= Muscle spindle
A <muscle spindle> is a specialized stretch-sensitive receptor within a <skeletal muscle>. It contains small <intrafusal muscle fibres> with sensory endings that report changes in their mechanical state. This supplies <proprioception>: information about the body's own position and movement, even without seeing the limb.
The receptor is adjustable. Motor input to the <intrafusal muscle fibres> changes tension within the spindle and can alter the response of its <sensory neurons>. In a cat soleus experiment, researchers electrically stimulated an individual motor fibre supplying a <muscle spindle>, then applied a controlled ramp stretch while recording the sensory output. The prior stimulation changed the subsequent burst of impulses.<After-effects of fusimotor stimulation on the response of muscle spindle primary afferent endings by M. C. Brown and others>[1969, abstract: preparation, single-fibre stimulation and ramp-stretch response]{ref}
The same imposed movement can therefore produce different sensory signals depending on the spindle's previous activation. A <muscle spindle> is an actively regulated sensor, rather than a passive ruler reading muscle length independently of neural state.
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