Editing Muscle spindle

{{short description|Innervated muscle structure involved in reflex actions and proprioception}}
{{for|the class of neurons characterized by a large spindle-shaped body|Spindle neuron}}
{{Infobox anatomy
| Name        = Muscle spindle
| Latin       = fusus neuromuscularis
| Greek       =
| Image       = muscle spindle model.jpg
| Caption     = Mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in spinal cord (middle) and expanded schematic (right).  The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibres. The sensory endings of a primary (group Ia) afferent and a secondary (group II) afferent coil around the non-contractile central portions of the intrafusal fibres. Gamma motor neurons activate the intrafusal muscle fibres, changing the resting firing rate and stretch-sensitivity of the afferents. {{efn|Animated version: https://www.ualberta.ca/~aprochaz/research_interactive_receptor_model.html Arthur Prochazka's Lab, University of Alberta}}
| Width       =
| Image2      =
| Caption2    =
| Precursor   =
| System      = [[Musculoskeletal]]
| Part_of     = [[Muscle]]
| Artery      =
| Vein        =
| Nerve       =
| Lymph       =
}}
'''Muscle spindles''' are [[stretch receptor]]s within the body of a [[skeletal muscle]] that primarily detect changes in the length of the muscle. They convey length information to the [[central nervous system]] via [[afferent nerve fiber]]s. 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 [[muscle contraction|contraction of muscles]], for example, by activating [[motor neuron]]s 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 fiber]]s which spiral around muscle fibres within the spindle, and secondary [[type II sensory fiber]]s
* Activation of [[muscle fibre]]s within the spindle by up to a dozen [[gamma motor neuron]]s and to a lesser extent by one or two [[beta motor neuron]]s <ref>{{Cite journal |last=Stifani |first=Nicolas |date=2014-10-09 |title=Motor neurons and the generation of spinal motor neuron diversity |journal=Frontiers in Cellular Neuroscience |volume=8 |page=293 |doi=10.3389/fncel.2014.00293 |doi-access=free |issn=1662-5102 |pmc=4191298 |pmid=25346659}}</ref> 
''Recent research has also uncovered unique immune cell populations (e.g: muscle spindle macrophages) and detailed the genetic and molecular pathways regulating spindle formation, offering new insights into proprioceptive regulation and clinical implications in neuromuscular disorders.''

==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>

== Function ==
=== Stretch reflex ===
When a muscle is stretched, primary type Ia sensory fibers of the muscle spindle respond to both changes in muscle length and velocity and transmit this activity to the [[spinal cord]] in the form of changes in the rate of [[action potentials]]. Likewise, secondary type II sensory fibers  respond to muscle length changes (but with a smaller velocity-sensitive component) and transmit this signal to the spinal cord. The Ia afferent signals are transmitted [[Reflex arc#Monosynaptic vs. polysynaptic|monosynaptically]] to many [[alpha motor neurons]] of the receptor-bearing muscle. The reflexly evoked activity in the alpha motor neurons is then transmitted via their efferent axons to the extrafusal fibers of the muscle, which generate force and thereby resist the stretch.  The Ia afferent signal is also transmitted polysynaptically through [[interneurons]] (Ia inhibitory interneurons), which inhibit alpha motorneurons of antagonist muscles, causing them to relax.<ref>{{Cite journal |last1=Mukherjee |first1=Angshuman |last2=Chakravarty |first2=Ambar |date=2010 |title=Spasticity Mechanisms – for the Clinician |journal=Frontiers in Neurology |volume=1 |page=149 |doi=10.3389/fneur.2010.00149 |doi-access=free |issn=1664-2295 |pmc=3009478 |pmid=21206767}}</ref>  

===Sensitivity modification===
The function of the gamma motor neurons is not to supplement the force of muscle contraction provided by the extrafusal fibers, but to modify the sensitivity of the muscle spindle sensory afferents to stretch.  Upon release of [[acetylcholine]] by the active gamma motor neuron, the end portions of the intrafusal muscle fibers contract, thus elongating the non-contractile central portions (see "fusimotor action" schematic below).  This opens stretch-sensitive [[ion channels]] of the sensory endings, leading to an influx of [[sodium]] [[ion]]s.  This raises the [[resting potential]] of the endings, thereby increasing the probability of [[action potential]] firing, thus increasing the stretch-sensitivity of the muscle spindle afferents.

