Osame, "Extrafusal and intrafusal muscle
effects in experimental botulinum toxin-a injection," Muscle & Nerve, vol.
Each muscle spindle is built around 3-10 small intrafusal muscle fibres attached to the sheaths of the surrounding extrafusal skeletal muscle fibres, the ends of which are excited by small gamma efferent motor nerve fibres.
The receptor portion of the muscle spindle is located midway between its ends where the intrafusal muscle fibres have no contractile element.
Its are composed of 3-12 intrafusal muscle
fibers, of which there are three types:
If botulinum toxin is administered over a prolonged period of time, however, true muscle atrophy can occur, both in the extrafusal and intrafusal muscle
fibers within the muscle spindle organ.