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WASHINGTON (AP) -- Sen. Tim Johnson has shown significant improvement after brain surgery and doctors say "everything is going to be just fine," Senate Democratic leader Harry Reid said Sunday.

Yet when asked whether the 59-year-old South Dakota Democrat was conscious, Reid said in a television interview: "I'm not a doctor. I have heard and talked to his family. You should talk to them. It's not appropriate to talk to me about that."

Reid, who has visited Johnson frequently after the surgery Wednesday following a brain hemorrhage, said "he's doing very well. ... His improvement has been significant."

Johnson has responded to voices, opened his eyes and moved his limbs.

Surgeons at the George Washington University Hospital have said Johnson was experiencing post-surgery swelling in his brain, but they said that was normal. (What happened in Johnson's brain? Video)

"Doctors tell us everything is going to be just fine," Reid said.

Johnson's spokesman, Noah Pinegar, said Sunday the senator remained in critical condition. "The goal has been rest," Pinegar said. He said he could not answer whether Johnson is conscious or sedated.

Johnson has been diagnosed with arteriovenous malformation, a condition that causes arteries and veins to grow abnormally large, become tangled and sometimes burst. The condition often is present from birth. (Interactive: What is congenital arteriovenous malformation?)

South Dakota's Republican governor, Mike Rounds, would appoint a replacement if Johnson's seat were vacated by his death or resignation.

A Republican appointee would create a 50-50 tie and effectively allow the GOP to retain Senate control because of Vice President Dick Cheney's tie-breaking vote. (What happens if Johnson has to be replaced? Video)

There is ample precedent for senators to continue to hold office while incapacitated.