Recent transcriptomic and proteomic studies have identified unique gene expression profiles specific to muscle spindle regions. Distinct macrophage populations, known as muscle spindle macrophages (MSMPs), have been observed, suggesting an immunological component in muscle spindle maintenance and function.<ref>{{Cite journal |last=Yan |first=Yuyang |last2=Antolin |first2=Nuria |last3=Zhou |first3=Luming |last4=Xu |first4=Luyang |last5=Vargas |first5=Irene Lisa |last6=Gomez |first6=Carlos Daniel |last7=Kong |first7=Guiping |last8=Palmisano |first8=Ilaria |last9=Yang |first9=Yi |last10=Chadwick |first10=Jessica |last11=Müller |first11=Franziska |last12=Bull |first12=Anthony M. J. |last13=Lo Celso |first13=Cristina |last14=Primiano |first14=Guido |last15=Servidei |first15=Serenella |date=2025-01-16 |title=Macrophages excite muscle spindles with glutamate to bolster locomotion |url=https://www.nature.com/articles/s41586-024-08272-5 |journal=Nature |language=en |volume=637 |issue=8046 |pages=698–707 |doi=10.1038/s41586-024-08272-5 |issn=0028-0836 |pmc=11735391 |pmid=39633045}}</ref> Immunostaining and sequencing have enabled tissue-level identification of novel markers, contributing to an advanced cellular atlas of the muscle spindle. Regarding the structural-functional correlation; muscle spindle density is not uniform across the musculoskeletal system. Recent biomechanical modeling suggests that spindle abundance correlates with muscle fascicle length and fiber velocity during dynamic movement, emphasizing the relationship between muscle structure and proprioceptive requirements.<ref>{{Cite journal |last1=Kissane |first1=Roger W. P. |last2=Charles |first2=James P. |last3=Banks |first3=Robert W. |last4=Bates |first4=Karl T. |date=2022-06-08 |title=Skeletal muscle function underpins muscle spindle abundance |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=289 |issue=1976 |doi=10.1098/rspb.2022.0622 |issn=0962-8452 |pmc=9156921 |pmid=35642368}}</ref>

How does the central nervous system control gamma fusimotor neurons?  It has been difficult to record from gamma motor neurons during normal movement because they have very small axons. Several theories have been proposed, based on recordings from spindle afferents.
* 1) ''Alpha-gamma coactivation.''  Here it is posited that gamma motor neurons are activated in parallel with alpha motor neurons to maintain the firing of spindle afferents when the extrafusal muscles shorten.<ref>{{cite journal |vauthors=Vallbo AB, al-Falahe NA |title=Human muscle spindle response in a motor learning task |journal=J. Physiol. |volume=421 |pages=553–68 |date=February 1990 |pmid=2140862 |pmc=1190101 |url=http://www.jphysiol.org/cgi/pmidlookup?view=long&pmid=2140862 |doi=10.1113/jphysiol.1990.sp017961}}</ref>
* 2) ''Fusimotor set:'' Gamma motor neurons are activated according to the novelty or difficulty of a task.  Whereas static gamma motor neurons are continuously active during routine movements such as locomotion, dynamic gamma motorneurons tend to be activated more during difficult tasks, increasing Ia stretch-sensitivity.<ref>{{cite book |last=Prochazka |first=A. |chapter=Proprioceptive feedback and movement regulation |editor1-last=Rowell |editor1-first=L. |editor2-last=Sheperd |editor2-first=J.T. |title=Exercise: Regulation and Integration of Multiple Systems |publisher=American Physiological Society |location=New York |year=1996 |isbn=978-0195091748 |pages=89–127 |series=Handbook of physiology }}</ref>
* 3) ''Fusimotor template of intended movement.''  Static gamma activity is a "temporal template" of the expected shortening and lengthening of the receptor-bearing muscle.  Dynamic gamma activity turns on and off abruptly, sensitizing spindle afferents to the onset of muscle lengthening and departures from the intended movement trajectory.<ref>{{cite journal |vauthors=Taylor A, Durbaba R, Ellaway PH, Rawlinson S |title=Static and dynamic gamma-motor output to ankle flexor muscles during locomotion in the decerebrate cat |journal=J. Physiol. |volume=571 |issue=Pt 3 |pages=711–23 |date=March 2006 |pmid=16423858 |pmc=1805796 |doi=10.1113/jphysiol.2005.101634 |url=http://www.jphysiol.org/cgi/pmidlookup?view=long&pmid=16423858}}</ref>
* 4) ''Goal-directed preparatory control.''  Dynamic gamma activity is adjusted proactively during movement preparation in order to facilitate execution of the planned action. For example, if the intended movement direction is associated with stretch of the spindle-bearing muscle, Ia afferent and stretch reflex sensitivity from this muscle is reduced. Gamma fusimotor control therefore allows for the independent preparatory tuning of muscle stiffness according to task goals.<ref>{{cite journal |last1=Papaioannou |first1=S. |last2=Dimitriou |first2=M. |title=Goal-dependent tuning of muscle spindle receptors during movement preparation |journal=Sci. Adv. |date=2021 |volume=7 |issue=9 |pages=eabe0401 |doi=10.1126/sciadv.abe0401 |pmid=33627426 |pmc=7904268 |doi-access=free |bibcode=2021SciA....7..401P }}</ref>

