When one voluntarily flexes the arm at the elbow, two things occur : first, there is sent to the biceps muscle an excitatory impulse from the motor cortex; second, there is sent simultaneously to the triceps muscle an inhibitory impulse. This association of an excitatory stimulus to a muscle or group of muscles with a simultaneous inhibitory stimulus to the antagonistic muscle or group was first discovered by Sherrington in 1893.1 He found that in the case of the flexors and extensors of the limbs excitation of one group was always accompanied by relaxation of the other, and the occurence of this phenomenon was sufficiently widespread to warrant the annunciation of a law, which he called the law of reciprocal innervation. This states that during active contraction of a muscle there occurs a simultaneous relaxation in its antagonist. "As contraction progresses in a muscle, for example an extensor, contractile
ADLER FH. RECIPROCAL INNERVATION OF THE EXTRAOCULAR MUSCLES. Arch Ophthalmol. 1930;3(3):318–324. doi:10.1001/archopht.1930.00810050070006
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