This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Physiology
which of the following is true regarding nuclear bag fibres of motorunit ? 
Select and explain. 
A. responds to length only, B. responds to both length and velocity changes, C. responds to contraction of muscle fibres, D. responds to reflex relaxation
B: responds to both length and velocity changes
Answer: B: responds to both length and velocity changes 
Explanation: 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