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. The reflex test is simple yet informative and can give important insights into the integrity of the nervous system at many different levels.
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.
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:
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 dimished 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 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. Available from: https://www.ncbi.nlm.nih.gov/books/NBK396/
(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.
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