ALL OR NOTHING 22 



zzo 



ill neurons forces us to look for some other way in wliich a strong 

 stimulus differs from a weak but effective one. The only possible 

 way in which to provide for a gradation is to consider the ninnber 

 of impulses per unit time, i.e., the frequency. Experiments have 

 shown that strong stimuli produce a large series of propagated 

 disturbances in nerve fibres. The stronger the stimulus, the 

 greater is the frequency up to a limit, but the " size " of each 

 impulse is invariable. 



5. Nature of the impulse. About the nature of the " disturb- 

 ance " propagated along a nerve we know nothing. It is accom- 

 panied by electrical changes similar in nature to the action potential 

 of muscle, and by metabolic changes, e.g. evolution of C02and heat. 



0. Electrical changes. The electrical changes have been much 

 studied by means of the capillary electrometer. Just as in muscle, 

 so in nerve, an electrical wave accompanies the nervous impulse. 

 The part excited becomes galvanometrically negative (zincative) to 

 the rest (see Muscle, p. 179), and this negative wave passes along 

 the nerve in the direction of and at the same rate as the nervous 

 impulse. It is followed by an electro-positive wave of greater 

 potential. The combined electrical changes are thus said to be 

 diphasic (cf. Fig. 43). That is, a potential difference is produced 

 which, if conditions permitted, would cause a current to travel in 

 the electrometer circuit first from B to A, and then from A to B 

 (Fig. 43). The relation between impulse and action potential at 

 any point are found experimentally by damaging the nerve at, 

 say, B, so producing a constant demarcation negative potential 

 difference (current of injury) which remains constant even when 

 the wave of negativity (current of action) reaches it (cf. Fig. 38). 

 The potential difference shown by the electrometer in an experi- 

 ment of this kind will, therefore, be in one direction or monophasic, 

 and will indicate the rise and fall of a potential difference under 

 one electrode. 



The monophasic response is brief, to 8 a 10 '* seconds in the 

 frog. Increase of temperature shortens, and decrease of tempera- 

 ture lengthens the duration of the stay of the electrically excited 

 region at any spot. The duration is, however, the same in all 

 similar fibres in a nerve trunk. 



7. Velocity of passage of electrical disturbance. The propagated 

 disturbance and the electrical disturbance pass along a nerve fibre 

 at the same rate and in the same direction. In the frog this rate 

 is somewhere about 28-33 metres per second. The rate varies, 

 however, from fibre to fibre. Even all the fibres in a nerve trunk 

 do not pass on the disturbance at the same rate, some having 

 velocities of propagation five times that of their neighbours. This 



B. 15 



