434 - Multicellular Animals, Especially Man 



upon the recording paper; and sometimes a 

 simultaneous tracing of the action current of 

 the muscle is made by means of a galvanom- 

 eter. 



The duration of a single twitch in a frog's 

 muscle, at room temperature, is about 0.1 

 second (Fig. 24-4). This time can be sub- 

 divided by careful measurements into three 

 periods: (1) the latent period, the very brief 

 (0.003 second) interval following stimulation, 

 before any sign of mechanical contraction 

 can be detected; (2) the contraction period, 

 the somewhat longer (0.047 second) time dur- 

 ing which the muscle is engaged in shorten- 

 ing; and (3) the relaxation period, the long- 

 est (0.05 second) period, during which the 

 muscle returns to its original length. Usually 

 the action potential, which is the first indica- 

 tion that the stimulus has excited the muscle, 

 reaches a peak and subsides during the latent 

 period, before there is any mechanical sign 

 of contraction (Fig. 24-1). Also before it con- 

 tracts, at the very end of the latent period, 

 the muscle displays a very slight, but defi- 

 nitely measurable relaxation (called the la- 

 tency relaxation), as was first shown in 1948 

 by Alexander Sandow, at New York Uni- 

 versity. 



Following a twitch, the muscle consumes 

 oxygen and produces carbon dioxide and 



heat in excess of the normal resting quanti- 

 ties. This indicates that there is a recovery 

 period, which restores the tissue to its orig- 

 inal state. For a single twitch, the recovery 

 period endures for about one minute. If a 

 muscle is stimtdated repeatedly and rapidly, 

 so that the successive excitations occur be- 

 fore the muscle completes recovery from the 

 preceding twitches, fatigue begins to appear. 

 In this case, the twitches become feebler and 

 feebler (Fig. 24-5), and the fully fatigued 

 muscle will not respond to further excitation 

 until it is allowed to rest in the presence of 

 an adequate supply of oxygen. 



Contraction: The Tetanus. The duration of 

 a single muscle twitch varies in different 

 animals, being about 0.1 second in the frog; 

 0.05 second in man; and 0.003 second in cer- 

 tain insects. However, most muscular move- 

 ments in intact animals are not twitches, but 

 more prolonged contractions, called tetani. 

 A prolonged contraction, or tetanus, involves 

 not one, but a volley of excitations. During 

 any tetanus the excitations follow each other 

 so rapidly that relaxation cannot occur be- 

 tween the successive contractions. Conse- 

 quently the muscle remains in a contracted 

 state until the volley ceases. Each separate 

 excitation is, however, accompanied bv its 

 own electric discharge; and a continuous 



LATENT CONTRACTION 

 PERIOD PERIOD 



RELAXATION 

 PERIOD 



RECOVERY 

 PERIOD 



MOMENT OF 

 STIMULATION 



V 



HEAT PRODUCTION IN ABSENCE OF 0, 



V 



HEAT PRODUCTION IN PRESENCE OF 0, 



Fig. 24-4. Analysis of a mus- 

 cle twitch. The dotted line 

 represents the electrical re- 

 sponse (change of electrical 

 potential); the heavy line, the 

 mechanical response (contrac- 

 tion and relaxation) of the 

 muscle. 



