434 PRINCIPLES OF GENERAL PHYSIOLOGY 



Various excitable tissues differ in the rate of incidence of their "optimal 

 stimuli, that is, the stimulus which excites with the least expenditure of energy. 

 This is Waller's " characteristic," and is included in the modified Nernst formula. 



The function of the medullary sheath is still problematical, although it appears 

 to have a function in connection with the growth of fibres. The axis cylinder 

 is probably of a liquid nature, but colloidal. There is no evidence of the existence 

 of " neuro-fibrils " in the living state. 



The nerve impulse, as it travels along a fibre, seems to be a reversible, physico- 

 chemical process, not associated with metabolic changes ; but the question is not, 

 as yet, altogether decided. 



A distinction must be made between the local process at the spot stimulated 

 and the propagated change. The former is confined to the stimulated spot, and 

 requires a certain small expenditure of energy to set it up. When set up, if 

 sufficiently intense, it produces a propagated disturbance. There is no convincing 

 evidence that the latter is attended with any consumption or evolution of energy. 



In muscle there is an excitatory process essentially like that in nerve, and, 

 superadded to this, a contractile process, which has a latent period and is associated 

 with metabolism, with its accompanying heat production and fatigue. The 

 former process may be present without visible contraction. 



There are certain other excitable substances which intervene between nerve 

 and muscle. These are called " receptive substances " or the " myo-neural junction." 

 Their existence can be shown by their reaction to various drugs, and by their 

 different optimal rates of stimulation. The muscle fibre has a low optimal rate, 

 the nerve trunk one of a rather higher value, while the intermediate substance 

 has a very high one. These optimal rates have been shown by Keith Lucas to 

 be functions of the rate of diffusion of the ions concerned in the excitation process, 

 according to the conception of Nernst. 



There is a membrane intervening between the nerve ending and the muscle 

 fibre supplied by it, as also between one neurone and the fibre connecting it 

 with another neurone. This membrane is called by Sherrington the "synaptic 

 membrane." 



The essential processes connected with inhibition must be of an opposite kind to 

 those connected with excitation. Various theories have been propounded, from 

 different points of view, as to what is the basis of inhibition. 



In the text, a number of illustrations are given to show the different aspects of 

 inhibition, as it affects peripheral muscular organs directly, and as it affects nerve 

 centres acting on these organs through nerves. Also on the nerve centres con- 

 trolling those peripheral organs, such as skeletal muscle, which have no automatic 

 activity of their own. 



That the excitatory and inhibitory influences act on the same cell is shown, 

 amongst other facts, by the capability we have of exactly neutralising the effect 

 of stimulation of the one nerve by simultaneous stimulation of the one having the 

 opposite effect. 



The state of excitation of a nerve centre causing peripheral inhibition can be 

 itself inhibited by nerves acting on this centre, so that we have "inhibition of 

 inhibition." It seems probable that an intermediate neurone, inhibiting a 

 particular motor neurone, may itself be inhibited by an afferent neurone and 

 thus the inhibition of the motor centre may be taken off. 



Higher centres in the nervous system influence lower centres, either increasing 

 or decreasing their excitability. Hence the phenomena of " spinal shock " and of 

 " decerebrate rigidity." 



Inhibitory effects can be brought about by direct action of chemical agents or 

 by the anode of the electrical current. 



The phenomena of physical interference of wave motion are incapable of 



