Answer: fibrillating

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