By experimenting with reflexes in cats, Sherrington discovered that motor neurons receive and respond selectively to one of two very different signals: excitatory signals and inhibitory signals. Excitatory signals trigger the action of the motor neurons that initiate extension of a limb, for example, while inhibitory signals tell the motor neurons that control flexion, the opposing movement, to relax. Thus, even a simple knee-jerk reflex requires two simultaneous and opposite commands: the muscles that extend the knee must be excited, while the opposing muscles that flex the knee must be
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