POLAR EXCITATION OF CRA YFISH NERVES. 



5i3 



Polar excitation of the nerves of the crayfish. — The response to 

 the excitation of the nerves supplying the claw is remarkable, inas- 

 much as a weak adequate stimulus, such as the faradic current, evokes 

 augmentation of the tonic contraction of the abductor or opening 

 muscle, at the same time that it causes relaxation of that of the 

 adductor or closing muscle ; on the other hand, a more intense stimulus 

 of a similar kind causes an augmentation in the adductor contraction, 

 accompanied by the relaxation of the abductor. 



To account for this, we must assume the existence of four sets of 

 nerves differing in the degree of their excitability, and in their 

 susceptibility to respond to different external stimuli ; the four sets 

 comprise an augmentor and inhibitory set for each muscle. 



The polar excitation of such a nerve trunk by galvanic currents is 

 thus extraordinarily complicated, since to the inhibitory influences of 

 the poles themselves must be added those produced by the polar 

 excitation of the inhibitory nerves. 



The simplest experiments are those made upon a preparation in 

 which one muscle is dissected free of its insertion, and employed when 

 it has lost its tonicity. In such a preparation all the three stages of 

 Pfliiger's law can be demonstrated, but there is this difference between 



771 & 



J I I I I I I I I I I I / " 



Fig. 267. 



the results and those observed in the nerve-muscle preparation of the 

 frog, the muscular contractions evoked at opening and closing are in all 

 cases prolonged. 1 If, on the other hand, the muscle retains its tonicity, 

 then the passage of a current in either direction evokes more complex 

 effects. In the case of the adductor muscle (Fig. 267) the results are as 

 follows : — 



When the nerve is excited by weak currents, three changes are 

 seen — a, an initial contraction on closure (m in Fig. 267), or an augmen- 

 tation of tonicity, due to the excitation of the augmentor fibres ; b, a 

 persistent relaxation succeeding a, slowly subsiding during the closure, 

 due to the inhibition of the effect ; c, the return of the muscle to its 

 normal state of tonicity on opening the current. 



With more intense currents these effects are profoundly altered 

 (see Fig. 267, right-hand figure), (a) The initial augmentation is fused 

 with a persistent one, due to a prolonged excitation of the augmentor 

 overpowering that of the inhibitory fibres ; (b) this may be succeeded 

 during closure by relaxation, and this again by contraction, so that a 

 conflict between the response due to persistent excitation of augmentor 

 and inhibitory nerve fibres occurs, in this conflict the augmentor effect 

 has the advantage ; (c) on opening, there is a relaxation of the muscle 



1 Biedermann, " Electrophysiologic. " 1895, S. 602-604. 

 VOL. II.— 33 



