MICROSCOPIC ANATOMY



In general, two functional cell types are present in cardiac tissue: those responsible for electrical impulse generation and transmission and those responsible for mechanical contraction. Nodal cells are thought to be the source of normal im­pulse formation in the sinus node and are richly innervated with adrenergic and cholinergic nerve fibers. Like the sinus node, the AV node and His bundle regions are innervated with a rich supply of cholinergic and adrenergic fibers. Purkinje cells are large clear cells found in the His bundle, bun­dle branches, and their arborizations. They have particularly well-developed end-to-end connec­tions that may facilitate rapid longitudinal con­duction.
Atrial and ventricular myocardial cells, the con­tractile cells of the heart, contain numerous cross-banded bundles termed myofibrils that traverse the length of the fiber. Myofibrils are composed of longitudinally repeating sarcomeres (Fig. 1-6). Thick filaments composed of myosin comprise the A band, while thin filaments composed pri­marily of actin extend from the Z line through the I band into the A band, ending at the edges of the central H zone, which is the central area of the A band where thin filaments are absent. Thick and thin filaments overlap in the A band, and inter­action between the thick and thin filaments pro­vides the force for contraction of the heart.
The surface membrane of the cell is termed the sarcolemma, and adjacent myocardial cells are connected end to end’by a thickened portion of the sarcolemma termed the intercalated disc. Near the Z lines of the sarcolemma are wide invagin­ations called the T system that traverse the cell. Not continuous with the T system is the sarco­plasmic reticulum that surrounds each myofibril and participates in the excitation of the muscle. When the sarcolemma is depolarized electrically, the impulse conducts through the T system to cause calcium release from the sarcoplasmic re­ticulum and therefore activates the myofibrils to contract. The thick fibers in the myofibrils are composed of myosin molecules that have the abil­ity to split ATP and interact with the thin actin filaments when activated by calcium. Regulatory proteins, troponin and tropomyosin, inhibit the interaction of actin and myosin unless a calcium-troponin complex is present that then allows the actin-myosin interaction to proceed. The sarco-lemma possesses the ability to control the flux of various ions (especially sodium, potassium, and calcium) into and out of the cell via specific ionic channels located within the membrane. The se­lective permeability of the membrane establishes ionic gradients and the electrical forces that create and maintain the resting transmembrane potential and generate the action potential (see below).





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