[How do you counter takedowns?]

Sorry if this question has already been asked many times. If a BJJist or a wrestler tries a takedown technique, what would be a MT practitioner's counter? A knee to the head of the one using a takedown? The hardness of a MT practitioner's knee would be enough to knock out the other person right?

[What's the secret to just shrugging off blows to the head?]

What's the secret to just getting a punch to the head or face and then treating it as if it was nothing? I've been punched in the forehead before and the pain is like the same pain as you the headache you get have when you have a fever, only it hurts twice as much. But I've seen Muay Thai fighters just shake their head in a playful style or even smile after receiving punches to either the head or face. Is it because they have gotten used to so many blows to the head and face and gotten used to it over the years or are they taught some sort of mental method to ignore pain?