Ariel Sharon is able to breathe independently, say doctors who have started to bring the Israeli prime minister slowly out of a coma.
Sharon's own surgeon says he will not be able to work again
As the anaesthetic was reduced, Mr Sharon began responding to pain, but staff at the Hadassah hospital warned the revival process would take days.
Despite the positive developments, they said, Mr Sharon was not out of danger.
Mr Sharon has been in an induced coma since being operated on for a major stroke, which he suffered on Wednesday.
A full assessment of any brain damage Mr Sharon may have suffered can only be made once the process of weaning him off the sedation is complete.
Brain activity signs
However, the fact that he is able to breathe is encouraging, Hadassah hospital director Dr Shlomo Mor-Yosef said.
"This is the first sign of some sort of activity in his brain," he said.
Nonetheless, the prime minister remains hooked up to a ventilator.
He is still heavily sedated, Dr Mor-Yosef said, but doctors have been able to begin testing Mr Sharon's response to pain in his right arm and leg by applying pressure to certain points.
As a result of the stimuli, the prime minister moved both the arm and leg slightly, movements which increased in significance as the day wore on, Dr Mor-Yosef said.
Peter Sanderco, professor of medical neurology at Edinburgh's Western General Hospital, told the BBC's Newshour programme that while the prime minister's ability to breathe spontaneously was a vital development, the limb movement was less so:
ARIEL SHARON'S HEALTH
Sharon suffers minor stroke on 18 December 2005
Doctors discover small hole in heart, schedule operation for 5 January
Sharon rushed to hospital one day before scheduled surgery with major stroke
Undergoes two operations overnight on 4/5 January, followed by third on 6 January
"In a sense, if he moved his hands or he doesn't move his hands, that is relatively unimportant," he said.
Prof Sanderco said the next milestone doctors will be hoping for is if Mr Sharon is able to open his eyes and follow movement around the room, or make purposeful limb movement such as grasping hand, although he stressed it could be some time before such skills return.
The neurosurgeon who operated on the 77-year-old prime minister, Jose Cohen, has said that although his chances of survival are "very high", Mr Sharon will not be able to continue as prime minister.
Once Mr Sharon is out of the coma, induced to allow him to rest and heal, his doctors will begin testing his cognitive responses.
The medics will then pass their assessment of brain damage to Attorney General Meni Mazuz.
"They will inform us the moment they wake him up from the sedation and they will know what systems were damaged and what his situation is," justice ministry spokesman Jacob Galanti said.
If the assessment declares that Mr Sharon has been permanently incapacitated and is unable to return to office, a cabinet meeting will be called to choose a caretaker leader to be prime minister until the general election.
The five cabinet members from Mr Sharon's Kadima party are the only eligible candidates, but of those, acting Prime Minister Ehud Olmert is seen as the real contender as the other four are backing him to assume the role.
One of Mr Olmert's first important decisions as acting prime minister has been whether to allow Palestinian campaigning in East Jerusalem for elections on 25 January.
Israel announced that Palestinian candidates would be allowed to campaign in East Jerusalem, but must submit requests in advance to Israeli police and militant groups like Hamas would be barred.
Palestinian politicians have rejected these conditions.
Israel's own general election is to go ahead as planned on 28 March.