Editing Muscle spindle (section)

{{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}}
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| Image2      =
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| 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.''