Externally visible expression of a disorder of the nervous system. A reflex is a muscular contraction caused by a certain stimulus. Absence of a reflex or even a particularly strong reflex can provide information about neurological conditions or disorders of the spinal cord and brain. Reflexes used to test the nervous system include the knee tendon reflex, pupil reflex (narrowing of the pupil when a light is shone in the eye) or foot reflex (crooking of the toes when the sole of the foot is tickled). A reflex abnormality can appear on both sides of the body, or just on one. A certain stimulus (such as striking the tendon below the kneecap) directs a signal to the spinal cord by means of a sensory nerve; from there the signal is passed to the nerve which finally stimulates the muscle concerned to move. This so-called reflex pathway is limited by the brain to prevent overactivity of certain muscles. If this moderating influence fails – as a result of a brain disorder, for example – then the reflex is unduly strong, often associated with spastic paralysis. Low reflex activity is generally caused by a break in the reflex pathway itself. This indicates a disorder of one or more sensory nerves, motor nerves (which stimulate the muscles), the spinal column or the muscles themselves. An example is extensive neuropathy, slack paralysis or a muscular disorder. The nature and extent of reflex abnormalities are most important in the diagnosis of neurological conditions.