= Intrafusal muscle fibre
An <intrafusal muscle fibre> is a specialized <skeletal muscle> fibre within a <muscle spindle>. Its mechanical state is monitored by endings of <sensory neurons>, contributing to <proprioception>, and motor input adjusts that state. The ordinary force-producing <extrafusal muscle fibres> lie outside the <muscle spindle>. Intrafusal contraction therefore helps regulate a sensory apparatus as well as changing local tension.
In a cat soleus preparation, researchers repetitively stimulated a single motor fibre supplying a <muscle spindle>, then imposed a ramp stretch while recording a primary sensory ending. Prior motor stimulation produced an extra burst of impulses at the start of the stretch. A subsequent stretch abolished this after-effect, whereas without that stretch it could persist for more than a minute.<After-effects of fusimotor stimulation on the response of muscle spindle primary afferent endings by M. C. Brown and others>[1969, abstract, observations 1–4]{ref}
Two stretches with the same imposed movement can thus produce different sensory signals because the <intrafusal muscle fibres> have different recent activation histories. The authors proposed persistent interactions between <actin> and <myosin> as a mechanism; that proposal should be distinguished from the directly recorded impulses.<After-effects of fusimotor stimulation on the response of muscle spindle primary afferent endings by M. C. Brown and others>[Abstract, observation 5, proposed intrafusal filament mechanism]{ref}
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