Reciprocal Inhibition and Reciprocal Innervation
In the section paragraphs, we pointed out several times that excitation of one group of muscles is often associated with inhibition of another group. For instance, when a stretch reflex excites one muscle, it often simultaneously inhibits the antagonist muscles. This is the phenomenon of reciprocal inhibition, and the neuronal circuit that causes this reciprocal relation is called reciprocal innervation. Likewise, reciprocal relations often exist between the muscles on the two sides of the body, as exemplified by the flexor and extensor muscle reflexes described earlier.
Figure 54–11 shows a typical example of reciprocal inhibition. In this instance, a moderate but prolonged flexor reflex is elicited from one limb of the body; while this reflex is still being elicited, a stronger flexor reflex is elicited in the limb on the opposite side of the body. This stronger reflex sends reciprocal inhibitory signals to the first limb and depresses its degree of flexion. Finally, removal of the stronger reflex allows the original reflex to reassume its previous intensity.