The Rev. Jesse L. Jackson, the longtime civil rights leader and former Democratic presidential candidate, said Friday he has Parkinson’s disease.

In a letter posted on Twitter on Friday afternoon, Mr. Jackson, 76, shared the news and his struggle to accept it.

“Recognition of the effects of this disease on me has been painful, and I have been slow to grasp the gravity of it,” he wrote. “For me, a Parkinson’s diagnosis is not a stop sign but rather a signal that I must make lifestyle changes and dedicate myself to physical therapy in hopes of slowing the disease’s progression.”

Parkinson’s is a movement disorder. Its symptoms include muscle tremors and stiffness and poor balance and coordination. It typically begins after age 50 and can cause difficulty sleeping, chewing, swallowing or speaking.