Quoted by T. Quick, 'Disciplining Physiological Psychology: Cinematographs as Epistemic Devices, 1897-1922', Science in Context 30 (4), pp. 423-474.
Description:

'McDougall's explanation of reflex action was heavily indebted to Sherrington's studies of nervous action. Moreover, it identified the critical variable in the alteration of nervous response in functionally significant 'gaps' between cells. Though previous studies had characterized inhibition as a phenomenon inherent to cellular physiology (for example in Hering's appeal to contradicting physiological forces within cells) such conclusions were not universally accepted . In opposition to such claims, McDougall ascribed the inhibition of all nervous processes to the predomination of others. Inhibition was, he suggested, 'always the result of the setting in of some other mental process' (McDougall 1903, 155-160, 169). There were not two abstract and opposing forces working within cells: rather, inhibition occurred at point of diminution in the number of nervous paths. Thus, given two 'antagonistic' nerve arcs entering a synaptic gap, the manifestation of one of these would prevent the other from gaining expression. Similarly, when an initially-predominating reflex diminished in strength, the nerve arc that had not yet found expression would take over (McDougall 1903, 174-176).'

 

Relevant passages from McDougall:

 

'Both Hering and Verworn tell us very explicitly that the two processes, assimilation and dissimilation, go on simultaneously in the same cell or mass-unit of living substance, and that neither one diminishes or in any way prevents the other, but that either one merely tends indirectly to accelerate the other. Yet simultaneously-occurring assimilation is supposed to prevent in some way the vital manifestations which in its absence result from dissimilation. The result of dissimilation is admittedly the transformation of the potential chemical energy of the loosely-linked atom-groups of the biogen into living energy, heat and molar motion in the case of muscle. We are asked, then, to believe that assimilation prevents the appearance of this living energy. But we know that it does not and cannot do so, for we know that the total result of a cycle of metabolism, involving equal amounts of assimilation and dissimilation, is the conversion of a definite quantity of the chemical energy of the food into living energy, and that this is the purpose of the whole process.' (155-156)

 

'Dr. Gaskell has put forward, independently of Hering and Wundt, an anabolic theory of inhibition. The main points are presented in his article on "The Contraction of Cardiac Muscle," in Prof. Schafer's " Text-book of Physiology." [note: 'Vol. ii., London, 1900.']... Gaskell supposes increased anabolism to inhibit the vital manifestations, the effects of katabolism, only by checking the rate of katabolism... That such direct checking of katabolism is the essential nature of inhibition in both muscle and nerve seems to be the view somewhat vaguely held by many, perhaps most, physiologists.' (158-160)

'It appears, in fact, that the inhibition of a mental process is always the result of the setting in of some other mental process, and, if we consider the underlying physiological processes, we see that this means that the inhibition of the excitation of one neural system is always the result of the excitement of some other system, that inhibition appears always as the negative or complementary result of a process of increased excitation in some other part. This fact suggests that inhibition is essentially the result of a process of competition, and many psychologists have given expression to this conception in some such vague phrase as : The mind has only a limited quantity of energy, which will not suffice for the simultaneous maintenance of two mental processes.' (169)

 

 'Of all spinal inhibitions the reciprocal inhibition of antagonistic muscle-groups is the best-defined class and may serve as the type. Sherrington has shown that this relation obtains between numerous pairs of antagonistic muscle-groups, and it seems probable that it obtains between most, if not all, such pairs.

It is easy to suggest a scheme of the connections between two such arcs that will explain how the inhibition of the less-intensely stimulated arc by the more-intensely stimulated arc may be the result of a process of competition. Let us imagine each arc in simple schematic form as a chain of three neurones, [note: It is asserted that some branches of afferent neurones of the cord make direct connection with motor neurones, while others do so only indirectly by means of an intercalated neurone. See Barker, "The Nervous System," p. 952, &c.'] afferent (a1), central (a2), efferent (a3), and let us call them a1, a2, and a3, and b1, b2, and b3, in the two arcs respectively. In the resting state each arc maintains a gentle tonic contraction of its muscle-group, its afferent end being constantly subject to gentle stimulation partly by the muscles themselves, partly, but to a less extent, by stimuli affecting the skin. [note: 'Sherrington, op. cit.,p. 870.'] The synapses of either arc present, in the resting state, a certain degree of resistance determined by the past history of each synapse in the individual and in the species, a resistance that is diminished by the charging of the neurones between which it mediates, and is increased by the fatigue that results from the continued transmission of the impulse. The neurin generated in either arc we may conceive as escaping into the motor neurone through the resistant synapses of the resting arc by leakage. [note: 'I have displayed in some detail in a former paper the evidence that justifies the conceptions introduced in this scheme. BRAIN, Winter Number, 1901.']

When a strong stimulus is applied to the afferent neurone of arc a it generates neurin rapidly, so that it becomes very rapidly charged, and the resistance of synapse a1-a2 lowered, until a series of discharges takes place from a1 to a2, and again from a2 to a3. The problem then is to imagine such a mode of connection between arc a and arc b as will cause the arc a during stimulation to drain off from the afferent and central neurones of arc b the smaller quantities of neurin generated in them. Several forms of such a connection may be imagined, but I think that probably it takes the form of a collateral fibre coming from neurone b2, and taking part with the axon of a2 in forming a synapse with a3. It may be that this synapse takes the form of a basket applied to the body of the motor neurone. [note: 'Our knowledge of the basket-like synapses is still very imperfect. Held, after stating that they have been observed in numbers both in the cord and brain, writes : " Entweder eine hauptsächliche Nervenfaser oder mehrere und zahlreiche sogar können zu dieser korbartigen Hülle der Ganglienzellen zusammenkommen, die somit als eine von Nervenfaser-verzweigungen gebildete reizumleitende Einrichtung aufgefasst werden kann."—H. Held, "Ueber den Bau der grauen und weissen Substanz," Archiv. f. Anat. u. Phys. Anat. Abth., 1902.'] But whatever the exact constitution of this synapse may be, we may assume that, when its resistance is lowered by the stimulation of a1, and consequent charging of a2, the collateral of b2, making connection with a3 through this synapse, becomes the path of least resistance for the escape of neurin from b1 and b2. These neurones are therefore drained by a3, while b3 ceases to receive any neurin from b2, and the tone of the muscle-group supplied by it is abolished.

In a similar way, if a collateral from a2 joins with the axon of b2 in forming a synapse with a3, a strong stimulus applied to b1 will cause charging and discharging of the arc b, with lowering of resistance of synapse b2-b3, and consequent drainage of a2 into b3, i.e., inhibition of tonus of muscle-group supplied by a3. In a similar way, if both a1 and b1 be stimulated, but one more strongly than the other, the more-strongly stimulated arc will drain the afferent and central neurones of the less-strongly stimulated arc, because the resistance of synapses of the former will be reduced to a lower level than that of the synapses of the latter.' (174-176)