<prompt> which of the following is true regarding nuclear bag fibres of motorunit ?responds to length onlyresponds to both length and velocity changesresponds to contraction of muscle fibresresponds to reflex relaxation  <response>: Two types of intrafusal fibres -- nuclear bag fibre primary la afferent -- supplies annulospiral ending in the centre -- provide information on muscle length and velocity phasic response fast stretch reflex -- nuclear chain fibre primary la and secondary II afferent -- supplies flower spray ending -- monitor the length of the muscle tonic response --slow stretch reflex ref : guyton and hall 