INTEGRATION OF ACTION WITHIN THE REFLEX ARC 941 



The Action of Strychnine and Tetanus Toxin on Reciprocal Inhibition 



Under certain conditions reciprocal action may fail to occur, as, for 

 example, at certain stages of strychnine poisoning and during the action 

 of tetanus toxin. In order to demonstrate this failure of reciprocal ac- 

 tion, it is necessary to examine muscles which act on one joint only, and 

 to observe their behavior when an afferent nerve is stimulated which un- 

 der ordinary conditions would throw them into inhibition. Such a 

 preparation can be obtained in the hind limb of a dog by cutting all the 

 muscles that act on the knee joint except the vastus crureus, which in a 

 normal animal invariably undergoes inhibition when the central end of 

 the internal saphenous nerve is stimulated. If a suitable dose of strych- 

 nine is injected, it will be found that stimulation of the internal saphenous 

 nerve, in place of inhibition, causes contraction of the vastus crureus 

 muscle. The same result is obtained by injection of tetanus toxin. 



The failure of the reflex inhibition explains the symptoms produced 

 by these substances. It explains, for example, the well-known rigidly 

 extended condition of the limbs in strychnine poisoning, and the dis- 

 tressing symptom of lockjaw in tetanus infection. In this latter con- 

 dition the sufferer is subjected to extreme torture with every endeavor 

 that he makes to open the jaw for the purpose of taking food or drink. 

 Firmer closure is the result because the normal inhibition of the temporal 

 and masseter muscles does not occur, but instead they become excited 

 and the jaw all the more firmly closed. Not only does, the inhibition fail 

 to occur, but the above muscles are usually in a state of constant hy- 

 perexcitability, which it is impossible for the patient to restrain ; indeed, 

 whenever he attempts to do so the opposite occurs and the excitation 

 becomes heightened. Chloroform acts on reciprocal innervation in an 

 opposite way from strychnine and tetanus; namely, it paralyzes the ex- 

 citation of the contracting muscles. 



The Reflex Figure 



We have seen in the preceding paragraphs that the afferent fibers of 

 a single reflex arc make connection with the motor neurons of a con- 

 siderable group of muscles, exciting some and having an inhibitory in- 

 fluence on others to the end that all may cooperate in producing an or- 

 derly movement of some functional significance. The effects of stimulating 

 a single afferent path may not be limited to the antagonistic muscles ar- 

 ranged about a single joint, but may extend to the flexors and extensors of 

 all the joints in a single limb and, further, to each of the other limbs and to 

 muscles of the head, trunk, and tail as well. The response which follows 

 the application of a strong stimulus to the receptors of a reflex arc may 

 consequently involve nearly the whole musculature of the animal. The par- 



