SANFORD, Fla. >> Trayvon Martin’s mother and George Zimmerman’s mother clashed on the witness stand today over whether the screams for help that can be heard in the background on a 911 call came from the teenager or the neighborhood watch volunteer who shot him.

“I heard my son screaming,” Sybrina Fulton, Martin’s mother, said firmly after listening to a recording in which high-pitched wails followed by a gunshot could be heard.

Gladys Zimmerman listened to the same recording later in the day and answered, “My son” when asked whose voice it was. Asked how she could be certain, she said: “Because it’s my son.”

The conflicting testimony over the potentially critical piece of evidence came on a dramatic, action-packed day in which the prosecution rested its case and the judge rejected a defense request to acquit Zimmerman on the second-degree murder charge.

The question of whose voice is on the recording could be crucial to the jury in deciding who was the aggressor in the confrontation that ended with Zimmerman killing the 17-year-old.

The question sharply divided the two families: Martin’s half brother, 22-year-old Jahvaris Fulton, testified that the cries came from the 17-year-old. And Zimmerman’s uncle, Jose Meza, said he knew it was Zimmerman’s voice from “the moment I heard it. … I thought, that is George.”

In asking the judge to acquit Zimmerman, defense attorney Mark O’Mara argued that the prosecution had failed to prove its case.

He said an “enormous” amount of evidence showed that Zimmerman acted in self-defense, and he argued that Zimmerman had reasonable grounds to believe he was in danger, and acted without the “ill will, hatred and spite” necessary to prove second-degree murder.

But prosecutor Richard Mantei countered: “There are two people involved here. One of them is dead, and one of them is a liar.”

Mantei told the judge that Zimmerman had changed his story, that his account of how he shot Martin was “a physical impossibility,” and that he exaggerated his wounds.

After listening to an hour and a half of arguments from both sides, Judge Debra Nelson refused to throw out the murder charge, saying the prosecution had presented sufficient evidence for the case to go on.

The prosecution rested late in the afternoon after calling 38 witnesses over two weeks.

Among them, earlier in the day, was Sybrina Fulton, who sat expressionless on the witness stand while prosecutors played the 911 recording of a Zimmerman neighbor urging a dispatcher to send police quickly.

“Who do you recognize that to be?” prosecutor Bernie de la Rionda asked her.

“Trayvon Benjamin Martin,” she replied.

When introducing herself to jurors, Fulton described having two sons, one of whom “is in heaven.”

During cross-examination, O’Mara suggested — haltingly, in apparent recognition of the sensitivity of the questioning — that Fulton may have been influenced by others who listened to the 911 call, including relatives and her former husband.

O’Mara asked Fulton hypothetically whether she would have to accept that it was Zimmerman yelling for help if the screams did not come from her son. The defense attorney also asked Fulton whether she hoped Martin didn’t do anything that led to his death.

“I would hope for this to never have happened and he would still be here,” she said.

O’Mara asked Jahvaris Fulton why he told a reporter last year that he wasn’t sure if the voice belonged to Martin. Jahvaris Fulton explained that he was “shocked” when he heard it.

“I didn’t want to believe it was him,” he said.

The doctor who performed an autopsy on Martin also took the stand. Associate Medical Examiner Shiping Bao started describing Martin as being in pain and suffering after he was shot, but defense attorneys objected and the judge directed Bao away from that line of questioning.

He later estimated that Martin lived one to 10 minutes after he was shot, and said the bullet went from the front to the back of the teen’s chest, piercing his heart.

“There was no chance he could survive,” Bao said.

With jurors out of the courtroom, Bao acknowledged under defense questioning he had changed his opinion in recent weeks on two matters related to the teen’s death — how long Martin was alive after being shot and the effect of marijuana detected in Martin’s body at the time of his death.

Bao said last November that he believed Martin was alive one to three minutes. He also said Friday that marijuana could have affected Martin physically or mentally; he said the opposite last year.

The judge ruled before the trial that Martin’s past marijuana use couldn’t be introduced, and so the jury did not hear Bao’s opinion about the drug’s effect.