Emergency exits

Is there anything to stop passengers from opening emergency exits on planes mid-flight? I’m referring to those ‘pull handle out and turn, then push panel out and away’ deals you get mid-cabin on both sides of the plane.

If there is some sort of ‘brake’ that needs to be tripped before they will open, who trips it? Cockpit? What if they are disabled?

The one thing that makes me think these exits aren’t ‘live’ all the time, is the fact that nobody has popped one yet.

Here’s the answer from Cecil himself.

Arjuna34