388 



ORGAN SYSTEMS OF MAN 



muscle 



Fig. 15-9. Muscle action can be studied by attaching an 

 isolated muscle, such as the gastrocnemius muscle 

 of the frog, to a lever which can scratch a line of 

 its path on a smoked moving drum (kymograph). 

 When the muscle is electrically stimulated, the nature 

 of the contraction can be recorded on the smoked 

 drum. 



pull of the muscles. Considerable energy 

 is utilized in maintaining this continuous 

 contraction and, as in all muscle contrac- 

 tions, a large portion of it is released in the 

 form of heat. This heat helps to keep a 

 constant body temperature. 



Muscles normally contract as a result of 

 impulses coming to them through nerves. 

 However, an isolated muscle can be made 

 to contract if stimulated directly by an 

 electrical current, even though all the 

 nerves have been destroyed. The nature of 

 the contraction can be studied by attaching 

 the muscle to a recording device (Fig. 

 15-9) and noting its action following stimu- 

 lation. When the muscle first receives a 

 very brief stimulus there is no visible evi- 

 dence of anything happening. This period 

 is known as the latent period (Fig. 15-10), 

 and lasts about 0.01 second in the fros; mus- 

 cle. Contraction then begins and continues 



stimulus 



I f 

 latent period 



0.0\ second 



Fig. 15-10. This record was made when the frog gastroc- 

 nemius muscle contracted and relaxed, using a re- 

 cording device as shown in Fig. 15-9. Note the time 

 required for each event to occur. 



for 0.04 second. This is immediately fol- 

 lowed by a relaxation period that lasts 0.05 

 second during which time there is a 

 chemical readjustment taking place in the 

 muscle (discussed below). If successive 

 stimuli are increased in their frequency 

 there will come a time when the contrac- 

 tions will be superimposed upon one an- 

 other until there is a sustained contraction 

 which is greater than any derived from 

 single stimuli (Fig. 15-11). This is called 

 tetanus, and is what usually happens in 

 most muscular contractions, however short. 

 If a stimulus is given to an isolated frog 

 heart muscle, contraction occurs, provided 



Fig. 15-11. This record shows that by applying stimuli 

 to a skeletal muscle with gradually increasing fre- 

 quency, contractions merge until there is a sustained 

 contraction called tetanus. The contraction is stronger 

 in tetanus than in the single contractions. 



