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.

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.