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.