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.^[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.

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]