THE CONFLICT BETWEEN EXCITATORY AND INHIBITORY EFFECTS IN A SPINAL CENTER

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Abstract
The balancing of excitatory and inhibitory central effects in the extensor center has been studied by isometric recording of contraction in the knee extensor, vasto-crureus, in the decerebrate cat. The effective strength of the inhibitory stimuli has been controlled by simultaneous recording of action currents in the ipsilateral afferent nerve. Maximal stimulation of the ipsilateral (peroneal or popliteal) nerve causes complete inhibition of the strongest crossed extension reflex. Submaximal stimulation causes complete inhibition of weak crossed extension reflexes, but as the strength of the excitatory stimulation is increased, the amount of inhibition passes through a maximum and then decreases, in this respect resembling the relation found by Creed and Eccles with the crossed inhibition of flexors. Increasing the excitatory background against which the the submaximal inhibitory stimulus operates, not merely adds more motor neurones to the number previously involved, but robs the inhibitory stimulus of the power to inhibit as many motor neurones as it did when acting against the weaker excitatory background. Weak stimulation of the popliteal nerve, which causes inhibition against a weak excitatory background, sometimes, in the presence of a strong excitatory background, gives instead additional excitation. The authors'' observations of the rebound contraction which follows strong inhibitory stimuli, tend to support the interpretation of this phenomenon, proposed by Sherrington and Sowton, as the delayed expression of activity of excitatory fibers in the ipsilateral nerve. The threshold of these fibers is relatively high. Evidence is also adduced for the presence of inhibitory fibers of high threshold in the contralateral sciatic nerve. The balance between excitatory and inhibitory influences is rarely steady. One or the other usually tends to dominate. The intrinsic tendency of the spinal centers to exhibit a spontaneous alternation between extension and flexion, even in the absence of pro-prioceptive impulses, demands some postulate other than the simple grading of opposed excitatory and inhibitory states, acting like chemical substances. Evidence at present known, pointing to the view that central excitatory and inhibitory effects are qualitatively different from those associated with peripheral nerve impulses, is discussed and shown to be compatible with an interpretation based on known functions of peripheral nerve. Sher-rington''s theory, which assumes a "central inhibitory state" of undetermined character, requires fewer subsidiary hypotheses and is simpler in this respect, but must still be regarded as merely a working hypothesis.

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