Contraction of Cardiac Muscle Cells Is Triggered by an Electrical Action Potential
The heart is a muscular pump that propels blood through the blood vessels by alternately relaxing and contracting. As the heart muscle relaxes, the ventricles fill with venous blood.
During cardiac contraction, some of this blood is ejected into the arteries. Cardiac contraction takes place in two stages: (1) the right and left atria begin to contract, and (2) after a delay of 50 to 150 milliseconds (msec), the right and left ventricles begin to contract. Atrial contraction helps to finish filling the ventricles with blood. The delay allows time for this “topping up" of ventricular volume. Ventricular contraction ejects blood out of the left ventricle into the aorta and out of the right ventricle into the pulmonary artery. After ventricular ejection, the heart relaxes, and the ventricles begin to refill. The entire contractile sequence is initiated and organized by an electrical signal, an action potential, which propagates from muscle cell to muscle cell, through the heart.This chapter begins with a brief description of how cardiac muscle contracts, followed by a detailed description of the action potentials that initiate and organize the heart’s contractions. Several common electrical dysfunctions of the heart are then discussed.
Throughout this chapter, comparisons are made between cardiac and skeletal muscle (Table 19-1). In both cardiac and skeletal muscle, an electrical action potential in each muscle cell is necessary to trigger a contraction. The molecular mechanisms that carry out the contraction are also similar in both types of muscle. However, important differences exist between cardiac and skeletal muscle in the characteristics of the action potentials that initiate contractions.