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.[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.[ref]
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
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