THE FRAMEWORK OF THE BODY 43 



second, relaxation taking somewhat longer than contraction. It is only 

 in laboratory experiments that the effects of a single impulse can be 

 studied; in life no voluntary movement is due to a single impulse. Even 

 the quickest and briefest movement is produced by a stream of nerve 

 impulses sent into the muscle at a rate which varies from 40 per second for 

 some of the leg muscles to more than 100 per second for the jaw muscles. 

 The rate at which the impulses come is quite constant for a given muscle. 

 Following one another so rapidly, these impulses do not give the cell time 

 to relax but keep it in a continuous state of contraction known as tetanus 1 

 until the stream of impulses stops. 



As we have said, the actual mechanism by which the chemical energy 

 of the contracting muscle is converted into mechanical energy is quite 

 unknown. It seems clear, however, that the transformation must be more 

 or less direct. It is probably a physical process — comparable, let us say, to 

 the release of a coiled spring. All the chemical changes seem to be subse- 

 quent to contraction and to restore the energy for the physical process — 

 to rewind the spring, as it were. The nerve impulse is the trigger which 

 releases the spring. The most likely hypothesis as to the nature of the 

 physical change that causes contraction is that it is the sudden alteration 

 of elongate protein molecules into shorter, thicker spirals. This, however, 

 is only a guess, and no chemical mechanism for causing such a change is 

 known. 



The source of the energy for contraction and the process by which it is 

 stored have been partially worked out. According to a widely held theory 

 supported by much evidence, the actual energy-storing substance in the 

 fibrils is a compound called adenosine triphosphate (or ATP for short). 

 When the trigger is pulled by a nerve impulse, an enzyme (organic 

 catalyst) 2 is liberated in the fibrils, causing a part of the stored ATP to 

 unite with water and to split into adenylic acid and pyrophosphoric acid, 

 with explosive release of energy. The exact nature and source of the 

 activating enzyme and the means by which the released energy is trans- 

 formed into mechanical tension and shortening of the muscle are two of 

 the important gaps in our knowledge. 



During the period of relaxation the ATP is resynthesized, thus "rewind- 

 ing the spring" in the fibrils. This synthesis requires a large amount 

 of energy, which comes from a complex series of chemical transforma- 

 tions of other substances, each reaction being controlled by enzymes 

 and related to those above and below it in the chain. At the base of the 

 series, and ultimately furnishing most of the energy used by muscle, is 



1 From the Greek, tetanos, "stretched." The disease commonly known by this name 

 is characterized by intense and prolonged tetanus of the voluntary muscles, especially 

 those of the lower jaw, whence the alternative name "lockjaw." 



2 The nature of enzymes is discussed in connection with the digestive system. 



