Editing Muscle spindle (section)

==Structure==
Muscle spindles are found within the [[muscle belly|belly]] of a [[skeletal muscle]]. Muscle spindles are [[fusiform]] (spindle-shaped), and the specialized fibers that make up the muscle spindle are called [[intrafusal muscle fiber]]s. The regular muscle fibers outside of the spindle are called [[extrafusal muscle fiber]]s. Muscle spindles have a capsule of [[connective tissue]], and run parallel to the extrafusal muscle fibers unlike [[Golgi tendon organs]] which are oriented in series.{{cn|date=August 2024}}

=== Composition ===
Muscle spindles are composed of 5–14 [[intrafusal muscle fibers|muscle fibers]], of which there are three types: dynamic [[nuclear bag fiber]]s (bag<sub>1</sub> fibers), static nuclear bag fibers (bag<sub>2</sub> fibers), and [[nuclear chain fiber]]s.<ref>
{{cite book | editor-last1 = Mancall | editor-first1 = Elliott L | editor-last2 = Brock | editor-first2 = David G | year = 2011 | title = Gray's Clinical Neuroanatomy: The Anatomic Basis for Clinical Neuroscience | chapter = Chapter 2 - Overview of the Microstructure of the Nervous System | publisher = Elsevier Saunders | isbn = 978-1-4160-4705-6 | pages = 29–30 }}</ref><ref name="PearsonGordon2013">
{{cite book | last1 = Pearson | first1 = Keir G | last2 = Gordon | first2 = James E | year = 2013 | title = Principles of Neural Science | edition = 5th | editor-last1 = Kandel | editor-first1 = Eric R | editor-last2 = Schwartz | editor-first2 = James H | editor-last3 = Jessell | editor-first3 = Thomas M | editor-last4 = Siegelbaum | editor-first4 = Steven A | editor-last5 = Hudspeth | editor-first5 = AJ | location = United States | publisher = McGraw-Hill | isbn = 978-0-07-139011-8 | chapter = 35 - Spinal Reflexes | pages = 794–795 }}</ref>

[[File:Muscle Spindle LM HE stain.jpg|thumb|right|alt=A|Light microscope photograph of a muscle spindle with [[H&E stain]]]]

Primary [[type Ia sensory fiber]]s (large diameter) spiral around all intrafusal muscle fibres, ending near the middle of each fibre.
Secondary [[type II sensory fiber]]s (medium diameter) end adjacent to the central regions of the static bag and chain fibres.<ref name="PearsonGordon2013" />
These fibres send information by stretch-sensitive mechanically-gated [[ion-channels]] of the [[axon]]s.<ref>{{cite book | year = 2018 | title = Neuroscience | edition = 6th | editor-last1 = Purves | editor-first1 = Dale | editor-last2 = Augustine | editor-first2 = George J | editor-last3 = Fitzpatrick | editor-first3 = David | editor-last4 = Hall | editor-first4 = William C | editor-last5 = Lamantia | editor-first5 = Anthony Samuel | editor-last6 = Mooney | editor-first6 = Richard D | editor-last7 = Platt | editor-first7 = Michael L | editor-last8 = White | editor-first8 = Leonard E | publisher = Sinauer Associates | isbn = 9781605353807 | chapter = Chapter 9 - The Somatosensory System: Touch and Proprioception  | pages = 201–202 }}</ref>

The motor part of the spindle is provided by motor neurons: up to a dozen [[gamma motor neuron]]s also known as ''fusimotor neurons''.<ref name="Macefield">{{cite journal |last1=Macefield |first1=VG |last2=Knellwolf |first2=TP |title=Functional properties of human muscle spindles. |journal=Journal of Neurophysiology |date=1 August 2018 |volume=120 |issue=2 |pages=452–467 |doi=10.1152/jn.00071.2018 |pmid=29668385|doi-access=free }}</ref> 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.{{cn|date=August 2024}}

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 bag<sub>2</sub> 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 bag<sub>1</sub> intrafusal muscle fibers. They increase the stretch-sensitivity of the Ia afferents by stiffening the bag<sub>1</sub> intrafusal fibers.

[[Efferent nerve fiber]]s of [[gamma motor neurons]] also terminate in muscle spindles; they make [[synapse]]s 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.<ref>{{cite book|author=Hulliger M|title=Reviews of Physiology, Biochemistry and Pharmacology, Volume 86|year=1984|chapter=The mammalian muscle spindle and its central control|journal=Rev. Physiol. Biochem. Pharmacol.|volume=101|pages=1–110|doi=10.1007/bfb0027694|pmid=6240757|isbn=978-3-540-13679-8}}</ref>