Giving evidence for just over an hour, James Dunleavy, 40, repeatedly insisted he loved mother Philomena, 66, and denied murdering her.
He also told a jury at the High Court in Edinburgh he did not have mental health problems and claimed doctors had been swayed by the serious charge he was facing.
He told how he believed she would "miraculously" turn up again after she left his Edinburgh flat without warning early one morning.
Mrs Dunleavy, a mother-of-five from Dublin, had been staying with her son in Balgreen Road, just a few minutes walk from where her remains were found on Corstorphine Hill, a local nature reserve.
Mr Dunleavy said his mother was in the habit of "going walkabout" without saying where she was going. She had even gone on holidays abroad without warning.
Defence QC Gordon Jackson asked him: "Did you do anything that would have caused the death of your mother?"
Mr Dunleavy replied: "No."
Mr Jackson said: "Were you responsible for what happened to her before she got buried?"
Mr Dunleavy told him: "No".
The QC said three psychiatrists agreed Mr Dunleavy was suffering from some sort of mental disorder.
"I think the gravity of the crime I am accused of may have coloured their perception," Mr Dunleavy said. "They are entitled to their opinion."
The trial heard Mr Dunleavy moved to Edinburgh to work on the construction of the city's tram lines. His mother had visited him in the capital on a previous occasion.
Mr Dunleavy denied arguing with his mother just before she is believed to have died, saying a neighbour who described a row had misinterpreted "a wordy discussion" between them.
"We were just having a bit of banter, that's all," he said.
Asked if he was surprised by his mother's sudden departure, he said: "That was my mother's MO."
Mr Dunleavy said he had not seen any of the publicity that followed the discovery of her remains because he did not read newspapers or watch television.
"I did nothing to my mother. I thought she would miracu- lously appear again," he said.
Advocate depute Alex Prentice, QC, pointed out that when police searched the flat in Balgreen Road they found €870, Mrs Dunleavy's identity card and clothing belonging to her.
Mr Dunleavy's father, also James, said he got a phone call from his son in late April or early May last year.
"James rang me to say she was on her way home, that she would be there that night. But she never returned." He added: "It was no surprise."
Mr Dunleavy Jnr denies battering Mrs Dunleavy to death between April 30 and May 7 last year and trying to cover up the alleged murder and destroy evidence.
The murder charge alleges it was there that he inflicted "blunt force trauma" by means unknown, compressed his mother's throat and cut off her head and legs with a blade and something like a saw.
A second charge accuses Mr Dunleavy of pretending his mother - also known as Phyllis - was unwell and had returned to Ireland.
The charge further alleges that Mr Dunleavy put his mother's torso, severed legs and head into a suitcase and took the dismembered body to Corstorphine Hill where he buried her.
Prosecutors also claim Mr Dunleavy vacuumed and washed his flat to remove blood stains and set fire to a bed and mattress.
The trial is expected to hear closing speeches today.