Muhammad Ali's endurance in face of Parkinson's

In the 1970s, Muhammad Ali was heavyweight champion and probably the world's most famous athlete.

Today is his 70th birthday, which he celebrated over the weekend in his hometown of Louisville, Ky.

Ali has battled Parkinson's disease for nearly 40 years.

Dr. Jon LaPook, who treats patients with neurological disorders including Parkinson's disease, discussed the condition on "CBS This Morning."

Parkinson's is a chronic disease that affects millions of Americans, with 50,000 to 60,000 new cases diagnosed each year. The disease is the result of a lack of the chemical dopamine, a neurotransmitter that helps nerve cells talk to other nerve cells. With Parkinson's, that ability declines.

LaPook also discussed the early stages of the disease, which include shaking and rigidity, versus its advanced stages, which can affect cognition and behavior. Watch his full report in the video above.