Curiosities: Why does your knee jerk when the doctor hits it with a rubber hammer?

By Eddy Montilla.


If a doctor hits your knee with an iron hammer, then he just wants to kill you. But if he uses a rubber one (better called reflex hammer), his intentions are good and different: He wants to test your reflexes, and in general, to check your nervous system. When the doctor hits a patient’s knees, this action will force the tendons to relax, but his or her body’s answer will be to tell the tendon to contract, which is the origin of the involuntary kick from the patient (or voluntary kick if the doctor’s fees are high!). If you put pressure on a tendon, this causes the tendon to constrict and will send a message to the spinal cord. From the nervous system, the body will be informed that the tendon is too tight and a new message will be sent out to relax the muscle. This process is known as reflex arc.

Copyright 2016 All rights reserved.


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