Pressure Tensions of Tissues

Another important group of insistent afferent excitations which bombard the central nervous system are produced by the pressure of substances contained in certain of the hollow viscera. The most obvious of these effects are found in the bladder and the colon. When the bladder becomes distended by its liquid contents, pressure-sensitive receptors in the walls of the viscus discharge impulses until the appropriate sphincters relax and the pressure tension is relieved by micturition. In the colon the defecatory impulses are set up by the pressure of fecal matter. Since both of these tissue conditions can be relieved without reference to the environment, they led to well-defined reactions almost from birth. However, the undifferentiated, uncanalized character of the excitant becomes immediately apparent when the natural mictural and defecatory responses are inhibited and, through the process of learning, their release is associated with definite external situations. The entire mechanism is then kept in a condition of disequilibrium and unrest until the appropriate environment is found for satisfying the tissue need. It may be argued that persistent afferent stimulation from these sources is a secondary consequence rather than a primary condition of tissue needs. But the fact that inhibition of the essential reflexes occurs in animals as well as in man suggests that these conditions fit the general concept of motivation excitant as we have defined it. Cats, dogs, and apes apparently develop a natural inhibition to unrestrained defecation and micturition at a certain age. There is no particular reason to believe that this would not be a maturation effect in man as well. One might even conceive that the spinal centers of defecation and micturition react to a summation of afferent impulses, and that the restlessness aroused by the internal excitants leads the animal to seek an appropriate external situation, the sight of which produces the requisite sum mational effect.

There are probably many other instances of pressure tensions of internal tissues giving rise to persistent excitants of the nervous system. Gas in the stomach and intestinal tract seems to act as a persistent excitant at times. At least, there is general restlessness and random movement until some posture or activity provides relief for the intestinal pain. Patients suffering from other internal ills are often unable to remain quiet. If they are in bed because of their indisposition, they toss and turn, get up and lie down, take one medicine after another, try first this therapeutic application and then that, until finally relief is obtained. What better example could be found of random activity occasioned by internal excitants? That the case is one of pathology does not alter the principle involved. Just as the child who is stuck with a pin twists and turns in an unsuccessful attempt to relieve the pain, so we adults flounder more or less helplessly in an effort to relieve and eliminate many of our internal ills.