a reflex mechanism that coordinates motor activity and aids in the coordination of antagonistic muscles: examples are the simultaneous contraction of a joint’s flexors and the relaxation of its extensors. In reciprocal innervation, reflex excitation in a group of nerve cells that innervate particular muscles is accompanied by reciprocal inhibition of activity in other cells functionally related to the antagonists, a process causing the antagonists to relax. Thus, the centers of antagonistic muscles—flexors and extensors—act in opposition during the performance of many motor acts. The mechanism of reciprocal innervation also enables the organism to make coordinated movements, such as those involved in walking, scratching, moving the eyes, and working.

Reciprocal innervation was discovered in 1876 by P. A. Spiro, a student of I. M. Sechenov, and studied in detail by the English physiologist C. Sherrington, who also introduced the term itself. N. E. Vvedenskii and A. A. Ukhtomskii demonstrated that reciprocal innervation is not a rigidly fixed mechanism but a dynamic one: muscles that are antagonistic when making certain movements simultaneously contract when making others, that is, they are synergistic.

The study of excitation and inhibition in solitary nerve cells using microelectrodes began in the 1950’s. This research elucidated the mechanism of reciprocal innervation at the cellular level. Interneurons, which function in the nervous system as relay switches and integrating elements, play a major role in the development of reciprocal relations between the motor neurons that innervate antagonistic muscles.