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.

The Functions of Various Nerve Centers

In order to understand the functions of the cerebrum, it is necessary, however, to have a clear knowledge of the general nature of the lower nerve centers. The spinal cord, as the lowest of these centers, contains the essential junction points for a large number of bodily reflexes which can occur quite perfectly without any participation by higher regulative centers, although they may be subject to interference or reinforcement through the action of the latter. The cord also acts as a conduit through which impulses are conducted from the body surface and internal organs to the brain. The functions of the medulla oblongata, which connects the spinal cord to the brain, are similar to those of the cord, but involve reflexes of the head-end of the body and the regulation of the more vital processes, such as those of circulation and respiration. The cerebellum, which is a portion of the brain adjacent to the medulla, receives afferent nerve impulses primarily from sense-organs located in the motor apparatus (muscles, tendons and joint-surfaces) and from the equilibrium sense mechanism of the inner ears. The efferent impulses which leave the cerebellum pass to all portions of the voluntary musculature, and are concerned in the automatic maintenance of tension and coördination between the various muscular units. The cerebellum appears to be a device for adjusting the details of motor innervation, usually under the guidance of the cerebral cortex. It is possible that it is endowed by heredity with a stock of "records," which enable it to produce and reproduce specific types of motor reaction when circumstances demand them, although these reaction forms are not linked with any definite stimuli. As examples of such reaction forms, we may consider some of the items on James' list of simple instincts: "sucking, biting, chew ing, licking, grimacing, etc." However, the prime duty of the cerebellum seems to consist in the maintenance of tonus and balance throughout the voluntary musculature.

In the mid-brain, which is enveloped by the mass of the cerebrum, we find further regions of reflex transfer, which, however, are usually more complex and variable in their action than is the case with the centers of the spinal cord or medulla. The thalamus, which is an important portion of the fore-brain, forms a kind of vestibule to the cerebral cortex, since practically all of the sensory nerve currents which are destined for the cortex, pass through the thalamus. Here, also, is found a synaptic center for all of the pain nerves of the body, and many of the mimetic expressions of instinct or emotion are probably controlled directly from centers in this general region of the brain. There is a very definite interaction between the thalamus and the cerebral cortex in regard to pain impulses, and possibly also with reference to impulses which give rise to pleasure. Concerning this relationship we shall have a great deal more to say in later chapters.

The cerebrum is by far the largest portion of the brain in the human being although it is practically absent in many lower vertebrates. It consists of a very intricate network of conducting fibres, which have myriads of junction points, located for the most part in the surface of the organ, a large part of which is adjacent to the bony case of the skull. The convolutions and fissures in this surface appear to have the function of increasing its area to a maximum. The cerebrum is divided, right and left, into two halves known as the cerebral hemispheres. The right hemisphere is connected almost exclusively with the left side of the body, while the left hemisphere deals with the affairs of the right side.

The cerebral cortex receives a very large number of nerve fibres from all of the sensory surfaces of the body and also gives rise to fibres which pass to all of the skeletal, or so-called voluntary muscles. These fibres are segregated and distributed to special zones, known as projection areas. Thus, we have surfaces in the cortex which are exclusively for visual, for auditory, for olfactory, for tactual, for motor impulses, and so on. The motor area is devoted to the transmission of impulses along the pyramidal neurones. These sensory and motor projection areas by no means exhaust the entire surface of the cortex, and it is natural to suppose that the remaining and intervening parts will be employed for purposes of association between the sensory and motor zones. This supposition has already been corroborated to a convincing extent by empirical observation. It is evidently in the association areas of the cortex that we should look for the principal basis of specificity in voluntary behavior.