130 INTRODUCTION TO GENERAL PHYSIOLOGY 



instead of extension ; and so on. Inhibition is more prominent 

 than excitation, and appears independently of excitation of 

 antagonists. After-actions, such as tonic or rhythmic contractions, 

 are of various kinds, and affect the pairs of antagonists in a diversity 

 of ways. 



The Nerve Impulse 



As pointed out above, there is nothing to be seen in a nerve fibre 

 to indicate that a propagated disturbance is passing along it. 

 Moreover, only one physical or chemical accompaniment of the 

 impulse has been definitely shown to be present, that is, an 

 electrical change of such a nature as to indicate that a point in a 

 state of activity is electrically negative to one at rest. It has been 

 stated that an evolution of carbon dioxide occurs, but the experi- 

 ments are not altogether free from objection. The most sensitive 

 instruments have failed to show that any evolution of heat takes 

 place, and the absence of fatigue under normal conditions, referred 

 to previously, indicates an extremely small consumption of energy. 

 Indirect evidence suggests that what happens is a concentration of 

 ions of a certain sign at or near some membrane, and that this 

 concentration progresses as a wave along the fibre, the change at 

 a forward point being brought about by the electrical effect of that 

 behind it. The ions thus move backwards and forwards at any 

 particular point, somewhat as the molecules concerned in the pro- 

 pagation of sound waves do. 



Like other excitable tissues, nerve fibres exhibit a refractory 

 phase, at first of loss of excitability altogether and then of gradual 

 return to normal, or for a moment slightly beyond it. The whole 

 period is very short, 0.0025 sec - m the frog, for the period of 

 inexcitability. In man, it is propably about one-fourth of this 

 value. 



The rate of conduction of the impulse in man is about 120 

 m. per second. 



We have seen that muscle fibres are only able to manifest one 

 degree of activity, however the strength of the stimulus is varied. 

 As far as motor nerves are concerned, the same fact of "all-or- 

 nothing" has been found, and, in all probability, it holds for afferent 

 nerves also, since no other difference between the two kinds of 

 nerve fibres has been detected. 



The fact just mentioned is difficult to reconcile with a wave-like 

 displacement of ions or similar view of the nature of the nerve 

 impulse. If a nerve impulse passes through a region subjected to 

 the action of an anaesthetic, it may be abolished ; but if the 

 anaesthesia is not too deep or the length anaesthetised not too long, 



