Michael J. Fox won't be slowed down by his Parkinson's disease, which he compares to an unruly child within.

"It's like having a 4-year-old climbing on you all the time," the star, 47, tells Oprah Winfrey on her show that airs Today, "and so whatever you're trying to do, you've got this 4-year-old and you're … just trying to be patient and focus on what you need to do."

To show how he won't be held back, Fox dons a pair of skates and takes to the ice for a little hockey. Above all, the actor – who's promoting his new book, Always Looking Up – says that he refuses to let the disease pigeonhole him.

"I'm a dad, I'm a husband, I'm an activist, I'm a writer and I'm just a student of the world," he says. "This is one fact of my life, but it's not the totality of my life. It doesn't define me."

Fox continues to advocate stem cell research, which he hopes can help led to a cure for the disease. Traveling in Bhutan when the ban on stem cell research was lifted, he scrambled to get back to the States.

When asked by Winfrey if it was the day he had been waiting for, Fox responded, "It was absolutely crucial that this research be unfettered and allow us to go forward in a responsible way. It's an exciting day."