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, mechani- 

 cal, 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 contractions. 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 electronus. 



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 arg therefore ^nstifipH ir> the nflft-o-glr.iaacfl.1 

 stimuli not only for arousing the activity of excitably, tissue^ but also lor 

 determining the conditions of excitation of muscle and nerve. For this pur- 

 pose we may use either the make and break of a constant current, the induced 

 current of short duration 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 deter- 

 mine 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. 110. 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 and the electromotive force by which it is 

 charged, we can estimate the energy of the charge sent through the nerve. 



E (energy in ergs)* = 5FV a 

 (F = capacity in microfarads ; V = electromotive force in volts). 



* An erg is the amount of work produced or energy expended by the action of 

 one dyne through one centimetre. A dyne is the force which will give to a mass of 

 one gram an acceleration of one centimetre per second per second. 



262 



