SECTION V 

 THE EXCITATION OF NERVE FIBRES 



MANY different forms of stimuli may be used to arouse the activity 

 of an excitable tissue such as muscle or nerve. Thus we may use 

 thermal, mechanical, or chemical stimuli. If the temperature of a 

 motor nerve be gradually raised, no effect is noticed till about 40 C. 

 is reached, when the muscle may enter into weak quivering contrac- 

 tions. Sudden warming of the nerve always gives rise to excitation. 

 At about 45 C. the nerve loses its irritability and dies, On the other 

 hand, a nerve may be rapidly cooled without any excitation taking 

 place. 



A nerve may be excited mechanically by crushing or cutting. 

 These methods destroy the nerve. It is possible to excite a nerve 

 mechanically, without any serious injury to it, by carefully graduated 

 taps, and this method has been used in investigating the phenomena 

 of electrotonus. 



All chemical stimuli applied to the nerve have a speedy effect in 

 destroying its irritability. The chemical stimuli most used are strong 

 salt solutions, glycerin, or weak acids. If any one of these be 

 applied to a motor nerve, the muscle enters into an irregular 

 tetanus, which lasts till the irritability of the nerve is destroyed at 

 the part stimulated. 



None of these forms of stimuli can be adequately controlled either 

 as to strength or duration. Moreover, owing to their destructive 

 effects, any repetition of the stimulus will fall on a nerve or muscle 

 more or less altered by the first stimulus. We are therefore justified 

 in the use of electrical stimuli not only for arousing the activity of 

 excitable tissues, but also for determining the conditions of excitation 

 of muscle and nerve. For this purpose we may use either the make 

 and break of a constant current, the induced current of short dura- 

 tion produced in a secondary coil of an inductorium by the make or 

 break of the primary circuit, or the discharge of a condenser. 



The last-named method of stimulation is especially useful when we desire 

 to determine the total amount of energy involved in the electrical stimulation 

 of a nerve or muscle. The arrangement of such an experiment is shown in 

 Fig. 111. By means of the switch S the condenser can be put into connection 

 either with the battery from which it receives its charge or with the nerve 

 through which it can discharge. By knowing the capacity of the condenser 



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