The nerves of your body are a group of specialized fibers that are bundled together like the strands of a rope. These nerve fibers run throughout your body to carry sensory impulses from your body to your brain and motor impulses from your brain to muscles and other organs. According to Structure and Function of the Body, the one trillion routes of nerves in your body are responsible for providing the rapid communication necessary to maintain life.
According to the Mayfield Clinic, your nervous system is divided into the central nervous system, which includes your brain and spinal cord, and the peripheral nervous system, which is comprised of tiny branches of nerve bundles called spinal roots. These bundles branch off the spinal cord at each junction of the vertebrae in your spine. There are seven nerve bundles that branch through the cervical vertebrae, 12 that branch through the thoracic vertebrae, five that branch through the lumbar vertebrae, and five that branch through your sacrum.
The muscles that you use for movement are connected to your bones by thick fibers called tendons. When a tendon is stimulated by a reflex hammer, stretch receptors in the muscle send a signal through the sensory nerve of the spinal branch to the spinal cord. A “reflex” contraction signal is then sent from the spinal cord through a motor nerve, causing the expected jerking of the extremity being tested.
There are six primary locations for testing the reflex arcs all across the spinal cord. The biceps tendon, triceps tendon and brachioradialis tendon test the cervical nerves along the fifth, sixth and seventh vertebrae. The abdomen tests the nerves along the eighth through twelfth thoracic vertebrae. The patellar tendon tests the nerves along the second through fourth lumbar vertebrae, and the Achilles’ tendon tests the first and second sacral nerves.
According to Neuroexam, deep tendon reflexes are tested using a reflex hammer to stretch the muscle and tendon. To test the biceps reflex, place your finger on the biceps tendon in the fold of the elbow. Strike your finger and feel for the contraction inward. Test the triceps reflex by gently tapping the triceps tendon above the elbow and feel for the arm flexing outward. The brachioradialis is tested by striking the radius about 1 to 2 inches above the wrist while the forearm is relaxed on a table or lap. The abdominal reflex is tested by gently stroking the abdomen with a blunt object starting at the umbilicus and moving outward across the abdomen. The stomach muscles should contract in the direction of the stroke. The patellar tendon reflex is tested by tapping the area just below the knee cap while sitting down with your feet dangling. You should notice a knee-jerk, or flexion of the lower leg. The Achilles tendon reflex is tested by flexing the foot toward the knee and gently tapping the Achilles tendon. A short flexion of the foot should be felt.
There are three possible responses to a deep tendon reflex test. An absent or diminished response is called hyporeflexia, and an over reactive response is called hyperreflexia. A normal reflex response is merely considered normal. The results of reflex tests gives health care providers a clue to the location of possible damage within the nervous system. According to the National Institute for Health, hyporeflexia represents a problem within the reflex arc itself, and hyperreflexia indicates a lesion in the spine above the level of the reflex.
According to Orthopedic Physical Assessment, hyporeflexia or absence of reflexes is caused by damage to the peripheral nerve tissue due to trauma, impingement or entrapment, and hyperreflexia is caused by damage to the motor nerve fibers in the brain or spinal cord referred to as the upper motor neuron.
Caution should be used when performing reflex testing in the presence of underlying conditions, such as Multiple Sclerosis (MS) or any type of muscular atrophy, as these will give false response results.