Muscle spindle
Muscle spindle | |
---|---|
Musculoskeletal | |
Identifiers | |
Latin | fusus neuromuscularis |
MeSH | D009470 |
TH | H3.11.06.0.00018 |
FMA | 83607 |
Anatomical terminology] |
Muscle spindles are stretch receptors 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 fibers. 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 contraction of muscles, for example, by activating motor neurons 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 fibers which spiral around muscle fibres within the spindle, and secondary type II sensory fibers
- Activation of muscle fibres within the spindle by up to a dozen gamma motor neurons and to a lesser extent by one or two beta motor neurons[citation needed]
Structure
Muscle spindles are found within the
Composition
Muscle spindles are composed of 5-14
Primary type Ia sensory fibers (large diameter) spiral around all intrafusal muscle fibres, ending near the middle of each fibre. Secondary type II sensory fibers (medium diameter) end adjacent to the central regions of the static bag and chain fibres.[2] These fibres send information by stretch-sensitive mechanically-gated
The motor part of the spindle is provided by motor neurons: up to a dozen gamma motor neurons also known as fusimotor neurons.[4] 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.
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 bag2 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 bag1 intrafusal muscle fibers. They increase the stretch-sensitivity of the Ia afferents by stiffening the bag1 intrafusal fibers.
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
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
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.[6]
- 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.[7]
- 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.[8]
- 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.[9]
Development
It is also believed that muscle spindles play a critical role in sensorimotor development.
Clinical significance
After
Additional images
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Muscle spindle
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Gamma fiber
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1A fiber
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Alpha fiber
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schematic of fusimotor action
Notes
- ^ Animated version: https://www.ualberta.ca/~aprochaz/research_interactive_receptor_model.html Arthur Prochazka's Lab, University of Alberta
- Golgi tendon organs, which are oriented in series
References
- ^
Mancall, Elliott L; Brock, David G, eds. (2011). "Chapter 2 - Overview of the Microstructure of the Nervous System". Gray's Clinical Neuroanatomy: The Anatomic Basis for Clinical Neuroscience. Elsevier Saunders. pp. 29–30. ISBN 978-1-4160-4705-6.
- ^ a b
Pearson, Keir G; Gordon, James E (2013). "35 - Spinal Reflexes". In Kandel, Eric R; Schwartz, James H; Jessell, Thomas M; Siegelbaum, Steven A; Hudspeth, AJ (eds.). Principles of Neural Science (5th ed.). United States: McGraw-Hill. pp. 794–795. ISBN 978-0-07-139011-8.
- ISBN 9781605353807.
- PMID 29668385.
- )
- PMID 2140862.
- ISBN 978-0195091748.
- PMID 16423858.
- PMID 33627426.
- S2CID 17828664.
External links
- Muscle+Spindles at the U.S. National Library of Medicine Medical Subject Headings (MeSH)