Just 4 minutes after tipoff in the Elite Eight matchup, Bert Smith crashed to the floor in a terrifying scene -- triggering team trainers and other medical personnel to race to his aid.

Smith was positioned on the baseline near the Gonzaga bench ... when he suddenly fell backward. It looks like his head may have slammed on the court during the fall.

Trainers from both teams wasted no time sprinting to Smith's side to provide immediate medical attention -- while another person rushed over with a stretcher.

The good news ... Smith began moving around relatively soon after the incident -- and by the time he was loaded onto the gurney, he was able to sit up and cross his arms.

He was even chatting with the trainers as he was wheeled off the court.

No word yet on what caused Smith to collapse -- but video shows he was having difficulty standing up during the previous possession.

Smith was replaced by William Henderson -- an alternate referee who was at the game in case of an emergency. Obviously, alternate refs never hope to be called into action, but Henderson was ready and stepped up so play could resume.

Smith has been an NCAA ref for years ... and has previously spoken about his passion for basketball and being a part of the game.

"I love being part of the game," Smith said in an article on Referee.com. "The arrival at the arena, the pregame, being with various partners, the competition, the players, coaches, ambience, the focus and concentration needed on each possession."