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Thread: knee jerk

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    knee jerk

    Why did this reflex evolve? Surely it can't be so that doctors could test your reflexes!
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Reflex reactions to sudden sensation such as jerking your hand out of a fire have survival value, and the motor nerves are the same type all over your body. Evolution is not discriminating.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by Noclevername View Post
    Reflex reactions to sudden sensation such as jerking your hand out of a fire have survival value, and the motor nerves are the same type all over your body. Evolution is not discriminating.
    No, no it evolved to give doctors some fun.
    As above, so below

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    Quote Originally Posted by Tom Mazanec View Post
    Why did this reflex evolve? Surely it can't be so that doctors could test your reflexes!
    It's a posture-control thing. Your body constantly measures your limb position, and subtly adjusts muscle tension to correct any deviation from the desired posture. It goes on all the time without your noticing it. Under normal circumstances, if your patellar tendon increases its traction on the quadriceps muscles, the quads gently contract to hold the knee in position. But by tapping the tendon with a hammer we suddenly increase the tension, evoking a sudden contraction of the quads, and the lower leg kicks up. Because this is mediated by a reflex arc of sensory and motor nerves, evoking the reflex tests the integrity of the nerve pathway. And because our reflexes are moderated by inhibitory pathways descending the spinal cord, a very brisk reflex suggests that there's something wrong with the inhibitory pathways.

    Grant Hutchison

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    So if my leg goes straight out in a high-kick, something's wrong? What would that diagnose?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    So if my leg goes straight out in a high-kick, something's wrong? What would that diagnose?
    What do you mean? Surely, if you're kicking, that's a voluntary flexion of your leg.

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    Quote Originally Posted by DaveC426913 View Post
    What do you mean? Surely, if you're kicking, that's a voluntary flexion of your leg.
    I think Tom was describing an involuntary kick in response to the tap of the tendon hammer. (In the trade, straightening your leg at the knee is called extension; bending is flexion.)
    Hyperreactive tendon reflexes are either due to something that makes your nervous system generally excitable (anxiety, hyperthyroidism, amphetamine overdose, strychnine poisoning ...[long list]) or that interferes with the descending inhibitory pathways in the spinal cord (Multiple Sclerosis, Amyotrophic Lateral Sclerosis, spinal cord injury, specific kinds of stroke ...[long list]).

    Grant Hutchison

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    Can the reflex disappear? Is there a long list of what that would mean?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    Can the reflex disappear? Is there a long list of what that would mean?
    Yes. And yes.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    I think Tom was describing an involuntary kick in response to the tap of the tendon hammer.
    D'oh!

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    Quote Originally Posted by Tom Mazanec View Post
    Can the reflex disappear? Is there a long list of what that would mean?
    Lots of Googlage on this.

    "The absence or decrease of this reflex is problematic, and known as Westphal's sign. This reflex may be diminished or absent in lower motor neuron lesions and during sleep. On the other hand, multiple oscillation of the leg (pendular reflex) following the tap may be a sign of cerebellar diseases. Exaggerated (brisk) deep tendon reflexes such as this can be found in upper motor neuron lesions, hyperthyroidism[3], anxiety or nervousness. The test itself assesses the nervous tissue between and including the L2 and L4 segments of the spinal cord."
    https://en.wikipedia.org/wiki/Patellar_reflex

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    Thanks, DaveC426913.
    Now could somebody translate that into English?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    Thanks, DaveC426913.
    Now could somebody translate that into English?
    Basically it's reiterating what Dr. Grant said.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by Tom Mazanec View Post
    Thanks, DaveC426913.
    Now could somebody translate that into English?
    I would think the point is simply that testing your reflexes is a very quick, noninvasive way to quickly check for the possibility of a large number of potential problems. So it's not particularly diagnostic by itself, but once it provides an indication that there could be something wrong, you'd proceed to more specific (and probably more involved) tests to try to narrow down what the actual issue was.
    Conserve energy. Commute with the Hamiltonian.

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    Quote Originally Posted by Grey View Post
    I would think the point is simply that testing your reflexes is a very quick, noninvasive way to quickly check for the possibility of a large number of potential problems. So it's not particularly diagnostic by itself, but once it provides an indication that there could be something wrong, you'd proceed to more specific (and probably more involved) tests to try to narrow down what the actual issue was.
    Yes, the deep tendon reflexes (there are several to test) are part of a general neurological examination, not something done in isolation. Abnormalities of one reflex can have multiple causes, but if you put them together with the distribution of other abnormalities (or normalities) detected in other parts of the neurological examination, you can limit the number of options on the diagnostic Venn diagram until you come up with a specific diagnosis.
    Neurological examination is really quite a complicated intellectual exercise, a bit like solving Sudoku by going through a sequence of steps and guesses and eliminations. For a couple of years I wanted to be a neurologist, because of the abstract appeal of the diagnostic process, but after I worked in the job for six months I realized that after going through the satisfying mental exercise, you almost always ended up telling a person that they had something horrible wrong with them, and there was nothing you could do to help.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    Yes, the deep tendon reflexes (there are several to test) are part of a general neurological examination, not something done in isolation. Abnormalities of one reflex can have multiple causes, but if you put them together with the distribution of other abnormalities (or normalities) detected in other parts of the neurological examination, you can limit the number of options on the diagnostic Venn diagram until you come up with a specific diagnosis.
    Neurological examination is really quite a complicated intellectual exercise, a bit like solving Sudoku by going through a sequence of steps and guesses and eliminations. For a couple of years I wanted to be a neurologist, because of the abstract appeal of the diagnostic process, but after I worked in the job for six months I realized that after going through the satisfying mental exercise, you almost always ended up telling a person that they had something horrible wrong with them, and there was nothing you could do to help.
    Actually, that kind of leads to a question. When I was a kid my recollection is that doctors always did the leg knock thing, but nowadays when I get a physical they don't. I wonder if it is a difference between the US and Japan, or if it is something they do with children to detect something? Or is it just my memory being fuzzy?
    As above, so below

