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.