xlil] fatigue of nerve. 225 



LESSON XLIL 

 FATIGUE OP NERVE-SEAT OP EXHAUSTION. 



1. Can Nerve be Fatigued? — "We have seen tliat a muscle 

 manifests fatigue, ^.e., its store of material and energy are gradually 

 used up, so tliat it shows a diminished capacity to respond to 

 stimulation. Does a nerve manifest such phenomena? Reasoning 

 a priori^ from the fact that the only known sign obtainable during 

 the activity of a nerve is the " negative variation of the nerve- 

 current," one is led to suppose that very probably nerve-fibres 

 partake but little if at all in the phenomena of fatigue. In fact, we 

 shall find that nerve is practically inexhaustible. 



Suppose one stimulated a nerve of a nerve-muscle preparation 

 with maximal induction shocks until the muscle ceased to respond 

 to indirect stimulation. This would afford no proof that the muscle 

 itself was fatigued. Why ? Stimulate the muscle directly, and it will 

 respond. Therefore the seat of fatigue in this case is not primarily 

 in the muscle, but must be sought for either in the nerve itself or 

 at the end-plates where the. nerve comes into relation with the 

 muscular substance. 



2. Seat of Exhaustion — is it in Muscle, Nerve or End-Plates ? 



A. Not primarily in Muscle. — {a.) Arrange an induction coil for repeated 

 shocks. Connect the secondary coil with a Pohl's commutator without cross- 

 bars. 



(6.) Prepare a nerve-mascle preparation, with a straw flag, or use a crank- 

 myograph, and place its nerve over Du Bois electrodes attached to the com- 

 mutator. Pass two fine wires through the gastrocnemius and attach them to 

 the other two binding screws of the commutator. 



(c.) Tetanise the nerve until the tetanus ceases. Then reverse the commu- 

 tator and stimulate the muscle. It contracts. Therefore, ^Ae 5m< of fatigue 

 is not in the muscle. 



B. Not in the Nerve {Nerve is practically inexhaustible). — {a.) Arrange a 

 nerve-muscle preparation in connection with a coil for repeated shocks as 

 before. Place the nerve over the electrodes from the secondary coil. 



{h. ) Arrange a Daniell's cell connected to N.P. electrodes, and short-circuited 

 for a constant current— the ' ' polarising current " (Lesson XLVIII ) — and place 

 tlie N.P. electrodes next the muscle, so that the — pole is next the muscle, i.e.^ 

 with the i)olarising current descending. The ** polarising current" so lowers 

 the excitability of the nerve as to "block" the passage of a nerve impulse 

 through this part of the nerve. The tetanising electrodes are placed near the 

 upper cut end of the nerve. 



(c. ) See that the muscle responds when the stimulating current acts on the 

 nerve, then throw in the polarising current, when at once the muscle ceases 

 to respond, because the nerve impulse is Islocked. Go on stimulating the 

 nerve for an hour or longer. We know that if there had been no " block " the 

 muscle would long ere this have ceased to respond to indirect stimulation. 



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