Reciprocal innervation of antagonistic muscles. Fourteenth note. - On double reciprocal innervation

Abstract

I. Reflex excitation and inhibition when brought to play simultaneously on the motoneurones of an extensor muscle can be so balanced that there results in the muscle a contraction , the degree of which evidences algebraic summation is obtainable not only in the decerebrate animal but also in the purely spinal, for instance, after decapitaion. This circumstance much facilitates the physiological study of the phenomenon. In other words, the grading (fig.1) of reflex contraction of the extensor by varying intensity of inhibition acting along with reflex excitation can be studied in the animal freshly made spinal as well as in the animal in the decerebrate condition. The muscle which I have chiefly employed in the purely spinal preparation is the isolated extensor of the knee in the decapitated cat. A difference does indeed exist between the reactions of the preparation in the decapitated and in the decerebrate condition. In the decapitated preparation, as in the decerebrate, the reflex effect of any inhibitory afferent is easily seen if stimulation of that afferent is employed concurrently with stimulation of an excitatory afferent. If however the inhibitory afferent is stimulated during the ordinary resting condition of the preparation, there is usually no change in the muscle to show that the inhibitory afferent is producing any effect at all (fig. 2). This is because the extensor muscle in the decapitated preparation is not exhibiting tonus and lies relaxed, therefore affording no background of contraction against which an inhibitory reflex can reveal itself by causing relaxation. Even in this condition it can, however, easily be shown that the inhibitory afferent is then really producing a state of inhibition in the preparation, although that state is not made evident by any further relaxation. If a stimulus sufficient to cause reflex contraction of the muscle be applied to the excitatory afferent while the inhibitory afferent, although apparently without effect, is being stimulated, the excitatory afferent is found to be ineffective (or only partially effective) then; but it, immediately becomes effective (or more effective) on withdrawal of the concurrent inhibitory stimulation (fig. 2).