Why does a medical doctor often tests the knee for reflexes? What is it really testing for ? Surely not that single area for a single purpose (reflex). What am I missing?
In: Biology
When someone tests a knee reflex they hit the patella tendon. The tendon is attached to muscles which stretched slightly when the tendon was hit. Muscles have stretch receptors which detect the sudden stretch and relay this information to the spinal cord. In the spinal cord, the sensory nerve is directly connected to a motor nerve via a relay nerve. There is just a hard wired direct connection and no actual thinking is required… so when the muscle is stretched a signal comes straight back to say “Quick! Contract!” and the muscle contracts. This muscle stretch -> muscle contraction direct connection is the basis for tendon reflexes tested by doctors.
Reflexes can be demonstrated in hitting various other tendons and watching for muscle contractions such as the achilles tendon and biceps tendon. Reflexes can also sometimes be elicited without hitting tendons but instead quickly stretching a muscle (such as pointing someones ankle downward and then quickly jerking it upwards to stretch out the calf muscles which is an often used alternative to hitting the achilles with a tendon hammer).
If theres a problem with the sensory/motor circuitry that detects the stretching, sends the signal to the spinal cord and then sends the “contract!” signal back to the muscle… you’ll see a missing or weak muscle contraction.
Interestingly our brains and spinal cords quickly learn to dampen our reflexes as we grow up. But if we have damage to our brains or spines the dampening effect will be missing and suddenly reflexes become more pronounced. So increased reflexes are a sign of a brain/spinal problem.
TLDR: Tendon reflexes happen when muscles are suddenly stretched and watching for reduced or exaggerated reflexes gives us clues as to where in the body damage might have occurred.
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As many comments, it’s to rest neurological response. Hit the tendon and see the muscle react. If it jerks then everything is normal, if there’s a lack of movement then there might be some nerve issues.
My doctor always has the “ok let’s check your brain” part of the physical.
Somethings like writing a number in my hand with his finger with my eye closed, asked me to push against his hands, knee taps and such.
How often do doctors find an issue with these tests for an otherwise healthy young individual? Like, could I feel completely well, but I can’t detect what number he is writing in my hand?
Your knee reflexes tell me in a broad sense what your neurology is like beyond the bottom of your spinal cord. Any issue in the cord can present as a problem distally, and since your knees are beyond the end of the cord and easy to prompt a reflex from, they’re an ideal place to test to rule out a spinal issue.
I dont actually care about your knees when I test the reflex, and if you say you have a problem with your knee testing the reflex there isn’t going to help either of us very much. Rather, testing the reflex at the knee can do 1 of 2 things: it can either help establish whether there is a neurological deficit, or help to localise where one is.
Causing a reflex provokes a movement. Said movement requires several different systems to work together (muscles, nerves that carry information to the body, spine/brain, nerves that carry a response to the muscles involved, etc…)
At the same time, all of these different systems depend on a multitude of conditions to work properly – e.g. structural integrity, nutrients, etc…
Therefore, testing different reflexes can test many of these components and how they work together at the same time. This is a quick way to narrow down initial complaints which often can mean various different things (e.g. “my arm feels numb” could be due to hundreds of different things, but exaggerated arm reflexes will point you in one direction, decreased reflexes in another)
Under some conditions, some reflexes can appear or disappear, which are very useful diagnostic clues. In some cases, these clues can greatly narrow down what could be causing the issue (if you’re curious, look up the Babinski reflex)
Also, comparing reflexes (involuntary movements) to voluntary movements is useful in many situations – eg if someone has erectile dysfunction, if they can achieve a full, involuntary erection while asleep (reflex erection), this is a pretty good sign that there is nothing physically wrong with said dong.
Bonus: next time you have a male human (possibly yourself) you feel comfortable with while naked in front of you, stroke the inner part of his thigh while he is relaxed (can’t be looking down). You will see the testicle on that side go up. This is called the cremasteric reflex, after the cremasteric muscle which causes the testicle to rise. If someone comes to the ER with testicular pain and an absent cremasteric reflex, you can be pretty sure that the person has testicular torsion (though nowadays it’s better to diagnose using other, more refined means – this is a “classic” sign)
How did I do for ELI5? Am currently a tired and stressed out medical student studying for exams 🙂 Also, I know the examples aren’t your typical knee-jerk reflex, but everyone knows about that one!
It’s actually just a straightforward test to see if the muscle and tendon and everything in between works fine, and for that area only. There are a lot of tests for various other areas too. I guess it gained popularity due to the simplicity and interesting effect, as it is fairly easy to do and looks cool on screen.
Source: I’m a medical student
This one time a doctor tested my knee reflexes, and I have really bad knees, so nothing happened even after him wailing on them with that stupid mallet. Afterward he asked if I was on the spectrum. Which confused me. Any relation to that?
I’ll try an Eli5 again.
Yes it is part of a Neuro exam.
The stretch reflex test is aimed at isolating a neurological lesion to a specific area.
HypOreflexia (lack of reflex) indicates an injury to a nerve outside of the brain and spinal cord.
HypERreflexia (the opposite) indicates an issue within the brain and spinal cord.
Furthermore, a lesion can be isolated to a specific spinal cord level by using a specific “myotome” (group of muscles associated with a spinal cord level) and the associated tendon reflex test.
Ie. patella tendon reflex is hypOreflexive = a potential lesion to L2-L4 spinal nerves peripheral to the spinal cord.
HypERreflexive = lesion within the brain/spinal cord
Hope that was Eli5 enough. It gets way more complicated.
When you hit the knee, it triggers nerves that do not travel up to your brain (that’s why your knee kicks without you thinking to do it).
This gives a measure of the conductivity of your nerves without involving your brain.
The reflex arch goes knee sensory neuron > spine > knee motor neuron.
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