The claim by attorneys of Michael Jackson's doctor that the singer killed himself with an injection of the powerful anesthetic propofol is a crazy scenario, a top anesthesiology expert testified Thursday.

Dr. Steven Shafer told jurors that the level of propofol found in Jackson's bloodstream at his 2009 autopsy was too high to be explained by the theory that he self-injected.

As the last prosecution witness at Dr. Conrad Murray's involuntary manslaughter trial, Shafer also sought to undercut defense attorney theories that Jackson swallowed a fatal number of sedative lorazepam pills.

Murray admitted to police that, while at Jackson's mansion, he injected him with propofol and lorazepam as a sleep aid. But Murray's attorneys have argued Jackson could have given himself an extra, fatal dose of propofol while alone in his bedroom.

Shafer discounted that idea, saying the 50 year-old Thriller singer would have had to inject himself several times to achieve a cumulative rise of the propofol in his bloodstream.

People just don't wake up from anesthesia like that, Shafer told jurors, after flailing his arms to mimic a patient regaining consciousness and then injecting himself.

People don't wake up ... hellbent to give themselves another dose, Shafer said. It's a crazy scenario.

Shafer used charts to explain that while propofol can stop a person from breathing, the heart continues beating for 10 minutes and circulates blood.

Even if Jackson injected himself with the large dose of 100 milligrams of propofol six times, and stopped breathing with the last shot, his heart would still beat long enough to lower the propofol concentration below the level found in the bloodstream at autopsy, Shafer said.

ONLY 25 MILLIGRAMS?

Murray told police that he gave Jackson only 25 milligrams of propofol and two, 2 milligram injections of lorazepam on June 25, 2009, the day the pop star died.

But prosecutors have argued he gave Jackson a higher amount of lorazepam and followed up his initial injection of propofol with a continuous intravenous drip of that drug.

Medical examiners found that propofol was the main cause of Jackson's death, and the sedative lorazepam played a contributing role.

Murray's defense attorneys, in addition to suggesting Jackson self-administered more propofol, have argued the singer could have swallowed more lorazepam than the 4 milligrams that Murray said he gave as a sleep aid.

But Shafer Thursday also criticized that theory. He said the amount of actual lorazepam found in Jackson's stomach -- as opposed to the harmless, metabolized form of the drug -- was minuscule.

The results ... prove that Michael Jackson did not swallow lorazepam, Shafer said.

He later explained his modeling only ruled out Jackson popping lorazepam pills up to four hours before his death.

At the same time, Shafer said, the amount of lorazepam found in Jackson's blood was too high to be explained by two, 2 milligram injections of the drug. So Murray may have given the singer additional injections, Shafer said.

Shafer also said numerous studies show propofol is not fatal when swallowed, undercutting a former, controversial defense theory that Jackson guzzled the drug by himself.

The defense is expected to cross examine Shafer later on Thursday and begin presenting its case Friday. Murray, who has pleaded not guilty, faces a maximum sentence of four years in prison if convicted.