system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What information does the doctor get out of hitting my knee with a tiny hammer?
gpt
That the L2, L3, and L4 segments of your spinal cord are working. If your leg doesn't kick out, or doesn't kick out well (absent mobility issues), something is wrong (Westphal's sign) and can mean motor neuron lesions (the neurons that control muscular contractions), cerebellar diseases, and even thyroid problems. So that little tap actually tells the doctor a lot about what's going on in your braincase.

There are many other reflexes that are used to measure brain function such as the plantar reflex (see Babinski's sign) which is a great way to determine if a comatose ER patient suffered damage to the central nervous system.