Gamma Loop: Fusimotor Control of Muscle Spindle Sensitivity

Muscle spindles don't just passively report length — the nervous system actively tunes them via gamma motor neurons. This simulator renders the spindle's bag1 (dynamic) and bag2/chain (static) intrafusal fibers nested inside a bundle of extrafusal muscle fibers, and lets you drive gamma-static and gamma-dynamic fusimotor input independently to see how contracting the intrafusal fiber's polar ends stretches its sensory equatorial region and raises Ia/II afferent firing — even with the whole muscle held at constant length. A "trigger voluntary contraction" control demonstrates the payoff: with alpha-gamma coactivation on, gamma drive rises together with the contraction and the spindle stays sensitive throughout; with it off, the shortening muscle slackens its own spindle and the Ia afferent falls silent, exactly the failure alpha-gamma coactivation exists to prevent.