Editing Muscle spindle (section)

===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>