284 



BIOPHYSICAL STUDIES ON NERVE AND MUSCLE 



tissue, as revealed by the light and electron microscopes, and a bit about the 

 chemistry of muscle proteins, to see where myosin fits in. 



Structure of Muscle Tissue 



Figure 10-12 illustrates what is seen by means of higher- and higher- 

 resolution microscopic examination of muscle. A muscle is made up of 

 fibers, which appear striated under the light microscope. Phase contrast and 

 interference attachments reveal that a fiber is composed of myofibrils, along 

 the side of which lie mitochondria and nuclei (not shown). The electron 

 microscope reveals that a myofibril appears segmented because of a repeat- 

 ing pattern of light and dark bands throughout. Repeating patterns, 

 bounded by the end- or "Z"-lines contain a faint "M"-line in the middle, 

 bounded first by narrow H-zones and then the wider "A"- (for anisotropic) 

 bands which span the middle. Isotropic 'T'-bands span the "Z"-lines (see 

 Figure 10-12). 



fibres 



Tendon 



Uh 



i i r 



(a) 



Whole 

 Muscle 

 (■vl cm.) 



Muscle 

 Fibre 



i 50-100/j) 



mmm ) 



z z z z 



1 <..,.*> ' 



sarcomeres 



Myofibril (1-2 u) 



— I — 



(actin) 



Figure 10-12. Schematic Drawings of Muscle Under Higher and Higher Reso- 

 lution Microscopy, (a) Light microscope; (b) Electron microscope. During 

 shortening the horizontal thick and thin filaments slide farther into each other, 

 so that only the H and / bands shorten. 



Under great magnification (~ 300,000 x ) a rather comical contraption is 

 disclosed: an array of overlapping thick and thin filaments, which run paral- 

 lel to the myofibril, and which apparently slide back and forth over each 

 other as the muscle contracts and relaxes. Partial overlap of the thick and 



