Intensity Factors in Reflex Control

However, a study of even the simplest of reflexes shows that the existence of an appropriate nerve-net is not the only thing which underlies the process. In the first place, the intensity of the stimulus plays an important part. The reflex is not aroused by all intensities of the stimulus whatsoever, but requires a certain minimal intensity, which is known as the threshold. If the intensity is increased beyond this value, the vigor of the reaction will also be augmented until a limiting condition is reached. This quantitative variation in the reflex may not be considered to be of very great interest when only a single reflex tendency is in operation, but when two or more tendencies are aroused simultaneously and come into conflict, the ratio of their intensities may determine the outcome. In this case, we have to regard the intensities of the stimuli, or of the reflex tendencies themselves, as the determining factors for the resultant response.

In this connection we may note another complicating feature, namely, that the intensity of the reaction is not deducible from that of the stimulus by any simple law such as that of the conservation of energy. The nerve current does not consist merely in the propagation, along the network, of energy or tension which is applied to it by the stimulus. Frequently the energy of the latter is excessively minute, and is only sufficient to pull a trigger which liberates the inherent energies of the nervous apparatus, and these continue the process. Furthermore, the forcefulness of the response activity at any point along the conduction is to a very large extent a function of the latent energies of the nerve mechanism at this particular point. That this is true will be clear from a consideration of the simple fact that muscles exert forces of many pounds, whereas the forces of the nerve current can only be measured by the use of delicate electrical apparatus. Nevertheless, these considerations do not disturb our conception of the response process as a propagation of a disturbance from one point to another along a predetermined path. They mean only that we may be forced to compare the response chain to a telephone system involving a cascade of amplifiers, rather than a simple line of wire.

Anatomical Conjunction in Response Specificity

Let us therefore begin our analysis by an inquiry into the mechanism of a typical "simple reflex." Such reflexes are exhibited in a particularly clear manner in the "preparations" which result from surgical operations upon the nervous system of such animals as the dog or cat. It is readily possible to sever the spinal cord from other portions of the nervous system in these animals and yet to maintain life and functional activity in the lower nerve centers. In such an animal preparation--a spinal dog or cat--the spinal reflexes can be evoked with great regularity, and their properties can be studied quantitatively. If the higher nerve centers are entirely cut off, we are not disturbed by thoughts concerning any influence which might be exerted by the "psyche" of the animal. As an example of processes of this sort, we may consider the "scratch reflex" of the spinal dog, as studied by Sherrington 109 and others. This reflex is set off by a tactual stimulation (tickling or rubbing) of the skin of the dog's back and the reaction consists in a scratching movement of the hind limb which applies the claws to the stimulated point. Although the reaction is fairly reliable, the intensity of stimulus which is required to bring it out varies from time to time.

Before proceeding, however, to consider variations in the sensitiveness, or intensity of the reflex, we may first inquire as to what feature of the organism is responsible for the existence of this specific response under any circumstances. The answer to this question is quite simple and direct. It is that the evocation of the scratching movement, upon stimulation of the skin receptors, is referable primarily to the anatomical conjunction of appropriate afferent and efferent neurones in the spinal cord. This anatomical conjunction operates in the nervous system just as does spatial proximity in any conducting arrangement, such as a network of electrical wires. The nerve currents flow along the paths of least resistance, which are determined by the continuity, or relative continuity of the nerve tracts.

Although we may not be led finally to adopt an anatomical conjunction theory of motivation," nevertheless we must recognize that this relationship is a prerequisite of all specific response and must be regarded as one of its primary determining factors. When we come to consider the mechanism of the cerebral cortex, we shall see how it is possible for anatomical conjunction to become so ubiquitous that it defeats its own ends and makes another principle paramount. However, in the case of the spinal and other reflexes, there can be little doubt that it is an important selective factor in determining the motor result which follows from the given form of stimulation. The dependency of specific action upon proximity of parts is not only a feature of conducting networks, but also of machines in general. In the case of a single neurone, or conducting nerve unit, the continuity of the unit is really nothing but an intimate juxtaposition of its constituent molecules. The nerve units, in turn, are juxtaposed at the synapses, which are in the nature of switches, or contact points. Assuming the continuity of the individual units, their synaptic connections become the crucial determining features. Hence if anatomical conjunction were the whole story, the synaptic diagram would provide us with the entire explanation which we are seeking.

Determining Factors in Reflex Response

Let us continue to adhere strictly to our physical point of view regarding response, and endeavor to arrive at a preliminary understanding of the factors which must determine its specificity. The reader is reminded that the concept of response specificity (as defined by Holt) comprises the identification, or selection of a particular kind of reaction (behavior) as a constant function, or accompaniment of a particular object which acts so as to stimulate the nervous system. Reduced to mathematical form, the specificity is represented by a functional equation, such as R & f (O), where O is the object and R is the effector reaction. Although such response functions can seldom be expressed in quantitative form, their logical nature is not affected by this limitation. The statement: "Johnny always cries when he sees a dog, is a functional formulation of this sort which indicates the specificity of Johnny's response to a dog.