Here's why doctors always whack you in the knee — no, it's not what you think.

Yes, yes, reflexes. But why the knee? And what would it mean if your knee didn't jerk. Find out why you are beaten with a rubber mallet as part of a medical check-up.

There are plenty of tests doctors do that live in infamy; especially when it involves telling a patient to cough. Only one term has broken free of the doctor's office and become its own idiom. The 'knee-jerk response' is any kind of reaction that is done automatically, without thought. We all know what it means in the political or social arena, but few understand what it means in the medical one.

The hit to the knee causes the thigh muscle to stretch. This stretch sends a signal along a sensory neuron to the spinal cord where it interacts directly with a motor neuron which goes to the thigh muscle. The muscle contracts and the leg kicks. The brain never gets involved, which is helpful because what doctor trusts their patients' brain? More importantly, the knee-jerk reflex is what's known as a mono-synaptic response. The impulse only has to jump from one nerve to another once. There aren't many variables to be dealt with, so it's its own little controlled experiment. If there is no response to the knee tap, it indicates nerve damage that needs to be dealt with. Continual jerks after the tap can indicate cerebellar disease.

Either problem can lead to huge problems. Without the quick activation of muscles in response to a stretch, any unanticipated weight on the legs would cause them to collapse. Even walking would take concentration. A knee-jerk response is a good thing, as long as its not in debate.

And so your doctors will continue to assault you, long into the future.

Via Washington.edu and Ithaca.edu.

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