Physiology of Cardiac Muscle

Physiology of Cardiac Muscle

• The heart is composed of three types of cardiac muscle: atrial muscle, ventricular muscle, and specialized excitatory and conductive muscle fibers.
• Atrial and ventricular muscles contract similarly to skeletal muscle, but with a longer contraction duration.
• Specialized excitatory and conductive fibers have few contractile fibrils, instead exhibiting automatic rhythmical electrical discharge or conduction of action potentials.

Physiologic Anatomy of Cardiac Muscle

• Cardiac muscle fibers are arranged in a latticework, dividing, recombining, and spreading again.
• Cardiac muscle is striated, similar to skeletal muscle, with typical myofibrils containing actin and myosin filaments.
• Cardiac muscle fibers are made up of many individual cells connected in series and in parallel with one another, forming a syncytium.
• Intercalated discs are cell membranes that separate individual cardiac muscle cells, allowing rapid diffusion of ions through permeable "communicating" junctions (gap junctions).

Cardiac Muscle Syncytium

• The heart is composed of two syncytiums: atrial syncytium and ventricular syncytium.
• The atrial syncytium constitutes the walls of the two atria, and the ventricular syncytium constitutes the walls of the two ventricles.
• The atria and ventricles are separated by fibrous tissue, and action potentials are not conducted directly through this tissue.
• Instead, action potentials are conducted through the A-V bundle, a specialized conductive system.

Action Potentials in Cardiac Muscle

• The action potential in a ventricular muscle fiber averages about 105 millivolts.
• The intracellular potential rises from a negative value (about −85 millivolts) to a positive value (about +20 millivolts) during each beat.
• The membrane remains depolarized for about 0.2 seconds, exhibiting a plateau, followed by abrupt repolarization.
• The plateau in the action potential causes ventricular contraction to last about 15 times as long as in skeletal muscle.

Learn about the composition and function of cardiac muscle, including atrial, ventricular, and specialized excitatory and conductive muscle fibers.