EXCITATION AND INHIBITION 399 



" voluntary " muscles, with their rapid contraction, and the smooth, "involuntary" 

 muscles of the viscera and blood vessels, with their slow rate of contraction. 

 The latter are, in their natural, unstimulated condition, in a state of partial 

 contraction, so that two sets of nerves are required, one set to increase the 

 activity, which may therefore be called "excitatory," the other set to decrease 

 it, "inhibitory" nerves. The voluntary, skeletal muscles are, if unstimulated, 

 completely at rest. They are supplied with one set of nerves only, those causing 

 excitation, the other being needless. If continued tonic contraction is required, 

 it must be kept up by continued innervation from the nerve centres ; so that, 

 to inhibit this state of contraction, influences must be brought to bear on the 

 nerve centres themselves to stop their activity. It is unnecessary to remark that 

 the excitatory and inhibitory centres of the smooth muscles are also liable to 

 similar exciting and inhibiting influences. There are, then, at least two kinds 

 of inhibition, one exercised on muscle itself directly, the other on nerve cells, 

 when these are in a state of activity. Again, the inhibitory nerves of smooth 

 muscle arise from centres in the nervous system, and these centres, if in a state 

 of activity, can be inhibited by the play upon them of nerve impulses from other 

 sources. We have thus, in Sherrington's phrase, an "inhibition of inhibition," 

 that is, a central inhibition of nerve activity which was producing inhibition in 

 peripheral organs. We shall find evidence of the actual occurrence of this 

 phenomenon in the case of vasomotor reflexes. 



OTHER EXCITABLE SUBSTANCES 



The two excitable substances already discussed, muscle and nerve, are not 

 the only members of the class. It might be supposed that a nerve acting on a 

 muscle merely formed some kind of direct connection therewith, but a simple 

 experiment shows that there is something between them, itself an excitable 

 substance. 



Let us take two nerve-muscle preparations and arrange the nerves of both on similar 

 electrodes in series in the same circuit, so that they can be excited with the same strength of 

 stimulus. On the one nerve, between the seat of excitation and the muscle, we place a pair of 

 non-polarisable electrodes, through which we send a galvanic current of sufficient strength to 

 block the nerve impulses on this side. Both nerves are thus excited, one muscle only. After a 

 time, the muscle becomes fatigued and ceases to respond. At this moment, the galvanic 

 current causing the block is cut off ; the muscle on this side goes into tetanus. In this way, 

 it is seen that the fatigue was not localised in the nerve trunk. The next step is to apply 

 electrodes directly to the muscle which had ceased contracting ; it is found to be able to respond 

 vigorously. So that the seat of the fatigue is not in the actual contractile substance of the 

 muscle. The unavoidable conclusion is that there is some intermediate substance, more easily 

 fatigued than either nerve or muscle. 



The action of the arrow poison, curare, affords similar evidence. If the nerve 

 only is immersed in a solution of this drug, it is not paralysed. If the muscle is 

 immersed, excitation of the nerve has no effect upon it ; but it is not because the 

 muscle itself is paralysed, since placing the electrodes on it produces contraction. 



Under the microscope, there is to be seen, where the nerve enters the muscle 

 fibre, or rather comes into connection with it, what appears to be a specialised 

 structure, the " end-plate " ; but that this is not the substance for which we are 

 seeking is clearly shown in several ways. Adrenaline, as we shall see in more 

 detail in Chapter XXIV., is a secretory product of the suprarenal bodies and 

 has the property of exciting organs supplied by sympathetic nerves, and in 

 precisely the same way as excitation of these nerves themselves. When, therefore, 

 it is applied to arteries innervated by vaso-constrictor nerves from the sympathetic, 

 these arteries contract. On the other hand, if applied to arteries not supplied 

 by sympathetic vaso-constrictor nerves, no contraction results. It does not, 

 accordingly, produce its effect by direct action on the muscle cells. If the nerves 

 are cut and allowed to degenerate, the constrictor effect of adrenaline is un- 

 diminished. Now there is every reason to believe that the visible nerve endings 

 in muscle degenerate with the nerve fibre. 



Langley (1906, p. 179) finds that the nerve endings on the sartorius muscle of the frog 

 disappear in six weeks after section of the nerve to the muscle. There is, moreover, no 



