

EXCITATION AND INHIBITION 41; 



was allowed to return to the normal height. It seems that the " shock " effect 

 must be due to the removal of some influence exerted by the higher parts of the 

 nervous system, an influence of such a kind as to increase the excitability of 

 the cord. Sherrington (1906, pp. 240-248) had previously come to the conclusion 

 that spinal shock has nothing to do with the injury of the section, and that 

 it was probably caused by some influence exerted by the mid-brain on the lower 

 centres. He shows that, when recovery from a transection of the cord has taken 

 place, a second transection just below the first has practically no effect. 



The work of Pike (1909, 1912, 1913) shows that the phenomena are not due 

 to the traumatic stimulation of long inhibitory tracts from the higher centres 

 to the spinal cord. He looks upon the normal path for what are called spinal 

 reflexes as having been diverted in the course of evolution, so that the impulses 

 do not pass directly across the cord, but ascend to the higher centres first and 

 from these return to the motor neurones. The direct path has thus become more 

 or less impassable from disuse, but it can, after a time, regain the power of 

 conduction to a certain extent, when the path through the higher centres is cut. 

 This point of view will be better understood when Chapter XV. on the nerve 

 centres has been read. 



Decerebrate Rigidity. Sherrington found (1898, 1906, pp. 299-303) that when 

 the crura cerebri are cut through, the centres of certain groups of muscles are 

 so much increased in excitability that the ordinary slight stimuli arising from 

 the periphery are sufficient to maintain these muscles in a state of reflex tonic 

 contraction. The centres in question are situated somewhere between the crura 

 and the lower part of the spinal bulb, but not in the cerebellum. Under normal 

 conditions, their excitability is restrained by the inhibitory influence of the 

 cerebral cortex. 



Weed (1914) comes to the following conclusions as the result of extensive 

 investigation. The main reflex centre for decerebrate tonus is in the mid-brain, 

 probably the red nucleus. The cerebellum, although not the absolutely essential 

 pathway for the afferent impulses concerned, is the most important one. It is 

 also the link in the inhibitory channel from the cerebral cortex, which prevents 

 this rigidity in normal conditions. 



This condition of "decerebrate rigidity" is very useful in the investigation of 

 reflex inhibition, since we have centres in a state of tonic excitation, which can be 

 played upon by the stimulation of various nerves. We have seen an example of 

 this in Fig. 118 (page 4 10). 



The Action of the Anode. We have seen that excitation proceeds from the 

 cathode. The simplest way of showing the fact is by the use of the device of 

 J. S. New (1899). 



Since excitation is associated with increased concentration of cations at a 

 particular membrane, it is natural to suppose that the effect of the anode, being to 

 decrease this concentration, would result in inhibition. It is clear, however, that 

 this cannot show itself unless the tissue is in a state of excitation when the anode 

 is applied. Biedermann (1895, p. 227) has made use of the drug, veratrine, to 

 produce this state of excitation in voluntary muscle. A muscle under the 

 influence of veratrine gives a prolonged contraction in response to a single 

 stimulus and, during this state, a current can be sent through the muscle in such 

 a way that the effects of the anode and cathode can be observed separately. It 

 was found that the anode causes relaxation, the cathode, increased contraction. 

 A still simpler way of seeing the fact, as Biedermann points out (1895, p. 219), is 

 to place the anode on the beating ventricle of the frog's heart, the cathode on 

 some other part of the body. It will be seen that the neighbourhood of the 

 anode remains permanently in a state of relaxation, while the remainder of the 

 ventricle undergoes periodic contraction. 



Another aspect of the action of the anode is manifested in the case of nerve. 

 The exciting effect of a constant current is only shown when it is made or broken ; 

 in other words, while it is really constant no excitation occurs. It must change 

 its strength at or above a certain minimal rate in order to excite. But the nerve 

 is not unaffected during the period of a constant flow of current. In the 



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