Since normal reflexes depend on normal neuron functions, reflexes are commonly used to obtain information concerning the condition of the nervous system.
An anesthesiologist, for instance, may try to initiate a reflex in a patient who is being anesthetized in order to determine how the anesthetic drug is affecting nerve functions. Also, in the case of injury to some part of the nervous system, various reflexes may be tested to discover the location and extent of the damage.
If any portion of a reflex arc is injured, the normal characteristics of that arc are likely to be altered. For example, a plantar reflex is normally initiated by stroking the sole of the foot, and the usual response includes a flexion of the foot and toes.
However, in persons who have suffered damage to certain nerve pathways (corticospinal tract) there may be an abnormal response called the Babinski reflex.
In this case the reflex response is plantar extension, in which the great toe extends upward and the smaller toes fan apart. If the injury is minor, the response may consist of plantar flexion with failure of the great toe to flex, or plantar flexion followed by plantar extension.
The Babinski reflex is, however, present normally in infants up to the age of 12 months and is thought to reflect a degree of immaturity in their corticospinal tracts.
Other reflexes that may be tested during a neurological examination include the following:
Biceps-jerk reflex
This reflex can be elicited by bending a person's arm at the elbow. The examiner's finger is placed on the inside of the bent elbow over the tendon of" the biceps muscle, and the finger is tapped. The biceps contracts in response, and the forearm is flexed at the elbow.
Triceps-jerk reflex
This reflex can be caused by flexing a person's arm at the elbow and tapping the short tendon of the triceps muscle close to its insertion near the tip of the elbow. The muscle contracts in response, and the forearm is extended slightly.
Abdominal reflexes
These reflexes occur when the examiner strokes the skin of the abdomen. For example, a dull pin may be drawn from the sides of the abdomen upward toward the midline and above the umbilicus. Normally, the abdominal muscles underlying the skin contract in response, and the umbilicus is moved toward the region that was stimulated.
Ankle-jerk reflex
This reflex is elicited by tapping the Achilles tendon just above its insertion on the calcaneus. The response is plantar flexion, produced by contraction of the gastrocnemius and soleus muscles.
Cremasteric reflex
This reflex is obtained in males by stroking the upper inside of the thigh. As a result, the testis on the same side is elevated by contracting muscles.
Anal reflex
This reflex is elicited by stroking the skin surrounding the anus. The anal sphincter muscles contract in response.
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