Understanding the Knee-jerk Response Reflex Test

Ian C. Langtree - Writer/Editor for Disabled World (DW)
Published: 2017/06/04 - Updated: 2024/09/22
Publication Type: Instructive / Helpful
Topic: Medical Research and News - Publications List

Page Content: Synopsis - Introduction - Main

Synopsis: Reflex tests help assess the integrity of nerve circuits and are performed to confirm the integrity of the spinal cord, or specific nerve root function.

Why it matters: The information on reflex testing explains a critical diagnostic tool used by medical professionals to assess neurological function. Reflex tests, particularly the knee-jerk response, offer valuable insights into the integrity of the nervous system, including the spinal cord and specific nerve roots. These simple yet informative tests can help identify potential neurological disorders, nerve damage, or spinal cord issues by evaluating the speed, strength, and symmetry of reflexes. Understanding the grading scale and interpretation of reflex responses enables healthcare providers to detect early signs of progressive diseases or localized nerve damage, making reflex testing an essential component of neurological examinations. - Disabled World (DW).

Introduction

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

Main Item

What Does Knee-jerk Reflex Mean?

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.

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:

Reflex Test Response Scale (i)
0 : No evidence of contraction (Hyporeflexia) - Weak or absent response may indicate damage to the nerves outside the spinal cord (peripheral neuropathy), damage to the motor neurons just before or just after they leave the spinal cord (motor neuron disease), or muscle disease. It usually indicates a disease that involves one or more of the components of the two-neuron reflex arc itself.
1+ : Decreased, but still present (hyporeflexic) - A slight but definitely present response; may or may not be normal (ii).
2+ : Normal - A normal reflex response is merely considered normal.
3+ : Super-normal (hyperreflexic) - A very brisk response; may or may not be normal (ii). Excessive response may indicate spinal cord damage above the level controlling the hyperactive response. Usually indicates an interruption of corticospinal and other descending pathways that influence the reflex arc due to a suprasegmental lesion, that is, a lesion above the level of the spinal reflex pathways.
4+ : Clonus: Repetitive shortening of the muscle after a single stimulation. Always abnormal

(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.

Author Credentials: Ian was born and grew up in Australia. Since then, he has traveled and lived in numerous locations and currently resides in Montreal, Canada. Ian is the founder, a writer, and editor in chief for Disabled World. Ian believes in the Social Model of Disability, a belief developed by disabled people in the 1970s. The social model changes the focus away from people's impairments and towards removing barriers that disabled people face daily. To learn more about Ian's background, expertise, and achievements, check out his bio.