Answer: fibrillates

AnswerCrossword Clue
fibrillatesBreak (a fiber) into fibrils
fibrillates(of a muscle, esp. in the heart) Make a quivering movement due to uncoordinated contraction of the individual fibrils
fibrillatesFIBRILLATE, to undergo fibrillation
fibrillates(of a fiber) Split up into fibrils