GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 



61 



PHENOMENA FOLLOWING A MUSCLE STIMULATION 

 PHYSICAL PHENOMENA 



Physiologic investigation has made it apparent that when a nerve impulse 

 reaches a muscle, it occasions a disruption of certain complex energy- 

 holding compounds and their subsequent oxidation to simpler compounds. 

 Coincidently with the chemic changes there is a transformation of the 

 potential energy of the molecules into kinetic energy which manifests 

 itself under three forms, heat, electricity and mechanic motion, or a change 

 of shape of the muscle. These phenomena vary in extent in accordance with 

 the intensity of the impulse as well as the frequency of its repetition. 

 Though the chemic changes are the first effects of the action of the nerve 



Fro. 21. SHOWING THE CHANGES IN A MUSCLE AND MUSCLE-FIBER DURING 



CONTRACTION. 



impulse and the ones on which other phenomena depend, it will be found 

 convenient to consider the most evident effect, the physical change in the 

 shape of the muscle, first. 



Change of Shape. The most obvious change in a muscle following the 

 arrival of a nerve impulse is that relating to its form. The muscle not only 

 becomes shorter, but at the same time thicker. The extent to which it may 

 shorten when unopposed may amount to 30 per cent, or more of its original 

 length. The increase in thickness practically compensates for the diminu- 

 tion in length, for there is no observable diminution in volume. The change 

 in form of the entire muscle results from a corresponding change of form of 

 its individual fibers as determined by microscopic examination, each of 

 which becomes shorter and thicker. The successive changes in both the 

 muscle and the individual fibers are represented in Fig. 21. 



When the contraction begins both the dim and bright bands diminish in 

 length, but at the same time increase in breadth. This continues until 

 the contraction reaches its maximum. The diminution in the length of the 

 bright band is greater proportionally than the diminution in the length of the 

 dim band, a fact which gave rise to the supposition on the part of Englemann 

 that there is at the time of the contraction a passage of fluid material from 

 the bright into the dim band or from the sarcoplasm into the sarcostyles. 

 When the relaxation begins, a reverse change in the dim and bright bands 

 sets in and continues until they regain their former shape and volume. Coin- 

 cidently there is a passage of fluid material from the sarcostyles to the 



