Question: How do human reflexes work and why?

Rebecca Dewey answered on 23 Jun 2015:

Reflexes protect your body from things that might have harmed it. As an example, if you put your hand on something hot, your reflexes tell you to move your hand immediately. This happens a lot faster than the message about the thing being hot even gets to your brain. Another example is the blink reflex – this is to protect your eyes from irritating things – or if you protect your face from something flying towards it, such as a ball. They are all just mechanisms to protect you from anything that might cause you harm, but the message doesn’t need to go all the way to the brain as this would take too long. Messages sometimes get sent to (and processed by) the spinal cord, as this is faster than the message going to the brain.

Another example is the knee-jerk (patellar) reflex – this is the one the doctor checks with a tendon hammer. This is one that gets processed in the spinal cord – the nerve sends the stimulus from your knee to the spinal cord, and the spinal cord sends the instruction back to kick. This is useful to help you keep your balance. If when you are standing, your knee to bend very slightly, then gravity might cause you to fall over forward and hurt yourself. Your reflexes protect you in this case by telling you to straighten your knee to keep you standing up.