Electrical Activity of the Heart
Objective 3
State the anatomical and physiological principles underlying the electrical activity of the heart. Explain the electrical activity at all levels, from the electrocardiogram to the molecular and cellular structures which explain the heart’s electrical activity.
Many of you may have seen that an animal’s heart keeps beating even after the animal is dead. This video shows that cardiac muscle cells, even when grown in a Petri dish, exhibit a spontaneous, rhythmic contraction. How can this be possible?
Pacemaker Cells of the Heart
The rhythmic electrical activity of the heart is in part due to the inherent electrical properties of each individual cell. The same principles we learned about electrical activity of excitable cells in Unit 13 still apply here. (You might want to take time to review these principles before diving into the rhythmic activity of the heart in this objective.) The same three ions will be discussed: sodium (Na+), potassium (K+), and calcium (Ca2+). Because of the special way these are managed in the excitable cells of the heart, the healthy heart responds to different stimuli, and the unhealthy heart can be treated with drugs that do not affect other excitable cells. (Drugs which affect the rhythm and excitability of the heart are beyond the scope of this course but will be a big part — you might even say “the heart” — of a pharmacology course you might take later.)
The spontaneous electrical activity of the heart is triggered by two groups of pacemaker cells: the main pacemaker in the sinoatrial (SA) node, and a backup pacemaker in the atrioventricular (AV) node. The SA node, as the name implies, is located near both the cardiac sinus and the right atrium, near the entry of the superior vena cava; the AV node is located near the junction between the right atrium and the right ventricle.
Several specialized cells, neither nerve nor muscle but a little bit of both, help spread electrical activity over the heart so that the atria can contract simultaneously, and the ventricles can contract simultaneously. Normally, electrical activity spreads from the SA node to the right and left atria, causing them to contract and push blood through the tricuspid and mitral (bicuspid) valves simultaneously. Then, the AV node pacemaker, activated by this electrical spread, sends a wave of voltage through the atrioventricular bundle (AV bundle; bundle of His). The AV bundle divides into right and left bundle branches, and these divide further, spreading the electrical signal into Purkinje fibers which are interwoven with the cardiac muscle cells of the ventricle in the myocardium. In all, it takes about 75 milliseconds (1/13 of a second) for the voltage wave to travel over the surface of the heart.
In this image, the purple shading indicates a wave of electrical depolarization. The SA node initiates the electrical wave (1) which passes over the surface of the right and left atria (2). Positive ions (Na+