page_title: Muscle spindle
section_title: Composition
hierarchical_section_title: Muscle spindle / Structure / Composition
caption_attribution_description: English: Light microscope photograph of a muscle spindle. HE stain.
context_page_description: Muscle spindles are stretch receptors within the body of a muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be processed by the brain as proprioception. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, for example, by activating motor neurons via the stretch reflex to resist muscle stretch.
  The muscle spindle has both sensory and motor components.
  Sensory information conveyed by primary type Ia sensory fibers which spiral around muscle fibres within the spindle, and secondary type II sensory fibers
  Activation of muscle fibres within the spindle by up to a dozen gamma motor neurons and to a lesser extent by one or two beta motor neurons
context_section_description: Muscle spindles are composed of 5-14 muscle fibers, of which there are three types: dynamic nuclear bag fibers (bag₁ fibers), static nuclear bag fibers (bag₂ fibers), and nuclear chain fibers.
  Primary type Ia sensory fibers (large diameter) spiral around all intrafusal muscle fibres, ending near the middle of each fibre.
  Secondary type II sensory fibers (medium diameter) end adjacent to the central regions of the static bag and chain fibres.
  These fibres send information by stretch-sensitive mechanically-gated ion-channels of the axons.
  The motor part of the spindle is provided by motor neurons: up to a dozen gamma motor neurons and one or two beta motor neurons, collectively called fusimotor neurons. These activate the muscle fibres within the spindle. Gamma motor neurons supply only muscle fibres within the spindle, whereas beta motor neurons supply muscle fibres both within and outside of the spindle. Activation of the neurons causes a contraction and stiffening of the end parts of the muscle spindle muscle fibers.
  Fusimotor neurons are classified as static or dynamic according to the type of muscle fibers they innervate and their effects on the responses of the Ia and II sensory neurons innervating the central, non-contractile part of the muscle spindle. 
  The static axons innervate the chain or static bag₂ fibers. They increase the firing rate of Ia and II afferents at a given muscle length (see schematic of fusimotor action below).
  The dynamic axons innervate the bag₁ intrafusal muscle fibers. They increase the stretch-sensitivity of the Ia afferents by stiffening the bag₁ intrafusal fibers.
  Efferent nerve fibers of gamma motoneurons also terminate in muscle spindles; they make synapses at either or both of the ends of the intrafusal muscle fibers and regulate the sensitivity of the sensory afferents, which are located in the non-contractile central (equatorial) region.
caption: Light microscope photograph of a muscle spindle. HE stain.
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