HISTOLOGY OF THE CARDIAC MUSCLE Pamela BL

Introduction  The cardiac muscle forms the muscular wall of the heart that is it forms the myocardium.  Some of the cardiac muscle is also present in the walls of the aorta, pulmonary vein and superior vena cava.

 Contractions of the cardiac muscle is not under voluntary control.  Heart rate is regulated intrinsically by a pacemaker composed of special cardiac muscle fibers that are influenced by the autonomic nervous system.

 Cardiac muscle fibers are chains of cardiac muscle cells joined end to end by cell junctions.  Cardiac muscle is also a type of “striated” muscle because of its appearance under the microscopy.  Most often the terms “striated” or “striped” refer to voluntary skeletal muscle.

 Cardiac muscle fibers have the same general pattern of striations as the skeletal muscle fibers, however, they are traversed at intervals by intercalated discs(disks)- specialized end to end junctions that are unique to cardiac muscle fibers.  Most cardiac muscle cells have a single nucleus, but some contain two.

 During embryonic development, the splanchnic mesoderm cells of the primitive heart tube align into chain like arrays.  Rather than fusing into syncytial cells as in the development of skeletal muscle, cardiac cells form complex junctions between their extended processes.

 Cells within the chain bifurcate, or branch and bind to cells in adjacent chains.  Consequently, the heart consists of tightly knit bundles of cells interwoven in a fashion that provides for a characteristic wave of contraction that leads to wringing out of heart ventricles.

 Mature cardiac cells are approximately 15 nanometers in diameter and from 85 to 100 nanometers in length.  Mature cardiac cells do not divide and thus destroyed cells are not replaced by new cells.  An injury to cardiac muscle tissue is repaired by formation of fibrous connective tissue.

 Unlike multinucleated skeletal muscles with their nuclei under the sarcolemma at the periphery of the cell, each cardiac muscle cell possesses only one or two centrally located nuclei.  Surrounding the muscle cells is a delicate sheath of endomysial connective tissue containing a rich capillary network.

 A unique and distinguishing characteristic of cardiac muscle is the presence of dark- staining transverse lines that cross the chains of cardiac cells at irregular intervals.

 These intercalated disks represent junctional complexes found at the interface between adjacent cardiac muscle cells.  The junctions may appear as straight lines or may exhibit a step like pattern.

 The structure and function of the contractile proteins in the cardiac cells are virtually the same as in the skeletal muscle.  The T-tubule system and the sarcoplasmic reticulum, however, are not as regularly arranged in the cardiac myocytes.  The T tubules are more numerous and larger in ventricular muscle than in the skeletal muscle.

 Cardiac muscle cells contain numerous mitochondria, which occupy 40% or more of the cytoplasmic volume, reflecting the need for continuous aerobic metabolism in heart muscle.  By comparison, only about 2% of skeletal muscle fiber is occupied by mitochondria.

 Fatty acids, transported to cardiac muscle cells by lipoproteins, are the major fuel of the heart.  Fatty acids are stored as triglycerides in the numerous lipid droplets seen in cardiac muscle cells.

 A small amount of glycogen is present and can be broken down to glucose and used for energy production during periods of stress.

Clinical notes  Hypertrophy of the myocardium: Hypertrophy is an increase in the size of cells resulting in increase in the size of the organ. Thus the hypertrophied organ has no new cells just larger cells. The increased size of cells is due not to cellular swelling but to the synthesis of more structural components.

 In the heart, the stimulus for hypertrophy is usually chronic hemodynamic overload resulting from either hypertension or faulty valves.  Synthesis of more proteins and filaments occurs, achieving a balance between the demand and the cell’s functional capacity.

Summary  Location: muscle of heart and adjacent portion of the great vessels.  Appearance: branching and anastomosing shorter fibers with transverse striations running parallel and connected end to end by intercalated discs, single central nucleus

 Type of activity: strong, quick continuous rhythmic contraction, acts to pump blood from the heart  Stimulation: involuntary; intrinsically(myogenically) stimulated and propagated, rate and strength of contraction modified by autonomic nervous system.

 Thank you for listening!