RECIPROCAL INNERVATION 819 



than the particular level of that reflex, will also be able to induce the 

 inhibition. In the case of the decerebrate cat this can be demonstrated 

 by stimulation of the lateral columns of the spinal cord; inhibition of 

 the extensor muscles of the elbow joint occurs, which is all the more 

 marked because in such a preparation these muscles are in a state of 

 hypertonicity. We shall see later also that through the pyramidal tract 

 impulses may descend from the cerebrum which exercise a marked in- 

 hibitory influence over the reflex activities of the cord. Similarly the 

 inhibition itself may be terminated by impulses from other sources, and 

 the motor neuron thus thrown from a state of inhibition into one of ex- 

 citation. This fact can perhaps best be demonstrated by exciting the 

 central end of the homolateral peroneal nerve (which produces a reflex 

 extension of the leg) while the leg is being held in a flexed position by 

 stimulation of the contralateral peroneal nerve. This will be clear from a 

 study of Fig. 214. 



Such alternating excitation and inhibition of an active motor neuron 

 serve to make it possible for rhythmic discharges to occur through the 

 neuron, as in the action of the muscles of the leg in walking or during 

 the scratching movement. In order to insure that the same final com- 

 mon path may be occupied at one time by but one kind of stimulus, either 

 inhibitory or excitatory, it is further of importance that the after-dis- 

 charge (see page 810) of the first stimulus should be capable of imme- 

 diate inhibition ; otherwise, while one reflex was in progress, it would be 

 impossible to start another of a different type employing the same motor 

 neuron without confusion of movement. That this occurs can be demon- 

 strated in the case of the after-discharge of the flexion reflex by stimula- 

 tion of the proper afferent nerve. 



In view of all these facts it is probable that the seat of the reciprocal 

 innervation is at or about the synapsis. In other words, the synapsis at 

 the termination of one collateral will allow a stimulating impulse to pass 

 to the cells of one motor neuron, whereas that at the end of another col- 

 lateral of the same afferent fiber will allow an inhibiting impulse to pass 

 to an antagonistic motor neuron, these conditions being, however, readily 

 interchangeable and thus making even rapid rhythmic contraction and 

 relaxation a possibility. 



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 



