the A-V node and ventricular systole. The ventricles are very large. You saw how much larger the ventricles are in lab. If we had to wait for the ventricular syncytium to be activated from the one point where the A-V node lies, it would take too long and never be properly coordinated to cause the entire ventricles to contract. Somehow, we have to spread the activity throughout the ventricles faster. So, we'll use some specialized cardiac muscle fibers again... but this time they need to be fast (unlike the junctional fibers).
Therefore, a group of fast-conducting fibers carry the A-V node activity to the interventricular septum really quickly. This group is called the A-V bundle (it used to be called the bundle of His, and still may be occasionally on board exams). The A-V bundle splits into two halves when it reaches the interventricular septum, in order to supply both the left and the right ventricles with the activity information. The two halves are the bundle branches (left bundle branch and right bundle branch). The bundle branches run all the way down the septum to the apex of the heart. Then they run back up along the outer edges of the ventricles. Once they start running up the outer edges, they are called Purkinje fibers, since they are named for a scientist who discovered them (His was one, too).
Both the bundle branches and the Purkinje fibers communicate with the regular cardiac muscle fibers throughout the ventricles. Therefore, all the cardiac muscle fibers in the ventricles are excited after the A-V node is activated. And, the contraction starts off first at the apex of the heart, helping to shove the blood toward either the pulmonary trunk or the aorta.
© 2011 STCC Foundation Press