Why do my muscles twitch during an MRI?

I have to have an MRI tommorow (brain) and the last time I had one (on spine) I had involuntary muscle twitching the whole time and it freaked me out, I mentioned it to the tech and she acted like I was weird. I mentioned it to the neurologist this time and he told me to take flexeril before the scan tommorow, I'm just curious why my body does that?