42 



THE INDIVIDUAL ORGANISM 



The finer structure of the muscle cell is so extremely minute, and the 

 sequence of physicochemical events that takes place at each contraction 

 is so complex and so rapid, that the exact mechanism of contraction is still 

 an unsolved problem. Theories are numerous, but none is yet established. 

 Nevertheless a great deal has been learned about what goes on in muscle 

 cells, and the brief account that follows is accurate so far as it goes. 



blood vessels 



nerve 



Fig. 3.9. Cross section of a fasciculus of a skeletal muscle. A single cell bundle is shown 

 magnified. 



Each muscle cell receives the end of a tiny motor nerve that comes from 

 the central nervous system. When a nerve impulse enters the cell, there is 

 a delay of only a few thousandths of a second, and then each of the min- 

 ute, string-of-beads-like fibrils suddenly contracts, causing the cell as a 

 whole to shorten and thicken, though its volume remains almost the 

 same. This contraction occurs simultaneously with the occurrence of an 

 explosively sudden energy-releasing change of an energy-storing sub- 

 stance in the fibrils. The nature of this change will be discussed a little 

 farther on. If only a single nerve impulse is received by the muscle cell 

 (fiber), relaxation follows immediately, and the fibrils resume their 

 former shape. The whole response is completed in a few hundredths of a 



