Successive Reflex Combinations

Sometimes reflexes which are antagonistic to one another, when simultaneously aroused, can combine harmoniously in succession. Some of the commonest movements, such as walking, involve a rhythmic succession or alternation of antagonistic reflexes: flexion followed by extension, or the like. It has been shown that, in certain cases, the motor process of extension itself arouses the reflex of flexion, or vice versa, so that an orderly sequence results. The notion of a chain of reflexes, so arranged that the motor phase of one member generates the stimulus to the next member, has frequently been advocated as an explanation of complex acts of a seemingly instinctive nature. Reflexes may be combined into more complex response systems either in this manner or because their respective stimuli are simultaneously provided by the environment.

Reflex Prepotency

From our present point of view, the fourth determinant of the issue of a conflict, as listed by Sherrington, is probably the most interesting of all. It consists in the species or kind of reflex which is involved. Certain classes of reflexes exhibit very superior strength when they come into conflict with reflexes belonging to a different class. Among such very potent--or "prepotent"--reflexes are those which are aroused by stimulation of nociceptive sense-organs. These are receptors, or senseorgans which respond specifically to injurious stimuli, their most important exemplars being the receptors of the pain sense. It is natural that the reflexes which are set off by pain stimulation should dominate over those which are aroused by innocuous stimuli, such as mere contact, since an appropriate reaction to the former is of vital importance to the welfare of the organism. The dominance of the flexion over the scratch reflex, which we have already mentioned above, is explained by this principle of the prepotency of nociceptively aroused reactions, since the flexion reflex is a response to a needle prick, whereas the scratch reflex follows from a harmless touch. However, if the pain stimulus is sufficiently weak and the tactile one sufficiently strong, the scratching action may appear, and become dominant over the continuous flexion process. Nociceptive reflexes show a tendency to persist in the face of general disturbances of nervous activity which obliterate many other reflex processes, thus showing the peculiar vitality of the former types of response.

Sherrington points out, furthermore, that any reflex arising from the stimulation of receptors which can produce strongly pleasant or unpleasant (affective) consequences in consciousness, tends to prevail over other types of reflexes in the case of conflict. He calls such reflexes in general "pseudaffective." The sexual reflexes, such as the embracing reaction in the case of the male frog during the breeding season, show a prepotency as great as, or greater than that which is exhibited by nociceptive reflexes. In order to complete our terminology, it seems advisable to invent the adjective, beneceptive, to designate sensory processes and attached reflexes which are of the character of the sexual responses, as contrasted with those of pain. Nociceptors may be defined as sensory cells, or afferent mechanisms, which are particularly attuned to injurious agencies; whereas beneceptors may be considered as similarly qualified with respect to especially beneficial environmental, or organic, conditions. We can then say that reflexes which are aroused by the stimulation of either nociceptors or beneceptors tend, other things being equal, to dominate over the motor tendencies appertaining to other forms of sensory excitation.

All of the factors which control the outcome of conflicts between reflex arcs apparently can be interpreted as different grounds of response intensity, presumably in the adjustor stage of the reflex. This applies not only to the influence of spinal induction, fatigue, and stimulus magnitude, but also to the species of reflexes, since it seems highly probable that the nociceptively and beneceptively aroused adjustor processes have a high degree of nervous power. This is evidenced by the fact that their normal, unimpeded expressions show great strength and persistence. The conflict of response tendencies is therefore fundamentally a quantitative affair, in which the result is dependent upon the comparative magnitudes of the positive and negative forces which are operative. For it seems that each response tendency must be capable of acting both positively and negatively: positively in seeking its own expression, and negatively in opposing the expression of its antagonists.

Factors Deciding Reflex Conflict

The most interesting relationship, from our present point of view, is, however, that which obtains between antagonistic reflexes. The adjustor mechanisms of such reflexes are set to use the same final common path, in different ways, so that it is physically impossible for them to operate simultaneously--at least to any good effect. However, their interference is based upon a nervous rather than a physical incompatibility. The evolution of the nervous mechanism has evidently been such as to guard against the simultaneous arousal of inconsistent reactions, which could lead to no useful result. As an example of such interference we may consider what happens when the scratch reflex and the so-called flexion reflex of the hind limb of the dog are simultaneously excited. One of these reactions demands a steady flexion of the limb, while the other requires alternate extension and flexion, so that the two cannot occur at the same time. As a rule, the scratch reflex is suppressed, and steady flexion becomes dominant when these two mechanisms are simultaneously aroused. This is attributed to one of the fundamental principles which determine the outcome of reflex conflicts, and which we shall consider below.

Four different classes of factors which govern the outcome of a conflict between opposed reflex tendencies. The first of these consists of "spinal induction," of which there are two kinds, immediate and successive. Immediate spinal induction is a process of alliance and reinforcement operating between similar reflexes. In accordance with the principle of this process, the reflexes which are in conflict may be assisted to win by their allies. "In union there is strength." In a complex organism, a contest is usually staged, not between physiologically simple reflexes, but between groups of associated reflex mechanisms. Successive spinal induction is a rather different kind of process, which has the status of an after-effect of inhibition. If a given reflex tendency has been inhibited, it is liable to reappear with augmented strength when the inhibiting force is removed. For example, stimulation of the flexion reflex lowers the threshold of excitation for the extensor muscles, which are inhibited during flexion. This "rebound" action can also be observed quite readily in the more complex forms of human behavior.

Another important determinant of the issue of a conflict between reflexes is found in the factor of fatigue. Continued repetition of a reflex process decreases the facility with which it is aroused, so that a reflex which is in the fatigued state will be more readily overcome by a second reflex agency. Two reflex mechanisms which are unequally fatigued will tend, in a conflict, to be successful in inverse proportion to the degree of their fatigue. The locus of the fatigue is probably in the synapse or adjustor, although in some cases it is sensory. In the situation which we are considering, the fatigue is not localizable in the muscle, or in the motor nerves (final common path). The condition of refractory phase may also act in a manner similar to that which is characteristic of fatigue, although the former condition is of such shorter duration. Lack of oxygen or the presence of drugs may produce analogous effects, depressing or enhancing conflicting reflex forces unequally, and hence affecting the outcome of the struggle.

Sherrington regards the relative intensities of the conflicting reflexes as the most powerful determinants of the issue of a conflict between them. The intensity of the afferent process depends primarily upon that of the stimulus, so that a reflex which is aroused by a weak stimulus will tend to be dominated by an antagonistic one having a stronger stimulus as a cause. However, the sensitiveness of the sensory mechanism, and the size of the afferent nerve, must also be considered. The smaller the number of fibres in the nerve, the lower its power usually is over the effectors. Moreover, afferent currents which are aroused at a considerable distance from the effector, which they seek to control, are at a disadvantage when in conflict with reflexes having a shorter path. Other features can, perhaps, be found which enter into the determination of the intensity of the conflicting adjustor processes, and which may thus have a bearing upon the outcome of the combat.