202 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



FIG. 3. Striated muscle bundles. Electron micrograph of 

 longitudinal section of rabbit psoas muscle (by H. E. Huxley). 

 Note the clearly delineated bands (A band = 1.5 /j). The 

 sarcosomes are relatively few, small in size, and distributed 

 mainly in the region of the I bands. [From Perry (191).] 



muscle is of the order of 2 to 3 /j (190), whereas in 

 the heart the range is from i to 2 //. Myofibril diam- 

 eter for botli skeletal and cardiac muscles falls in the 

 range of 0.5 to 2.0 n (164). The cardiac muscle cells 

 (fibers) themselves are 16 to 20 /n in diameter (196), 

 and hence contain 200 to 300 myofibrils. 



The skeletal myofibril, when examined in the 

 electron microscope, is found to contain two types 

 of myofilaments (107, iii): the primary filaments, 

 approximately 100 A in diameter and 1.5 microns 

 in length, correspond to the A band of the fibril; the 



o 



smaller secondary filaments are about 50 A in di- 

 ameter and extend to the Z membrane toward the 

 center of the sarcomere where they are connected in 

 the region of the H zone by very fragile S filaments. 

 These relationships are shown in figure 5. The ultra- 

 structure of the cardiac musculature does not differ 

 significantly from that of skeletal muscle (123). 



Z^m^^!0i^R£k 



FIG. 4. Elcctronmicrograpli uf cardiac muscle from rat. 

 [From Bryant el al. (35).] 



• • • • 



» • • • • 



» • • • 



FIG. 5. Filament model of striated muscle. [From H. E. 

 Huxley (109J.] 



o 



Cardiac myofilaments also consist of thin 40 A fila- 

 ments and thicker iio A filaments closely related to 

 those seen in skeletal muscle and related to the A and 

 I bands in the same way. As in skeletal muscle the 

 Z membrane seems to be a continuous structure 

 across the fibril and forms a connecting bridge be- 

 tween individual filaments ijlending indistinguishably 

 into the plasma membrane at the cell margin. 



Cardiac muscle differs from skeletal muscle quanti- 

 tatively in the abundance of mitochondria (sarco- 

 somes) which lie crowded between the myofibrils as 

 shown in figure 4. They measure 0.3 to 1.7 microns 

 in length and 0.2 to 1.0 microns in width (164, 187). 

 They possess a double membrane in common with 

 other mitochondria, the inner membrane of which 

 ramifies in numerous cristae which are the seat of 

 the terminal respiratory chain of enzymes essential 

 for aerobic metabolism (fig. 6) (103). The endo- 