=== Development ===
Genetic pathways critical for spindle formation include neuregulin-1 signaling via ErbB receptors, which induce intrafusal fiber differentiation upon sensory innervation. Disruption of these pathways impairs proprioception, as seen in gene knockout models.<ref>{{Cite web |title=GEO Accession viewer |url=https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE1998 |access-date=2025-04-30 |website=www.ncbi.nlm.nih.gov}}</ref> 

It is also believed that muscle spindles play a critical role in [[Piaget's theory of cognitive development|sensorimotor]] [[developmental psychology|development]]. Additionally, gain-of-function mutations in HRAS (e.g: G12S) observed in Costello syndrome are associated with increased spindle number, providing insight into genetic regulation of spindle density.<ref>{{Cite web |title=VCV000012602.66 - ClinVar - NCBI |url=https://www.ncbi.nlm.nih.gov/clinvar/variation/12602/ |access-date=2025-04-30 |website=www.ncbi.nlm.nih.gov}}</ref> 

==Clinical significance==
Dysfunction in muscle spindle signaling has been implicated in sensory neuropathies and coordination disorders such as ataxia. Enhanced understanding of genetic mutations affecting spindle development (e.g. HRAS and Egr3-linked pathways) opens avenues for targeted therapies in proprioceptive deficits and neuromuscular diseases. 

After [[stroke]] or spinal cord injury in humans, spastic [[hypertonia]] ([[spastic paralysis]]) often develops, whereby the stretch reflex in flexor muscles of the arms and extensor muscles of the legs is overly sensitive.  This results in abnormal postures, stiffness and contractures. Hypertonia may be the result of over-sensitivity of alpha motor neurons and interneurons to the Ia and II afferent signals.<ref>{{cite journal |vauthors=Heckmann CJ, Gorassini MA, Bennett DJ |title=Persistent inward currents in motoneuron dendrites: implications for motor output |journal=Muscle Nerve |volume=31 |issue=2 |pages=135–56 |date=February 2005 |pmid=15736297 |doi=10.1002/mus.20261 |citeseerx=10.1.1.126.3583 |s2cid=17828664 }}</ref>

==Additional images==
<gallery>
Image:MuscleSpindle.svg|Muscle spindle
 Image:Muskelspindel3.png|Gamma fiber
 Image:Muskelspindel4.png|1A fiber
 Image:Muskelspindel5.png|Alpha fiber
Image:Fusimotor action.jpg|schematic of fusimotor action
</gallery>

== Notes ==
{{Notelist}}

== References ==
{{Reflist}}

==External links==
* {{MeSH name|Muscle+Spindles}}

{{Somatosensory system}}
{{Muscle tissue}}
{{Authority control}}

{{DEFAULTSORT:Muscle Spindle}}
[[Category:Sensory systems]]