A reflex is a muscle contraction or secretion resulting from nerve stimulation by a pathway from a stimulus via the nervous system to the effector organ without the interference of volition. Basic primitive reflexes are stylized responses to stress of protective value to an infant. Stretch or tendon reflexes (e.g. knee-jerk) are muscle contractions in response to sudden stretching of their tendons. Conditioned reflexes are more complex responses that follow any stimulus which has been repeatedly linked with a stimulus of normal functional significance. See also reflex arc.