Reflex Test: Knee-jerk Response - What the Test Does

advertisement

Main Digest

The reflex test is simple yet informative and can give important insights into the integrity of the nervous system at many different levels.

What Does Reflex Mean?

A reflex, or reflex action, is an involuntary and nearly instantaneous movement in response to a stimulus. A reflex is made possible by neural pathways called reflex arcs which can act on an impulse before that impulse reaches the brain. The reflex is then an automatic response to a stimulus that does not receive or need conscious thought.

The knee-jerk response is any kind of reaction that is done automatically, without thought, and is one of the fastest and most primitive in the human body: it uses only two nerve cells via the spinal cord, and does not use the brain. However, the brain does preset the strength of the reflex. It only takes about 50 milliseconds between the tap and the start of the leg kick.

The Reflex Test

The knee-jerk reflex is what's known as a mono-synaptic response, because there is only one synapse in the circuit needed to complete the reflex.

Article continues below image.

Continued...

Within each body tendon is a stretch receptor. The stretch receptor can be stimulated by tapping the tendon with a rubber mallet, whereupon the associated muscle contracts slightly. This simple reflex forms the basis of the test performed by doctors during the examination of a patient's central and peripheral nervous system.

The 6 primary locations for testing reflex arcs across the spinal cord:

• The Achilles' tendon tests the first and second sacral nerves (S1, S2).
• 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. (L2, L3, L4)
• The biceps tendon (C5, C6), triceps tendon (C6, C7, C8), and brachioradialis tendon test the cervical nerves along the 5th, 6th and 7th vertebrae (C5, C6, C7). (C1 to S5 vertebra body functions)

Babinski Test

Involves gently stroking the sole of the foot to assess proper development of the spine and cerebral cortex. An adult or older child who responds to the Babinski test with an extended big toe may have a lesion in the spinal cord or cerebral cortex.

The speed and forcefulness of the reflex response varies.

Damage to the brain or spinal cord can produce very brisk reflexes in the affected limb, whilst damage to a peripheral nerve produces diminished or absent reflexes.

Reflex test responses should be the same for both sides of the body. Different responses on the two sides of the body may indicate early onset of progressive disease, or localized nerve damage, as from trauma.

Reflex Test Responses Include:

• Continual jerks after the tap can indicate cerebellar disease.
• If there is no response to the knee tap, it may indicate nerve damage.
• One side of the body reacts slower to a reflex test - If your reflexes are slower on one side it may indicate early onset of progressive disease, or localized nerve damage.

Reflex tests use a typical scale from 0 to 4+.

While 2+ is considered normal, some healthy individuals are hypo-reflexive and register all reflexes at 1+, while others are hyper-reflexive and register all reflexes at 3+.

The vigor of muscle contraction during a reflex test is graded on the following scale:

(i) Walker HK. Deep Tendon Reflexes. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990. Chapter 72.

(ii) Whether the 1+ and 3+ responses are normal depends on what they were previously, that is, the patient's reflex history; what the other reflexes are; and analysis of associated findings such as muscle tone, muscle strength, or other evidence of disease. Asymmetry of reflexes suggests abnormality.

Share This Information To:
𝕏.com Facebook Reddit

advertisement