How does the reflex beneath the knee cap make your leg “swing” when you hit it?

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How does the reflex beneath the knee cap make your leg “swing” when you hit it?

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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.

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.

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.