

REFLEX ACTION 497 



Although this reciprocal relation of antagonistic muscles was present to the minds of various 

 previous physiologists in a certain way, as for example to Meltzer (1883, pp. 215-216), it was 

 not until the work of Sherrington (1892, etc.) that our knowledge of the mechanism became 

 clear and definite. 



The phenomenon is shown in a striking way in Fig. 156 (from the paper by 

 Sherrington, 1909, 2, p. 260). The extensor muscle of the knee (vasto-crureus), in 

 a decerebrate cat, is isolated and connected to a tracing lever, similarly the flexor 

 (semi-tendinosus). These muscles are connected to the nerve centres by their 

 nerves, but the connections of all other muscles which might cause movement of 

 the levers are severed. The upper one (c) of the two signal lines at the bottom 

 of the figure indicates, by its rise, stimulation of the central end of an afferent 

 nerve of the leg of the opposite side (contralateral peroneal) and the lower signal 

 (i) marks stimulation of the corresponding nerve of the leg itself under observation 

 (ipselateral). E marks the myograph tracing of the extensor, F that of the 

 flexor. The lever attached to the extensor writes a few millimetres to the right 

 of the lever attached to the flexor. As the preparation was decerebrate, the 

 extensor muscle was in tonic contraction, but not the flexor, since decerebrate 

 tonus affects the muscles of posture only (Sherrington, 1906, p. 302), which 

 counteract gravity. The first stimulation is that of the contralateral nerve, 

 which produces a flexion reflex. In this reflex we see that, along with the 

 contraction of the flexor muscle, there is a marked inhibition of extensor tone, 

 followed by a rebound (successive spinal induction), as described above. In the 

 third stimulation, that of the ipselateral nerve, producing extension, inhibition of 

 the flexor cannot show itself on account of the fact that the muscle is already in 

 a state of relaxation, but it can be shown, indirectly, that the centres are 

 inhibited. The middle stimulation will be referred to later. It is clear that the 

 afferent nerve fibres proceed to the motor neurones of both the antagonist muscles, 

 but, while exciting the one, they inhibit the other. Examination of the figures 

 will show also that, in the rebound contraction, sudden inhibition of the flexor 

 contraction coincides with excitation of the .extensor muscle. 



Similar phenomena are observed in the movements of the eye brought about 

 by stimulation of the cerebral cortex and in movements of the limbs produced in 

 the same way. Sherrington concludes (1906, p. 285) that the seat of the inhibition 

 in these particular reactions from the cortex is not in the cortex itself, but 

 probably at the ultimate synapse with the final common path, that is, the motor 

 neurone. Certain phenomena to be described later, however, indicate that, 

 although the seat is very near to the final synapse, it is very likely in some 

 intermediate synapse. The phenomena referred to relate to the action of 

 strychnine and of chloroform. It is not to be concluded that, in many other 

 cortical reactions, inhibition of one cortical element is not effected by other cortical 

 elements, in fact, there is every reason to believe that this is the case. 



Reciprocal co-ordination was observed by Sherrington (1906, p. 285) in the 

 " willed " movements of the eyeballs in the monkey. The external rectus muscle 

 is supplied by the sixth cranial nerve, so that, if on one side the third and fourth 

 nerves, which supply all the other muscles, are cut, any movements of this eye are 

 due only to changes in the state of contraction of the external rectus. If now an 

 object is moved horizontally in such a way that its movement is followed by means 

 of contraction of the external rectus of the normal eye, it is seen that the other 

 eye also follows the movement. Since any movement of this eye must be effected 

 by the external rectus alone, and the movement observed is such as to be brought 

 about by relaxation, it follows that the tonus of its centre must be inhibited in 

 accurate time and step with excitation of the external rectus of the opposite eye. 

 It is therefore to be presumed that a similar process is going on with regard to the 

 internal rectus of the normal eye, which works in conjunction with the external 

 rectus of the other eye. 



A kind of reciprocal innervation holds in two cases already dealt with. In 

 the first of these, the " myenteric reflex " of the intestine, although the mechanism 

 is peripheral, it appears to be of reflex nature. In the second, the opening and 

 closing of the claw of the crayfish, the mechanism is not reflex, but of peripheral 



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