EXCITATION AND INHIBITION 



409 



movements of the intestine, so that it is not unnatural to imagine that the effect 

 on the pancreas might be due to the relaxation and contraction of muscular fibres 

 in the ducts. That this is the explanation is shown by the experiments of von 

 Anrep, described on page 349 above. 



Jappelli (1908) states that, if an attempt is made to produce reflex salivary secretion by 

 exciting the central end of the lingual nerve, no effect is produced as long as the stimulation 

 lasts, but that the secretion appears subsequently. If secretion is in progress when the nerve 

 is excited, a temporary cessation occurs. Before we can accept these results as evidence of 

 inhibitory fibres, experiments on the blood flow through the gland must be made under the 

 conditions of the experiment. It is practically certain that reflex vaso-constriction was 

 produced in the gland by stimulation of the sensory nerve and the failure of blood supply 

 would be quite sufficient 

 to account for the results. 

 Reference was made 

 previously (page 349) to 

 the " anabolic " nerves 

 supposed to supply the 

 gland cells ; the function 

 of these fibres is supposed 

 to be to excite the build- 

 ing-up process in these 

 cells. According to the 

 theory proposed by Gas- 

 kell, to be discussed pre- 

 sently, anabolic nerves 

 are inhibitory. When the 

 chorda tympani is cut, 

 these tonic inhibitory im- 

 pulses are removed and 

 we have "paralytic secre- 

 ,tion." The basis of this 

 theory will be found not 

 to be a very solid one. 



Nerve Centres. We 

 come now to a very 

 important region in 

 which inhibitory pro- 

 cesses play as funda- 

 mental a part as those 

 of excitation. A few 

 examples will be given 

 here, and the nature 

 of the phenomena dis- 

 cussed later. 



The necessity of the 

 process will be appar- 

 ent, if we consider the 

 state of affairs in a 

 complex co-ordinated 

 act, in which the nerve 



cells of centres governing various muscles are called upon. Some or all of these 

 cells are certain to be occupied in other ways at the time ; that is, they are 

 already in a state of excitation due to the arrival of messages from other sources. 

 If they are to perform the new process properly, they must be freed from this 

 previous state of excitation, so as to be accessible to the fresh one. Further, 

 the activity of certain centres sets into contraction muscles which oppose those 

 required in the new act and which must be relaxed. 



In Fig. 107, on page 388 (from Sherrington and Sowtx>n, 1911, p. 443), we saw 

 that a state of tonic reflex contraction is produced in the vasto-crureus muscle, by 

 stimulation of the central end of the popliteal nerve with rheonome currents. This 

 tonic excitation of the nerve cells continues after the stimulus has ceased, as shown 

 by the line of the signal at the base of the figure. At the second mark of the signal, a 

 weak tetanising stimulation was applied to the same nerve. The tonic state of the 

 nerve cell is completely abolished, but returns when the inhibiting stimulus ceases. 



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FIG. 117. ELECTRICAL CHANGE PRODUCED IN THE QUIESCENT 

 VENTRICLE OF THE TOAD BY STIMULATION OF THE AUGMENTOR 

 NERVE. The first two stimulations, after the heart had been^ 

 brought to a standstill by application of muscarin to the 

 sinus. The last one, on another heart after the ventricle 

 had ceased to beat owing &o block of impulses from the sinus 

 by application of a clamp between the auricle and ventricle. 



The sign of the electrical effect is similar to that of spontaneous 

 contraction, and opposite to that produced by the vagus. 



(Gaskell, Journ. of Physiol., 8, 404-415). 



