For each description related to muscle spindles A–E, select the most appropriate option from the following list of terms (fusal=spindle)

A. Option 2 Motor region. A spindle is a modified muscle fibre and the striated region at each end is due to actin and myosin arrangements which retain the ability to contact.

B. Option 1 Sensory, non-contractile region. This is the nuclear bag region which detects stretching of the spindle and sends information along fast-conducting fibres to the central nervous system.

C. Option 4 Contraction of extra-fusal fibres. This is the reflex response whereby stretching of the spindle leads to contraction of the muscle containing the spindle.

D. Option 3 Increased activity in sensory nerve supply. The antagonistic muscles stretch the muscle and its spindles and the direct effect on the spindle is stimulation of its sensory region and initiation of activity in the nerve endings supplying it.

E. Option 4 Contraction of extra-fusal fibres. Motor activity to the striated spindle ends makes them contract and stretch the central sensory region; stretching the muscle also stretches the spindles within it; in both cases the increased sensory output leads to reflex contraction of the extra-fusal fibres of the muscle containing the spindles.