brain, spinal cord, and nerve disorders
Neurologic Examination
Reflexes
A reflex is an automatic response to a stimulus. For example, the lower leg jerks when the tendon below the kneecap is gently tapped with a small rubber hammer. The pathway that a reflex follows (reflex arc) does not directly involve the brain. The pathway consists of the sensory nerve to the spinal cord, the nerve connections in the spinal cord, and the motor nerves back to the muscle, causing the knee to jerk. Doctors test reflexes to determine whether all parts of this pathway are functioning. The reflexes most commonly tested are the knee jerk and similar reflexes at the elbow and ankle. The plantar reflex may help doctors diagnose abnormalities in the nerve pathways involved in the voluntary control of muscles. It is tested by firmly stroking the outer border of the sole of the foot with a key or other object that causes minor discomfort. Normally, the toes curl downward, except in infants aged 6 months or younger. Having the big toe go upward and the other toes spread out is a sign of an abnormality in the brain or spinal cord. Testing other reflexes can provide important information. For example, doctors learn the extent of injury in a comatose person by noting the following: Whether the pupils constrict when light is shined on them (pupillary light reflex) Whether the eyes blink when the cornea is touched with a wisp of cotton (corneal reflex) How the eyes move when the person’s head is turned or when water is flushed into the ear canal (caloric testing) Whether the person gags when the back of the throat is touched, for example, with a tongue depressor (gag reflex) Doctors also check whether the anus tightens (contracts) when it is lightly touched (called the anal wink). If this reflex is present in a person paralyzed after a spinal cord injury, the injury may be incomplete, and the chance of recovery is better than if the reflex were absent. A reflex arc is the pathway that a nerve reflex, such as the knee jerk reflex, follows. A tap on the knee stimulates sensory receptors, generating a nerve signal. The signal travels along a nerve to the spinal cord. In the spinal cord, the signal is transmitted from the sensory nerve to a motor nerve. The motor nerve sends the signal back to a muscle in the thigh. The muscle contracts, causing the lower leg to jerk upward. The entire reflex occurs without involving the brain. 