Haight M, Nguyen V. Haight M, Nguyen V Haight, Matthew, and Vinh Nguyen.Cardiac Electrophysiology. In: Freeman BS, Berger JS. Freeman B.S., Berger J.S. Eds. Brian S. Freeman, and Jeffrey S. Berger.eds. Anesthesiology Core Review: Part One Basic Exam New York, NY: McGraw-Hill; 2014. http://accessanesthesiology.mhmedical.com/content.aspx?bookid=974&sectionid=61590173. Accessed April 23, 2019.
Haight M, Nguyen V. Haight M, Nguyen V Haight, Matthew, and Vinh Nguyen.. "Cardiac Electrophysiology." Anesthesiology Core Review: Part One Basic Exam Freeman BS, Berger JS. Freeman B.S., Berger J.S. Eds. Brian S. Freeman, and Jeffrey S. Berger. New York, NY: McGraw-Hill, 2014, http://accessanesthesiology.mhmedical.com/content.aspx?bookid=974&sectionid=61590173.
Cardiac muscle, like skeletal muscle, contains myosin, actin, tropomyosin, and troponin in various isoforms. Even though cardiac muscle fibers resemble skeletal muscle fibers in that they are striated, they differ in that they form a functional syncytium, which means that all fibers are electrically connected via gap junctions. Pacemaker cells of the electrical conduction system can initiate depolarization, and thus contraction, throughout the myocardium without external neurohormonal control.
The cardiac conduction system consists of the sinoatrial (SA) node, atrioventricular (AV) nodes, AV bundle (Bundle of His), and left and right bundle branches and Purkinje fibers (Figure 149-1). A normal electrical impulse generally starts at the SA node and produces an action potential by allowing ions to cross the cell membrane to increase the resting membrane potential. As a result, electrical activation will cause adjacent myocardium to produce an action potential along the conduction pathway to elicit a normal heartbeat.
A, B: Cardiac conduction system.
Cardiac muscle cells have a resting potential of −90 mV, with the inside of cell being negatively charged and the outside being positively charged. Ions flow in and out of the cell by a concentration gradient, electrical gradient, or permeability of the membrane. Potassium is higher inside the cell (140 mmol/L) than outside (4 mmol/L), and has the highest permeability (more than sodium or chloride). Thus, potassium is the major determinant of the resting membrane potential.