STUDIES IN THE PHYSIOLOGY OF THE NERVOUS SYSTEM. XXVIII.: ABSENCE OF ALGEBRAIC EQUALITY BETWEEN THE MAGNITUDES OF CENTRAL EXCITATION AND EFFECTIVE CENTRAL INHIBITION GIVEN IN THE REFLEX CENTRE OF A SINGLE LIMB BY THE SAME REFLEX STIMULUS

¹ The Physiology Institute, Cardiff

I. Two reflex stimuli which evoke antagonistic reflexes in thle same pair of antagonistic muscles yield a compound reflex when applied synchronously. At certain intensities of the two stimuli both of the antagonistic muscles contract more in the compound reflex than they were contracted in the state preceding all stimulation.

2. Each reflex stimulus excites one half-centre (e.g. flexor), and tends to inhibit anv discharge present in the antagonistic half-centre (e.g. extensor). The magnitude of the inhibition which actually falls upon any discharging neurones in the inhibited half-centre may be termed the "effective inhibition" conditioned by the stimulus in question.

3. If excitation is measured in terms of number of synchronous nerveimpulses by which the discharge of a half-centre is increased, and effective inhibition in terms of the number of synchronous nerve-impulses suppressed in the discharge of the antagonistic half-centre, a direct deduction may be drawn from the observation in paragraph 1. At certain intensities of stimulation, and in the case of at least one of two antagonistic stimuli, the effective inhibition conditioned by it in one half-centre is less than the excitation conditioned by it in the antagonistic half-centre. This means that, in the case (for example) of an extension-reflex, the inhibition of the previously discharging flexor motor neurones is less than the excitation of the extensor motor neurones.

4. The magnitude of inhibition which falls upon any neurones in a half-centre (whether they be discharging or not) may be termed the "total inhibition" conditioned by a reflex stimulus.

5. If total inhibition is so distributed in a half-centre that it first falls only upon discharging neurones (and then upon non-discharging neurones only when all the discharging neurones have been inhibited), the above deduction applies to total inhibition as well as to effective inhibition. But this cannot at present be assumed, for there is no proof that inhibition is so distributed.

6. If total inhibition is so distributed that it falls proportionally upon discharging and non-discharging neurones in a half-centre, then some of the inhibition may be spent upon non-discharging neurones. In this case the effective and the total inhibition in a half-centre will be of different magnitudes (if the inhibition and excitation in it are submaximal), and inequality between the excitation (in one half-centre) and the effective inhibition (in the other) given by a single reflex stimulus will not necessarily imply inequality between the excitation and total inhibition given by the same stimulus. That problem remains at issue.