The normal knee-jerk reflex involves no input to or from the brain.
The normal knee-jerk or, "patellar jerk," reflex is elicited when the knee is tapped below the knee cap (patella). Sensors that detect stretching of the tendon of this area send electrical impulses back to the spinal cord. The spinal cord automatically sends a signal back along a nerve to muscles of the front of the thigh. These muscles are then activated to contract, causing a kick. The brain is never involved in the reflex.
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