The muscle spindle sensitivity arises from the activity of which fibers?

• A. Intrafusal fibers
• B. Extrafusal fibers
• C. Tendon fibers
• D. Epithelial fibers

Answer: (A)

Muscle spindle sensitivity comes from intrafusal fibers, the specialized fibers located inside the spindle that house the sensory endings. When the muscle length changes, the central region of these intrafusal fibers is stretched, increasing the activity of the sensory afferents ( Ia and II fibers) that report length and rate of change to the nervous system. This sensitivity is kept when the muscle contracts through gamma motor neurons, which tug on the ends of the intrafusal fibers to maintain tautness so stretch is still detected. Extrafusal fibers are the main force-generating fibers outside the spindle and don’t provide the sensory receptor function, while tendon and epithelial fibers aren’t part of the muscle spindle’s sensing mechanism.