For the second time this summer the top Republican in the Senate, Minority Leader Mitch McConnell, abruptly went silent at a news conference. He was about to answer a question from a reporter when he suddenly froze up. He seemed unable to speak. An aide then stepped in, trying to keep things moving along.The senator's silences have raised concerns about his mental fitness – and larger questions about an aging Congress.NPR's Mary Louise Kelly speaks with Dr. Ann Murray, the Movement Disorders division chief at the Rockefeller Neurosciences Institute at West Virginia University.