CARDIAC MUSCLE

A. Histogenesis: Cardiac muscle arises as parallel chains of elongated splanchnic mesenchymal cells in the walls of the embryonic heart tube. Cells in each chain develop specialized junctions between them and often branch and bind to cells in nearby chains. As development continues, the cells accumulate myofilaments in their sarcoplasm. The branched network of myobiasts forms interwoven bundles of muscle fibers, but cardiac myoblasts do not fuse.

B. Cardiac Muscle Cells: Cardiac muscle fibers are long, branched cells with one or 2 ovoid central nuclei. The sarcoplasm near the nuclear poles contains many mitochondria and glycogen granules and some lipofuscin pigment. Mitochondria lie in chains between the myofilaments. The arrangement of myofilaments yields striations like those of skeletal muscle.

1. Sarcoplasmic reticulum and T tubule system. The sarcoplasmic reticulum in cardiac muscle fibers is less organized than that of skeletal muscle and does not subdivide myofilaments into discrete myofibrillar bundles. Cardiac T tubules occur at the Z line instead of the A-I junction. In most cells, cardiac T tubules associate with a single expanded cisterna of the sarcoplasmic reticulum; thus, cardiac muscle contains dyads instead of triads.

2. Intercalated disks. These unique histologic features of cardiac muscle appear as dark transverse lines between the muscle fibers and represent specialized junctional complexes. With the EM, intercalated disks exhibit 3 major components arranged in a stepwise fashion. a. The fascia adherens, similar to a zonula adherens, is a half Z line found in the vertical (transverse) portion of the step. Its cr-actinin anchors the thin filaments of the terminal sarcomeres. b. The macula adherens (desmosome) is the second component of transverse portion of the junction. It prevents detachment of the cardiac muscle fibers from one another during contraction. c. The gap junctions of intercalated disks form the horizontal (lateral) portion of the step. They provide electrotonic coupling between adjacent cardiac muscle fibers and pass the stimulus for contraction from cell to cell.

3. Types of cardiac muscle fibers a. Atrial cardiac muscle fibers are small and have fewer T tubules than ventricular cells. They contain many small membrane-limited granules that contain a precursor of atrial natriuretic factor, a hormone secreted in response to increased blood volume that opposes the action of aldosterone and acts on the kidneys to cause sodium and water loss. b. Ventricular cardiac muscle fibers are larger cells with more T tubules and no granules.


C. Organization of Cardiac Muscle: Because of the abundant capillaries in the endomysium, cardiac muscle fibers appear more loosely arranged in histologic section than those of skeletal muscle. The whorled arrangement of cardiac muscle fibers in the wall of the heart accounts for the ability of the myocardium to "wring out" blood in the heart chambers.

D. Mechanism of Contraction: Although the arrangement of the sarcoplasmic reticulum and T tubule complex of cardiac muscle fibers differs from that of skeletal muscle, the composition and arrangement of myofilaments are almost identical. Thus, at the cellular level, skeletal and cardiac muscle contractions are essentially the same.

E. Initiation of Cardiac Muscle Contraction: Unlike skeletal muscle fibers, which rarely contract without direct motor innervation, cardiac muscle fibers contract spontaneously with an intrinsic rhythm. The heart receives autonomic innervation through axons that terminate near, but never form synapses with, cardiac muscle cells. The autonomic stimulus cannot initiate contraction but can speed up or slow down the intrinsic beat. The initiating stimulus for contraction is normally provided by a collection of specialized cardiac muscle cells called the sinoatrial node; it is delivered by other specialized cells, called Purkinje fibers, to the other cardiac muscle cells. The stimulus is passed between adjacent cells through the gap junctions of the intercalated disks. The gap junctions establish an ionic continuity among cardiac muscle fibers that allows them to work together as a functional syncytium.