A Holden man charged with second-degree murder in the death of his girlfriend pleaded guilty to manslaughter Tuesday in Livingston Parish, prosecutors reported.

Joseph D. Mixon, 28, pleaded guilty to the shooting death of Brenda Myers, 40, of Holden, according to the District Attorney’s Office.

Judge Zoey Waguespack of 21st Judicial District Court sentenced Mixon to 40 years in prison, prosecutor Matthew Belser said.

The guilty plea came as potential jurors waited for the beginning of Mixon’s trial, which had been expected to last all week.

Authorities said Mixon shot Myers once in the head with a pistol.

Mixon maintained that he accidentally bumped the trigger of the gun, causing it to fire, Belser said.

The shooting occurred on Sept. 7, 2010.

Belser said Mixon and Myers had argued earlier in the day and that Myers apparently had threatened to break up with Mixon.

After the initial argument, the couple, who had no children, returned to the house they shared in Holden, Belser said.

State law requires Mixon to serve 85 percent of his sentence before he becomes eligible for parole, Belser said.

The prosecutor said the victim’s family agreed with the District Attorney’s Office’s decision to accept the manslaughter plea.