Cardiac cells have two unique features that aid in contractions. The first feature is automaticity, which refers to the ability of a single cardiac cell to contract. Secondly, cardiac cells exhibit autorhythmicity. This refers to the ability of cardiac cells to synch their contractions. How do these cells know when to contract?
Signal to Contraction
Contractions of the heart begin with something called the Frank-Starling Reflex. This reflex is able to detect an excess of blood in the heart, which causes it to contract. This reflex also monitors the strength of the contraction. The contraction squeezes the excess blood out of the heart.
The SA node is located in the right atrium of the heart and contains specialized muscle cells. These cells behave very much like nerve cells rather than cardiac cells because they do not contract and release action potentials.
From the SA node, the signal travels across the atrium, through internodal fibers and arrives at the atrioventricular node, or the AV node. The AV node is located between the atrium and ventricles.
From this point, the signal travels through the AV bundles, which you may also see referred to as the bundle of His. The AV bundle (bundle of His) splits into the right and left AV bundles.
Finally, the signal arrives at the Purkinje fibers which are connected to contracting cells. Purkinje fibers, like the cells of the SA node, behave like neurons. The Purkinje fibers are located along the sides of the ventricles and lead to the apex (tip or bottom) of the heart. From the apex, the contraction travels up and through the rest of the heart.
Intercalated discs, or desmosomes, connect the cardiac cells and allows the contraction to travel.
Frank Starling Reflex →SA Node →Internodal Fibers →AV Node →AV Bundle (Bundle of His) →Right and Left AV bundles →Purkinje fibers →apex →contraction
This is the chart I made for myself in order to memorize the steps and it works well.
Another important aspect to remember is that the cells in the SA node and in the Purkinje fibers behave more like neurons than muscle cells.
If you remember these points, you will know the basics of a heart contraction.