l;l . DPBOCAL l\ \i lev \Tl< >\ Blfl 



than the particular lev£l of thai reflex, will also be able to induce the 

 inhibition. In the case of the decerebrate ea1 this can 1"- demonstrated 

 by stimulation of the Lateral columns of the Bpinal cord; inhibition of 

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

 marked because in Buch a preparation these muscles are iri a Btate of 

 hypertonicity. We shall sec later also thai through the pyramidal ti 

 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 ^tate of inhibition into one of 

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

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

 extension of the 1<'<_:' S; while the leg is being held in a flexed position by 

 stimulation of the homolateral peroneal oerve. This will be clear from ;i 

 study of Pig. 214. 



Such alternating excitation and inhibition of an active motor neuron 



serve to make it possible for rhythmic discharges t »cur through the 



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

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

 mon path may he occupied at one time by bu1 one kind of stimulus, either 

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

 charge (see page s l" of the firsl stimulus should 1"' capable of imme- 

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

 impossible to st art another of a different type employing the same motor 

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

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

 tion of the proper afferent nerve. 



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

 innervation is al or about the synapsis. Tn other words, the synapsis at 

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

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

 lateral of the same afferenl fiber will allow an inhibiting impulse to p 

 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 OCCUT, I 

 example, al certain st. •' sf nii-lni' Ing and during the action 



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

 t ion. it is necessary to examine muscles which acl on one joint only, and 



