Bobbin Bergstrom is the person apart from the original six cast members who has appeared in more episodes than any other actor or actress. She often appears on-screen as the Nurse character and is a Registered Nurse in real life. She has also been the on-set medical advisor for the series since the Season 1 episode Paternity. She appeared on-screen for the first time in Role Model. At the end of Season 6, she had appeared in 90 episodes.
Facebook video with Bergstrom describing her job on the show