Hole's Human Anatomy and Physiology   8/e   Shier/Butler/Lewis
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Uses of Reflexes


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, observing reflexes may reveal the location and extent of damage.

Injury to any component of a reflex arc alters its function. For example, a plantar reflex is normally initiated by stroking the sole of the foot, and the usual response is flexion of the foot and toes. However, damage to certain nerve pathways (corticospinal tract) may trigger an abnormal response called the Babinski reflex, which is a dorsiflexion, extending the great toe upward and fanning apart the smaller toes. 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 dorsiflexion. The Babinski reflex is present normally in infants up to the age of twelve months and is thought to reflect immaturity in their corticospinal tracts.

Other reflexes that may be tested during a neurological examination include the following:

Biceps-jerk reflex. Bending a person's arm at the elbow elicits this reflex. 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 flexes at the elbow.

Triceps-jerk reflex. 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 elicits this reflex. The muscle contracts in response, and the forearm extends slightly.

Abdominal reflexes. These reflexes occur when the examiner strokes the skin of the abdomen. For example, a dull pin drawn from the sides of the abdomen upward toward the midline and above the umbilicus causes the abdominal muscles underlying the skin to contract, and the umbilicus moves toward the stimulated region.

Ankle-jerk reflex. Tapping the calcaneal tendon just above its insertion on the calcaneus elicits this reflex. The response is plantar flexion, produced by contraction of the gastrocnemius and soleus muscles.

Cremasteric reflex. This reflex is elicited in males by stroking the upper inside of the thigh. In response, the testis on the same side is elevated by contracting muscles.

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