Muscle spindle
545782
218026909
2008-06-08T20:45:01Z
Archibald Tuttle
692814
+ fr
[[Muscle]] structure is innervated by both [[sensory neuron|sensory]] and [[motor neuron]] axons. The muscle spindle's functions are to send [[proprioception|proprioceptive]] information about the muscle to the [[central nervous system]], and to respond to muscle stretching.
== Anatomy ==
Muscle spindles are found within the belly of [[muscle]]s, embedded in [[extrafusal muscle fiber]]s. They are composed of 3-10 [[intrafusal muscle fibers]], of which there are three types:
* dynamic [[nuclear bag fiber]]s (bag<sub>1</sub> fibers)
* static [[nuclear bag fiber]]s (bag<sub>2</sub> fibers)
* [[nuclear chain fiber]]s and the [[axons]] of sensory neurons. [[Axons]] of [[motor neurons]] also terminate in muscle spindles; they make [[synapse]]s at either or both of the ends of the [[intrafusal fibers|intrafusal muscle fibers]] and regulate spindle sensitivity.
Muscle spindles are encapsulated by connective tissue, and are aligned [[Parallel_(geometry)|parallel]] to [[extrafusal muscle fiber]]s, unlike [[Golgi tendon organs]], which are oriented [[Series_(geometry)|in series]].
The muscle spindle has both sensory and motor components.
* [[Type Ia sensory fiber|Primary]] and [[Type II sensory fiber|secondary]] sensory fibers spiral around and terminate on the central portions of intrafusal fibers, providing the sensory component of the structure via stretch-sensitive [[ion-channels]] of the axons.
* In humans, the motor component is provided by [[gamma motoneurons]]; in many other species, beta motoneurons innervate the spindles. They cause a slight contraction of the end portions of the [[intrafusal fibers|intrafusal muscle fibers]] when activated. The gamma (fusimotor) axons only innervate the intrafusal muscle fibers, whereas the beta (skeletofusimotor) axons innervate both extrafusal and intrafusal muscle fibers.
These motorneurons are classified as static or dynamic according to their pattern of innervation and their physiological effects.
* The static axons innervate the chain or bag<sub>2</sub> fibers.
* The dynamic axons innervate the bag<sub>1</sub> fibers and increase the velocity sensitivity of the Ia afferents.
==Sensitivity Modification==
The function of the [[gamma motoneuron|gamma motor neuron]] [[neuromuscular junction]] is not to supplement the general muscle contraction provided by [[extrafusal fibers]], but to modify the sensitivity of the muscle spindle to stretch. Upon release of [[acetylcholine]] by the gamma neuron, the end portions of the [[intrafusal fibers|intrafusal muscle fibers]] contract, thus deliberately elongating the non-contractile central portions of [[intrafusal fibers|intrafusal muscle fibers]]. This opens stretch-sensitive [[ion channels]] of the centrally-positioned sensory axons, leading to an influx of [[sodium]] [[ions]]. This raises the [[resting potential]] of these axons, thereby increasing the probability of [[action potential]] firing, thus increasing the sensitivity of the muscle spindle.
== Stretch reflex ==
When a muscle is stretched, primary sensory fibers ([[Group Ia afferent neurons]]) of the muscle spindle respond to both the velocity and the degree of stretch, and send this information to the [[spinal cord]]. Likewise, secondary sensory fibers ([[Group II afferent neurons]]) detect and send information about the degree of stretch (but not the velocity thereof) to the CNS. This information is transmitted [[monosynaptically]] to an [[alpha motor neuron|alpha efferent motor fiber]], which activates extrafusal fibers of the muscle to contract, thereby reducing stretch, and [[polysynaptically]] through an [[interneuron]] to another [[alpha motoneuron]], which inhibits contraction in the antagonist muscles.
[[PNF stretching]], or proprioceptive neuromuscular facilitation, is a method of flexibility training that reduces the automatic reflex action in order to allow muscles to lengthen.
== Development ==
It is also believed that muscle spindles play a critical role in [[sensorimotor]] [[developmental psychology|development]].
== See also ==
* [[Type Ia sensory fiber]]
* [[Type II sensory fiber]]
==Additional images==
<gallery>
Image:MuscleSpindle.svg|Muscle spindle
Image:Muskelspindel3.png|Gamma fiber
Image:Muskelspindel4.png|1A fiber
Image:Muskelspindel5.png|Alpha fiber
</gallery>
==External links==
* {{MeshName|Muscle+Spindles}}
{{somatosensory system}}
{{Muscle tissue}}
[[Category:Sensory system]]
[[Category:Physiology]]
[[de:Muskelspindel]]
[[fr:Fuseau neuromusculaire]]
[[it:Fuso neuromuscolare]]
[[he:כישור השריר]]
[[nl:Spierspoeltjes]]
[[pt:Fuso muscular]]
[[sv:Muskelspole]]