PH^•SIOLOG^■ OF CARDIAC MUSCLE 20I 



Inlercoioled diskv,^ Muneus^ 



Sarcomere 



FIG. 2. A 3-dimen- 

 sional view of a cardiac 

 muscle cell. [From Licata 

 & Roberts (136).] 



cytium compo.sed of many branching fibers. A typical 

 cardiac muscle fiber is shown in figure 2. The location 

 of the oval nucleus is essentially central rather than 

 peripheral as in .skeletal muscle, and the intranuclear 

 material is condensed into a reticulated framework 

 which may change in appearance as the result of con- 

 traction and relaxation of the cell. One or more 

 nucleoli are present. Around each pole of the tapered 

 nucleus are accumulations of granular sarcoplasm. 

 The sarcoplasm or muscle cytoplasm consists of a 

 homogenous intracellular matrix which contains a 

 variety of organelles and the myofibrils (the smallest 

 anatomic contractile unit of the fiber). 



The sarcolemma is a well-defined external limiting 

 membrane which invests the muscle fiber but which 

 is thinner than that found in skeletal muscle and pre- 

 sumably provides less resistance to the diffusion of 

 substrates and gases in the highly aerobic cardiac 

 muscle. The sarcolemma seems to be a procollagen 

 which becomes collagenous at insertion points of the 

 muscle bundles. In the electron microscope the sarco- 

 lemma of the heart appears as a double membrane 

 separated by a clear space of 200 A (191, 195). The 

 inner membrane is more dense than the outer and 

 is known as the plasma membrane. 



Cardiac muscle is also characterized by inter- 

 calated discs which stain with silver nitrate and 

 which occur at variable distances throughout the 

 muscle, often at points of bifurcation of the fiber, 

 shown in figure 2. The intercalated discs, when viewed 

 in the electron microscope, appear to be jagged 

 membranes superimposed on or merged with the 

 Z membrane of the individual cell but spanning more 

 than one myofibril. The Z membranes mark the 

 boundaries of individual sarcomeres, the smallest 

 anatomic units of the myofiisril. The intercalated 

 disc then represents the invagination and fusion of 

 several plasma membranes. It would appear there- 



fore that, although in the light microscope the heart 

 appears to be a syncytium with cardiac muscle cells 

 branching throughout the myocardium, actually the 

 syncytial nature of the heart is more apparent than 

 real. The myofibrils appear to terminate at the inter- 

 calated discs and support the view espoused by several 

 anatomists (164) that the heart is in fact cellular and 

 not syncytial in its ultimate organization. 



The cross striations of skeletal and cardiac muscle 

 are visible in both the light and polarizing micro- 

 scope. These are due to periodic changes in the bire- 

 fringence of the myofibrils which gives an alternating 

 light and dark pattern to the muscle fiber as shown 

 in figures 3 and 4. The dark A (anisotropic) band is 

 light when viewed under a polarizing microscope, 

 and the light I (isotropic) band is uniformly dark 

 under polarizing conditions. The lower birefringence 

 of the I band appears to be due to a lack of myosin 

 in that portion of the myofibril rather than to the 

 crosswise orientation of lipids as once thought (54). 

 The dark A band is crossed centrally by a lighter H 

 band and the I band is seen to be crossed by the 

 dark Z membrane. The Z membrane appears to be 

 a more or less helicoid continuous structure, uniting 

 the fibrils to one another (probably with the aid of 

 the endoplasmic reticulum) keeping them in register, 

 and blending indistinguishably into the plasma mem- 

 brane at the cell margins. The myofilaments proceed 

 from one sarcomere to another through the Z band 

 without interruption, although they apparently stop 

 at the intercalated disc. When the myofibrils shorten, 

 the noncontractile sarcolemma is thrown into periodic 

 folds because of its points of attachment to the Z 

 membrane. 



As indicated earlier, the part of the fiber between 

 two Z membranes is designated as the sarcomere 

 and therefore consists of an A band and two half I 

 bands. The sarcomere length from resting skeletal 



