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Reference.org
Intrafusal muscle fiber
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<p>Intrafusal muscle fibers are <a href="/facts/Skeletal_muscle/en1tVYFN">skeletal muscle fibers</a> that serve as specialized <a href="/facts/Sense/n9kYQ3rf">sensory organs</a> (<a href="/facts/Proprioceptors/Ik79bA2x">proprioceptors</a>). They detect the amount and rate of change in length of a <a href="/facts/Muscle/dmxrL8pH">muscle</a>. They constitute the <a href="/facts/Muscle_spindle/KFgIC8Xe">muscle spindle</a>, and are innervated by both sensory (afferent) and motor (efferent) fibers. </p><p>Intrafusal muscle fibers are not to be confused with <a href="/facts/Extrafusal_muscle_fiber/oUnoxkwv">extrafusal muscle fibers</a>, which contract, generating skeletal movement and are innervated by <a href="/facts/Alpha_motor_neuron/9GMtJNtQ">alpha motor neurons</a>. </p>