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    Quote Originally Posted by Jens View Post
    Actually, that kind of leads to a question. When I was a kid my recollection is that doctors always did the leg knock thing, but nowadays when I get a physical they don't. I wonder if it is a difference between the US and Japan, or if it is something they do with children to detect something? Or is it just my memory being fuzzy?
    Maybe it's an effect of the specializations in the field. As our medical knowledge grows, it becomes increasingly difficult for one doctor to hold all that information in their heads. Primary physicians used to be more generalist in their work. Nowadays my neurologist always performs a battery of simple neuro evaluations each visit, including the tap test. My primary does not.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    I think it comes from an understanding that neurological examination is unlikely to be abnormal if an adult patient gives no history, and shows no gross signs, suggestive of neurological disease. Children are a little different, since they often don't have the vocabulary or understanding to report neurological changes, and parents are often quite insensitive to a slow changes in, for instance, their child's gait.

    Grant Hutchison

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    And how did this test originate in the first place? i would not think that it is the kind of thing you would try out if you did not know about it beforehand. Did some doctor just try randomly tapping places on the body and see what happened?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    And how did this test originate in the first place? i would not think that it is the kind of thing you would try out if you did not know about it beforehand. Did some doctor just try randomly tapping places on the body and see what happened?
    Hardly. Tapping air-containing cavities (lungs, abdomen) was an old technique doctors borrowed from brewers--the border between resonance and dullness tells you where the air ends and fluid begins. And patients with certain neurological conditions will report that their muscles contract spontaneously if tapped or stretched. And it is obvious to anyone who observes a person standing with their eyes closed that the muscles in their legs are in constant activity, maintaining posture. So doctors worked their way from tapping hollow bits, to tapping muscles, to tapping tendons to elicit postural reflexes.

    Grant Hutchison

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    Try the Cremaster reflex if you are male. Stroke inside of your thigh downwards (or upwards) and the testis on that side will retract.
    Perhaps this evolved as a protective mechanism.. there are many others.. including the 'corneal reflex' which protects the eye.
    Women have a similar reflex 'Geigel' (less pronounced)

    The nocioceptive reflexes that (for example) rapidly pull your hand away from a hot object before reaching consciousness as "[REDACTED] THAT'S HOT !!" are flexural/withdrawl reflex arcs.. e.g biceps. These are not posture-related .. IMHO.
    Last edited by PetersCreek; 2020-Jun-29 at 03:21 PM. Reason: masked language
    "It's only a model....?" :-)
    https://www.youtube.com/watch?v=m3dZl3yfGpc

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    Yes, withdrawal reflexes are more complicated than the simple deep tendon reflex that Tom Mazanec asked about--quite orchestral, in fact. If you burn your finger, you may invoke a finger flex, wrist flex, biceps flex, and a mixed shoulder rotation and abduction. And you'll invoke postural reflexes, because flailing your arm around like that would make you fall over if you didn't respond to the shift in your centre of gravity. So there's a thing called the "crossed extensor reflex" which makes the opposite limb shift in the opposite direction, though people aren't generally aware of it.

    The cremasteric and Geigel reflexes fall into the category of "superficial reflexes"--responses to skin stimulation, in contrast to "myostatic" reflexes, like the knee jerk, which are reflexes that keep muscles at a constant length, to maintain posture. The only superficial reflex commonly used in the routine neurological examination is the plantar reflex. I've never known anyone actually try to elicit the cremasteric reflex, except socially, and when I was a student we all used think that Richard Geigel should have been the subject of a disciplinary hearing.

    Grant Hutchison

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    Cremaster excellent to check L1 nerve root, L3 for the knee, S1 for the ankle.
    "It's only a model....?" :-)
    https://www.youtube.com/watch?v=m3dZl3yfGpc

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    Quote Originally Posted by plant View Post
    Cremaster excellent to check L1 nerve root ...
    It's just not particularly reliable--absent in a significant proportion of normal males--and people aren't that keen on having it done, for obvious reasons.
    We were taught that it should be part of the examination for testicular pain, as a good sign of testicular torsion (a surgical emergency), but I think it has fallen from favour somewhat in that regard, too.

    Grant Hutchison

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