Kinda weird when you think about it, a dream is sort of like a virtual reality thingy, it can simulate any sight, smell, feeling, even taste.
As for that wee thing, thats happened to me too before [img]smileys/smilies_09.gif[/img]but I guess it's your bodys natural reaction, sometimes i miss a step in a dream and i wake up and my leg jolts horribly as if preparing to take a fall.