The mutual influence of phasic and tonic systems

Let us now consider the mutual influence of phasic and tonic systems in specific patterns of response. Beginning at the lowest level of neural integration, we find evidence that postural adjustment antedates and sustains phasic adjustment. When the skin on the flank of the spinal dog is stimulated, exteroceptive impulses flow into the spinal cord and produce a scratching response. However, the motor units which respond to these impulses are already under the influence of proprioceptive excitants emanating from the associated muscles. The exteroceptive impulses produce their effects upon this background and in accordance with it. Since they are stronger and more nearly simultaneous than the proprioceptive excitants, they may arouse the motoneurone field to a momentary full discharge-but not without the aid of the proprioceptors. If a muscle is deafferented, so that it loses its tone, formerly effective exteroceptive impulses cause little or no response. The situation is very much like that of a rubber band, which, when held slightly taut, will respond vigorously to any pressure which may be applied to it, but which, when held without stretching, offers little or no resistance to the same pressure.

The proprioceptive impulses do more than merely sustain any exteroceptive impulses which find their way to motor units under their dominance. They exercise a certain restraint and determination over the effectiveness of the exteroceptive impulses as well. Thus, if the position of the limb of our spinal dog were such that the muscles normally giving the scratch response were dominated by inhibitory rather than by excitatory proprioceptive impulses, that response could not be evoked. This is the situation which is present when the same external stimulus gives first flexion and then extension.

Reactions integrated at the level of the medulla oblongata present another instance of the dependence of phasic upon tonic responses. The heart beat and respiration are regulated primarily by the tonic system. Exteroceptive impulses influence these activities only within prescribed limits. A dash of cold water will cause the breath to be held only momentarily. Phasic thalamic responses to sudden pain vary according to the general neuromuscular state of the organism. If the pain is expected, the exteroceptive excitants meet with a certain set of the neural apparatus, into which they must fit. If the pain is unexpected, the typical disoriented "startle" response is made. Cortically integrated behavior shows the same directive influence of proprioceptive excitants, albeit often in a somewhat modified and a more subtle form. Voluntary finger oscillations begin as a pattern of exaggerated cocontraction in the antagonistic muscles. This warming up stage is essentially a function of the tonic system. Tonic impulses are probably equally important in sustaining and determining the efficacy of the temporary patterns of sensory fusion and redintegration.