How does the reflex beneath the knee cap make your leg “swing” when you hit it?
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Your muscles have sensors that tell your brain where the muscle is and how flexed it is. Your tight muscle ends a bit under your knee cap. Hitting the tendon with a hammer stretches your muscle and your body has a reflex to keep the balance – which is why your muscle contracts.
The sensory nerve terminating at the knee runs all the way up the leg to the spinal cord, synapses with the motor nerve there, and then the motor nerve runs back down the leg to trigger the muscle. So the reason the reflex test is useful is that it shows how well your nerves are conducting over a relatively large distance.
So, to stay standing up, we have sensory nerves called stretch receptors in the muscles and ligaments of the leg. They send signals to a nerve cluster that has motor nerves for the leg muscles. When you stand on your leg, the tensing and relaxing of your leg muscles keeps your balance. The stretch receptors activate the motor nerves for the muscle that got stretched. This is so that, if the pull on s muscle suddenly increases (maybe you stumble), the stretched muscle contracts automatically. The nerve signal does not have to travel all the way to the motor cortex in the brain, it can do it’s job using a shortcut.
When the doctor hammers the ligament below your kneecap, a big signal gets sent through this shortcut to make the muscles in your thigh contract.
Spastic cerebral palsy is s disruption of this loop.
My leg has NEVER moved and inch from this test in my 37 years. Every doctor tries a few times and then just shrugs. I’m perfectly healthy, but I’ve never understood this test. Doesn’t seem to matter if you fail.
Fun fact: I have Adis Syndrome which means that I have no large tendon reflexes such as the knee tap reflex. I always wondered why my knee didn’t do anything when a doctor would test it. Many, many years later my neurologist discovered it and dx’d me.
The patellar ligament reflex makes your quadriceps muscles contract, forcing your leg to kick.
Other users explain how reflexes in general work.
What you’re talking about it the myotatic reflex – it sounds complicated, but it’s actually one of the simplest phenomena in your body!
There’s a ton of stuff your body does automatically so you don’t have to worry about it, like digesting, keeping your heart beating, breathing, etc. Another thing it does is help your body stay balanced and let you stand up or sit down without having to readjust manually all the time.
To be able to do this, your body has a bunch of sensors, that tell your nervous system what’s going on. In this case, the ones that matter the most are the sensors inside of your muscles that tell the rest of your body how stretched your muscles are.
When the hammer hits your knee, it pulls on a tendon, which is the bit that attaches your muscles to the bone (in that case, the quadriceps in front of your thighs and your kneecap). In turn, that pulls on your muscle, and the sensors light up because the muscle stretched. They send a message through your nerves, all the way to your spine, saying “hey, that muscle extended!”. Now because your body didn’t plan on doing that, it’s gonna try to compensate. So the cell that received the message is gonna send signals to the cells that tell your muscles to contract:
* It’s gonna tell the cell in charge of the quadriceps muscle “hey, you’re extended, go flex”
* It’s also gonna tell the cell in charge of the muscles on the other side (your hamstrings), that do the opposite movement, “hey, something was wrong with the other muscle, stop flexing”
And do your body will react with more force than they should and send your leg flying!
Of course, your body does that sort of operation all the time. It’s the basic process that keeps your body balanced. But when you hit your knee with that hammer, you trick your body into thinking a big stretch is happening, which is why you get that big reflex.
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