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.[ref]
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
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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