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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]

M. C. Brown coauthored experiments showing that a muscle spindle's sensory response depends on its recent motor input. In 1969, recordings from the soleus muscle of anaesthetized cats with cut ventral roots paired stimulation of a single motor fibre supplying a muscle spindle with a later stretch. A burst of action potentials that had been absent before stimulation appeared during the testing stretch. The effect could persist for more than a minute, but another stretch abolished it.^[ref]

An illuminating feature was the dependence on conditioning length: starting the test from a shorter muscle length delayed the burst until the stretch passed the length used during stimulation. Thus, identical current lengths need not imply identical sensory output when the mechanical histories differ. The authors proposed persistent bonds between actin and myosin inside the muscle spindle; this was an interpretation of the observations, rather than a direct measurement of those bonds.^[ref]

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.

A sensory neuron carries information about a stimulus toward the central nervous system. Its input can concern the external world or the body's own state. For example, stretching a muscle spindle changes the activity of a sensory neuron; the resulting action potentials help the spinal cord regulate muscle activity through a reflex.^[ref]

Many sensory neurons supplying the body have their soma in a dorsal root ganglion and a branching axon with peripheral and central processes. Thus the soma is not simply an obligatory relay between an input dendrite and an output axon. Other sensory pathways use different arrangements; a sensory function does not specify one universal neuron shape.^[ref]