Bundles of fibres in the body (6)
I believe the answer is:
nerves
'bundles of fibres in the body' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nerves that I've seen before include "They carry impulses between brain and body" , "They connect the brain with other parts of the body" , "Body fibres conveying sensations" , "Fibres that convey messages to the brain" , "Feeling of anxiety" .)