Do intrafusal fibers contract?

Do intrafusal fibers contract?

• A. True
• B. False
• C. Cannot contract
• D. Only during fatigue

Answer: (A)

Intrafusal fibers are not just passive sensors inside the muscle spindle. They have contractile elements at their polar ends and receive input from gamma motor neurons. When gamma neurons fire, these polar regions contract, which tightens the spindle and preserves tension on the central sensory region. This keeps the spindle sensitive to stretch as the muscle length changes, allowing accurate proprioceptive signaling. The main force generation for a muscle comes from the extrafusal fibers, but the intrafusal fibers actively contract to modulate spindle sensitivity. So saying they can contract is true, not true only during fatigue or never.

Intrafusal fibers are not just passive sensors inside the muscle spindle. They have contractile elements at their polar ends and receive input from gamma motor neurons. When gamma neurons fire, these polar regions contract, which tightens the spindle and preserves tension on the central sensory region. This keeps the spindle sensitive to stretch as the muscle length changes, allowing accurate proprioceptive signaling. The main force generation for a muscle comes from the extrafusal fibers, but the intrafusal fibers actively contract to modulate spindle sensitivity. So saying they can contract is true, not true only during fatigue or never.