why do pediatricians check for reflexes with those hammer knee things? What is it actually for?
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Reflexes work by acting before the signal fully reaches your brain. The nerves that pass signals between your limbs and your brain can’t *think* but there are sort of…”pre-programmed” responses. It’s like touching a very hot stove-top: your hand will move away from the stove before you ever feel pain, because the heat/pain signal only has to travel a little bit up your arm before the nerves in your arm cause your muscles to spasm. That way, you’re not going to get burned as badly since you don’t have to wait for a full round-trip signal to go all the way to your brain, bounce around in there long enough to make a decision, and then go back to your arm.
There are similar nerves in your legs. If you tap the right spot around your kneecap, it bumps one of the nerves responsible for causing that reflex and makes it fire, forcing your leg to spasm. You can do the same thing to a particular spot in your elbow, although that one is a little harder to hit.
The test is there to check for things like nerve damage that may not show up another way. Like, if the nerves that transmit pain are damaged, you may not *feel* like there’s anything wrong with your leg. But when the doctor taps your knee and it doesn’t move it shows that yeah, something is wrong down there. It’s also great for diagnosing problems in children and infants who may not be able to articulate that something is wrong.
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