The Interaction of Reflexes

If the neuromuscular system of an organism contained only a single reflex mechanism, the problem of motivation would be solved for it by an understanding of factors exclusively of the character above discussed. Possibly an organization as simple as this can be demonstrated in certain sensitive plants. In the case of such an organism, having but a single reflex, we should merely need to inquire concerning the presence or absence of the specific stimulus, its intensity and point of application, and the contemporary state of the adjustor mechanism of the reflex. Whenever the reaction was observed to occur, we could explain it by demonstrating the existence of the appropriate object or stimulus. If the reaction should fail in the presence of the object, we should look either for some peripheral factor rendering the latter inoperative, or for some reduction of sensitivity on the part of the adjustor, or other stages of the mechanism.

However, there is, perhaps, no animal organism--even among protozoa, which possess no true nervous system--in which the science of a single reflex would serve to account for all behavior. An organism such as that of any vertebrate, possessing a very large number of reflex mechanisms, will frequently be subjected to stimuli which tend to arouse a number of them simultaneously. In this case two general possibilities appear. First, two or more reflexes may occur together in comparative harmony or, second, they may interfere with one another in such a manner that certain of the stimulated reactions are suppressed. In this case we have a situation closely resembling that which is usually contemplated when the question of motives is raised in connection with human voluntary behavior. The individual is pressed by a number of different forces, and frequently has to make an exclusive choice between them as regards his behavior. However, it sometimes happens that two or more stimuli give rise not merely to reflex tendencies which do not interfere, but which are positively helpful to one another, so that we must speak of allied as well as of opposed and indifferently related reflex mechanisms.

It therefore becomes of great importance to understand the factors which control the combination of reflex tendencies, and, in particular, the outcome of interference between such tendencies. This topic is the one to which Sherrington has made so many contributions in his doctrines of the "integrative action of the nervous system." The alliance or opposition of reflexes is ordinarily attributable to their use of the same motor apparatus, including the efferent neurones. Such neurones constitute, in the Sherrington terminology, the "final common path" of the reflexes in question. When the reflex adjustor mechanisms tend to energize the common path in the same manner, there is a strong probability of alliance. As an example, we may consider what happens when the scratch reflex of the dog is stimulated from two different points on the skin of the back. In this case the reaction is usually stronger than it would be if only one of the points had been affected. This situation as involving two logically separable scratch reflexes, a view which may be justified by the fact that the reaction ordinarily applies the claws to the spot which is stimulated, and, so, is specific with respect to this particular region. The degree to which the two stimuli reinforce each other is in fact proportional to their proximity on the skin. However, it is possible to find widely divergent forms of sensory excitation which are capable of combining in the production of a single reaction. For example, simultaneous stimulation of the skin of the foot, and of the sensory nerve of the so-called ham-string muscle contribute to the same muscular action, which consists in a flexion of the leg. It is even possible to demonstrate a general tendency for sensory excitation at any point whatsoever in the body to reinforce any concomitant motor process, although of course this tendency has many specific exceptions. Inhibitory processes, also, may combine so as to reinforce one another in their repressive effects.

Reflex mechanisms which are not compelled to utilize the same final common path may occur simultaneously without either reinforcing or opposing one another. Thus, wagging of the tail and the scratch reflex may, in the case of the spinal dog, occur together without either aiding or interfering with each other. Similarly, in the normal human organism, constriction of the pupil of the eye bears no appreciable relationship to the reflex of coughing, so that the two may occur simultaneously without interacting.

However, in many cases we find that reflexes which are lacking in final common paths, nevertheless, do actually have a bearing upon one another, as a consequence of their organization into some more complex system of response. For example, the reflex movements of the stomach are inhibited when the "instinct" of fear is aroused, although the motor expressions of fear are not primarily gastric in nature. Moreover, as Sherrington points out, neutrality between reflex actions tends to disappear as the processes in question increase in intensity.