Qualitative Factors in Reflex Control: Chronaxy

Nevertheless, there are still further complications which must be introduced even in the case of a so-called simple reflex. Anatomical conjunction is not the only feature which determines the exact path which is taken by the response currents. Even in the stimulus stage of the process, we can demonstrate the dependency of the path upon the intensity or quality of the stimulating force. If we act upon the skin of the dog's back with forces different from those of rubbing or tickling, we are liable to bring out reactions which differ from scratching, since these different forces may be picked up by other kinds of receptors, and may not excite those which are connected with the scratch mechanism. If we subject the organism to the action of light, we shall naturally arouse only those reactions which can be set off through the optically sensitive organs, the eyes, regardless of the fact that the light may be incident uniformly upon the entire bodily surface. In the same way, a low degree of heat will evoke only reflexes which are associated with the heat-sensitive receptors. A high degree of heat, however, may involve the response of pain nerves, which are also aroused by other stimuli which threaten the skin with injury. Thus there is a filterinq action of the receptors with reference to different kinds of stimuli, at the very outset of the response process, so that the exact character of the reaction will depend not only upon the anatomical point of incidence of the stimulus but also upon its qualitative (and quantitative) nature.

This principle of the determination of the exact response path through filtration, resonance, or some similar action is of fairly obvious application at the stimulus stage, since the differences which are involved can usually be incorporated in our definition of the object,--of which we regard the reaction as a function. There is, however, plenty of evidence that similar principles are operative in subsequent stages of the response where their exact nature is not so apparent. It is necessary to have recourse to these principles to explain certain directional features of conduction which do not seem to be completely accounted for by anatomical conjunction. For example, in the case of the scratch reflex, we find that there is a general innervation of the class of muscles known as flexors, regardless of whether they are attached to the ankle, the knee, or the hip of scratching limb. In the case of a different reflex, which is known as the extensor thrust, another class of muscles, the extensors, is involved. It is likely that this selection of particular classes of motor apparatus is not wholly due to the more anatomical conjunction of the corresponding neurones, but depends also upon the special character of the afferent nerve currents, which spread semi-diffusely through the spinal cord and arouse only those output mechanisms which are particularly sensitized, or resonant to the given afferent currents. If we endeavor to picture more exactly the mechanism of this process, we may be tempted to utilize Hartley's notion of resonance in nerve centers, particularly since it has been shown experimentally that the nerve current is pulsatory, or intermittent in character, somewhat resembling an alternating electrical current (including radio frequency disturbances). Accordingly, it might seem that we have only to suppose that nerve centers, synapses, or points at which outgoing currents are set up, can be tuned to certain frequencies of nerve vibration, just as we tune a radio set. A selective radio receiver can be subjected simultaneously to a vast number of different waves, coming from many broadcasting stations, but will respond only to the one with which it is in resonance. Another radio receiver in exactly the same environment, but differently tuned, will respond to quite a different wave. Mechanical devices analogous to muscles, might readily be activated by the response of such radio receivers.

However, the nervous current is not exactly comparable to an alternating electrical current, and the principle of resonance cannot be applied to it in the same form which is applicable to alternating or radio currents. Nevertheless, a modified principle, that of syntony--as defined by Lapicque--can be utilized. According to this view, each neurone or muscle exhibits a natural temporal course, or speed of process which Lapicque calls chronaxy. If stimuli are applied to the unit at a rate which corresponds to this chronaxy, the unit is aroused much more readily than would be the case if the rate, or temporal character, were different. A muscle fibre and its attached motor neurones are said by Lapicque to have the same chronaxy; and an afferent nerve of similar chronaxy would more readily arouse this syntonized motor unit than would some other afferent nerve having a different chronaxy.

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.