44 



EFFECT OF SUCCESSIVE STIMULI. 



contraction, there is a contraction of the antagonistic muscles, whereby a part of 

 the intended movement is, as it were, cut off. During the most rapid voluntary 

 movement in human muscles, v. Kries found that 4 stimuli per second were active, 

 so that a voluntary contraction is really a short tetanus. 



Pathological. In secondary degeneration of the spinal cord after apoplexy, atrophic muscular 

 anchylosis of the limbs, muscular atrophy, progressive ataxia, and paralysis agitans of long 

 standing, the latent period is lengthened ; while it is shortened in the contracture of senile chorea 

 and spastic tabes {Mendelssohn). The whole curve is lengthened in jaundice and diabetes 

 (Edingcr). In cerebral hemiplegia, during the stage of contracture, the muscle-curve resembles 

 the curve of a muscle poisoned with veratrin, and the same is the case in spastic spinal paralysis 

 and amyotrophic lateral sclerosis ; in pseudo-hypertrophy of the muscles the ascent is short and 

 the descent very elongated. In muscular atrophy, after cerebral hemiplegia, and in tabes, the 

 latent period increases, while the height of the curve diminishes. In chorea, the curve is short 

 (Reaction of Degeneration, 339). In rare cases in man, it has been observed that the execution 

 of spontaneous movements results in a very prolonged contraction (Thomsen's disease), 

 such cases the muscular fibres are very broad, and the nuclei increased (Erb). 



II. Action of Two Successive Stimuli. Let two momentary stimuli be applied 

 successively to a muscle : (A) If each stimulus or shock be of itself sufficient to 

 cause a maximal contraction, i.e., the greatest possible contraction which the 

 muscle can accomplish, then the effect will vary according to the time which 

 elapses between the application of the two stimuli, (a) If the second stimulus is 

 applied to the muscle after the relaxation of the muscle following upon the first 

 stimulus, we obtain merely two maximal contractions, (b) If, however, the second 

 stimulus be applied to the muscle during the time that the effect of the first is 

 present, i.e., while the muscle is in the phase of contraction or of relaxation ; in 

 this case the second stimulus causes a new maximal contraction, according to the 

 time of the particular phase of the contraction, (c) When, lastly, the second 

 stimulus follows the first so rapidly that both occur during the latent period, we 

 obtain only one maximal contraction (v. Helmholtz). It is to be specially noted 



that a single maximal 

 stimulus never excites 

 the same degree of 

 shortening as tetanic 

 stimulation (III), but 

 only about J of the 

 height of the contrac- 

 tion in tetanus. 



(B) If the stimuli 



U ' jT \ be not maximal, but 



only such as cause a 



/ -^ v - \g J -^ - ^_ ^ , _^* medium or sub-max- 



Y X o 33i. imal contraction, the 



I, two successive sub-maximal contractions ; II, successive contrac- e ff ec ts of both stimuli 

 tions produced by stimulating a muscle with 12 induction shocks are superposed, or 

 per second; III, curve produced with very rapid induction shocks there is a summation 

 (complete tetanus). of the contractions (fig. 



331). It is of no consequence at what particular phase of the primary contraction 

 the second shock is applied. In all cases, the second stimulus causes a contrac- 

 tion, just as if the phase of contraction caused by the first shock was the natural 

 passive form of the muscle, i.e., the new contraction (b, c) starts from that point as 

 from an abscissa (fig. 331, 1, b). Thus, under favourable conditions, the contraction 

 may be twice as great as that caused by the first stimulus. The most favourable 

 time for the application of the second stimulus is ^th second after the application 

 of the first (Sewall). The effects of both stimuli are obtained even when the 

 second stimulus is applied during the latent period (v. Helmholtz). 



W 



