Intrafusal fibers are muscle fibers that comprise the muscle spindle. These fibers are walled off from the rest of the muscle by a collagen sheath. This sheath has a spindle or "fusiform" shape, hence the name "intrafusal." While the intrafusal fibers are wrapped with sensor receptors, their counterpart, extrafusal fibers are the ones responsible for the power-generating component of muscle and are innervated by motor neurons. A top-down view of skeletal muscle Muscle is a contractile form of tissue. ... For the meaning of fiber in nutrition, see dietary fiber. ... [[A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. ... Tropocollagen triple helix. ... Receptor may refer to: In telecommunication, a receiver. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ...

The first of the two main group of stretch receptors wrapping the intrafusal fibers are the Ia fiber, which are the largest and fastest fibers, and they fire when the muscle is stretching. They are characterized by their rapid adaption, because as soon as the muscle stops changing length, the Ia stop firing and adapt to the new length.

The second of the two main groups of stretch receptors are the II fibers, and they are slow-adapting, responding while the muscle is stretching and continuing this response .