
On Reciprocal I nnervation of Antagonistic Muscles. 161 

directly or indirectly induces in the extension arcs a super-excitability as 
tested by crossed extension just as when tested by the extensor thrust. 
But although this after-effect of the activity of the flexion arcs upon the 
antagonistic arcs, both direct and crossed, is one of increase of activity, 
the primary effect is, as shown previously, one of depression. Thus the 
crossed extension reflex is temporarily inhibited by the flexion reflex; the 
extensor thrust becomes inelicitable directly a pronounced flexion reflex sets 
in. Indeed, these mutually antagonistic reflexes are, like other similarly 
opposed ones, coupled together in such a way that provocation of the active 
state of discharge of one reflex checks the active state of discharge of 
the other reflex. It, therefore, becomes obvious from this, and other 
evidence given in the previous notes, that in such instances the spinal 
mechanism, temporarily depressed (inhibited), later enters into a_ state 
of exaltation manifested by enhanced tendency to active discharge, and by 
capacity to discharge with greater intensity than before. A like sequence 
is well seen when, using the knee jerk as index, the effect on the knee- 
extensors of stimulation (mechanical, kneading, etc.) of the antagonistic 
hamstring muscles freed from their attachments is observed. The depression 
(inhibition) of the antagonists is pronounced, but on discontinuing the 
stimulation the knee jerk regains not merely its previous briskness, but 
becomes temporarily more brisk and ample than before. The same is 
seen when the stimulation is direct faradisation of the afferent nerves 
from the hamstrings, or when, instead of using the “knee jerk” as an 
index to the tonus, the heightened reflex rigidity of the extensors in 
decerebrate rigidity is directly observed. In all these instances, and in 
others that can. be given in a fuller communication than the present, there 
supervenes on the spinal inhibition a rebound effect of augmentation.* 
This intraspinal rebound effect becomes especially evident on cessation of 
a stimulus of inhibitory character of prolonged application. The rebound 
can be experimentally shown to ensue even during the actual application 
of the stimulus that initially caused inhibition if the application of that be 
long continued. The exaltation after-effect may ensue with such intensity 
that simple discontinuance of the stimulus maintaining the one reflex is 
immediately followed by “spontaneous” appearance of the antagonistic reflex. 
These phenomena are well shown by the opposed reflexes at the knee of 
the dog. 
Thus, in the spinal arcs inhibited there supervenes on the state of 
inhibition a phase of super-excitability. In this after-effect central inhibition 
* Sherrington, Schafer’s ‘ Text-book of Physiology,’ vol. 2, p. 841, 1900. 
