Tonic and Phasic Reaction Systems

Let us lay aside for a moment the problem of set, which becomes so very difficult in connection with complex behavior, and return to the functions of the neuromuscular mechanism already discussed. Our previous descriptions of this mechanism have clearly shown that it is capable of reacting in two ways, phasically and tonically. We first met this distinction in our study of the effectors, where tonic muscular contraction was contrasted with phasic contraction. Later it became useful to extend the dichotomy to the action of receptors and adjustors. The proprioceptors were said to supply the background excitations upon which those from the exteroceptors were cast; their effects were noted in connection with the discussion of cortical vigilance. The section on integrated action was replete with reference to both phasic and tonic response patterns. At both spinal and cortical levels, long-maintained tonic adjustments or postures were shown to be as significant as were patterns of momentary phasic response. Whether these terms represent two mutually exclusive types of reaction, or whether they stand at the extremes of a continuous scale has never been adequately determined. We are beginning to realize, however, that they serve quite different functions in behavior. The phasic response is of short duration and usually represents only a temporary adjustment to some momentary and fleeting stimulus. The tonic response represents a more enduring type of adjustment, calculated to maintain a certain continuity in the organism's conduct and helping to sustain phasic reactions.

Voluntary and reflex reactions occur only when the muscles are in a state of slight contraction. Special types of response cannot be made unless the pattern of tonic contraction expresses itself in a given posture. Except for postural patterns, the inception of a voluntary movement would find the part to be moved in an unsuitable position, as is the case in locomotor ataxia. Phasic intraneural sequences are equally dependent for their maintenance and direction upon a steady backflow of afferent impulses from the tonic processes. Just as the taximeter translates distance into cents, so the final product of postural change is measured and new spatial orientation related thereto. Regardless of the particular type of neuromuscular action under investigation, the mutual contributions of the tonic or postural element and the phasic or kinetic element are always apparent.

What, we may ask, is the neural basis for this division of labor? the efferent part of the central nervous system possesses structuro-functional differentiation which extends as far as the highest neural organ. The great pyramidal system dominates phasic reaction, and the extrapyramidal system dominates tonic reaction. We have already discussed the role of such structures as the cerebellum, the tegmentum, the thalamus, and the corpus striatum in the regulation of posture and the distribution of tone. These form an intimate part of the extrapyramidal system. The relation of the great pyramidal tracts to voluntary and reflex movement has also been pointed out. But this is only one side of the picture. If we attach particular importance to the pyramidal decussation in man, the efferent divisions of the nervous system may be assumed to have much more significance than has heretofore been supposed. This decussation as a compensation for the crossing of the optic nerves, places the great pyramidal tracts in especially close relation with one of the most important sources of exteroceptive stimulation. Afferent fibers from the other exteroceptors maintain the same type of crossed relationship and hence form a part of the same system. Afferent fibers from the interoceptors, however, tend to follow a different plan, connecting with the motor apparatus by way of the extrapyramidal tracts. Although the extrapyramidals are not so well charted as are the pyramidals, their representation is mainly uncrossed. Proprioceptive fibers connect with the cerebellum, which is the "head ganglion" of the extrapyramidal system, on their side of origin. Beyond this point there is much crossing and recrossing of pathways as the extrapyramidals develop interconnection with the exteroceptive and pyramidal paths at higher neural levels.

According to the scheme outlined above, there are two rather distinct reaction systems, each with its own receptor, adjustor, and effector units. The phasic reaction system includes afferent fibers from the exteroceptors, crossed to connect primarily with adjustor centers in the cerebral cortex, and efferent fibers therefrom recrossing to connect with the musculature. The tonic or postural reaction system includes afferent fibers from the proprioceptors, continuing on the same side to connect primarily with adjustor centers in the cerebellum, and efferent fibers therefrom returning to connect with the muscles. At first glance there seem to be so many exceptions that this structuro-functional division of the neural apparatus appears entirely artificial. The oriented student will point out that while the majority of exteroceptive fibers cross the midline of the body, the decussation is by no means complete. Furthermore, both proprioceptive and extrapyramidal fibers attain cross connection, particularly at the higher neural levels. All this, however, should be expected. If the pattern of behavior is to be unitary, with each system making its special contribution to the total flux, an intimate relation must obtain between them at all levels of neural integration. This necessitates many auxiliary and complementary fiber associations, and in the cord and brain stem these make it well-nigh impossible to establish the principles of neural organization which distinguish the tonic from the phasic system. It is only in those evaginations of the primitive neural tube which have been developed to meet the special demands of each system--the cerebrum and the cerebellum-that characteristically different structural plans are clearly in evidence.