Reciprocal action and reciprocal innervation of antagonistic muscles

In the foregoing survey of the mechanisms involved in muscular activity, it has been pointed out that both reflex and voluntary responses may become very complex yet are coördinated and may be purposeful and protective in character. Such responses cannot be mediated through a single muscle, but by a nicety of balance between the activities of groups of antagonistic muscles. This balance is the resultant of a reciprocal activity existing between muscles which act upon the levers of a certain joint or series of joints in opposite directions. When one group of muscles is stimulated, the other is thrown into a state of inhibition in order that the action may take place without opposition. Such a condition is found at all joints where flexion and extension, abduction and adduction, and rotation take place. Since this power of coördination does not reside in the muscles, but in the central nervous system, a reciprocal innervation of the opposing muscles in any particular instance is required.

The mode of innervation is such that when one group of similarly acting muscles, as the quadriceps in the extension of the leg, is stimulated, the antagonistic flexor group (hamstrings) is inhibited. When the reverse action occurs (flexion) the hamstrings are stimulated while the quadriceps are inhibited. That this actually occurs has been proved experimentally. The nervous mechanism necessary for such an allied or synergic response is known as reciprocal innervation. The flexors (hamstrings) are supplied by their monomuscular arcs from the fifth lumbar to the second sacral nerves. The two sets of arcs are connected by connector neurones within the cord and the number of nerve fibers involved probably amounts to several thousands. Impulses which arise in the voluntary centers of the cerebrum may act through this local mechanism. Similar relationships are to be observed at many other joints and in the muscles of the eyes.

Inhibitory nerves for skeletal muscles have never been demonstrated and it is generally believed that they do not exist. If this is the case the mechanism for reciprocal innervation must then be looked for in the central nervous system. Motor nerve cells in the spinal cord may be either stimulated or inhibited, that is, their activity may be augmented or diminished. Inhibition may be produced by diminishing or blocking the motor discharge issuing from the nerve centers over the efferent nerves and may affect a few or all of the nerve fibers involved. In any given reflex action, one and the same stimulus will increase the activity of the flexor center and simultaneously inhibit that of the extensor center. Conversely, when the stimulus is such as to excite the extensor center the activity of the flexor center is inhibited. Thus the influence on the two centers for antagonistic groups of muscles are of opposite sign.

Reciprocal innervation is not limited to the muscles of a single part, but may extend to other parts as, for example, when two or more parts are employed in progressive movements, such as walking. These mechanisms become more complex. From the same diagram, it may be seen that when the extensors of the supporting limb are excited and enter into contraction those of the opposite limb (progressing) must be inhibited, that is, their activity is of opposite sign. Exactly the opposite order of events is occurring in the flexors, namely, they are inhibited in the supporting and excited in the progressing limbs.