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[[File:MuscleSpindle.png|thumb|Muscle Spindle, GTO, and the neurophysiological response to stretch<ref>https://s-media-cache-ak0.pinimg.com/originals/51/d7/26/51d7260400d494a1d14c44734029e65d.png</ref>]]
== Definition ==
[[File:303px-MuscleSpindle.svg.png|alt=|right|frameless]]Almost every muscle contains muscle spindles. These delicate sensory receptors inform the [[Introduction to Neuroanatomy|central nervous system]] (CNS) about changes in the length of individual [[Muscle|muscles]] and the speed of [[stretching]]. With this information, the CNS computes the position and movement of our extremities in space, which is a requirement for [[Motor Control and Learning|motor control]], for maintaining [[posture]] and for a stable [[gait]]. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motor neurons via the stretch reflex to resist muscle stretch<ref name="idiopathic">Kröger S, Watkins B. Muscle spindle function in healthy and diseased muscle. Skeletal muscle. 2021 Dec;11(1):1-3. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7788844/]</ref><ref name="senses">Proske U, Gandevia SC. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2754351/ The kinaesthetic senses]. The Journal of physiology. 2009 Sep 1;587(17):4139-46.</ref>. 


== Definition ==
Image 1: Muscle Spindle (located in muscle belly)  


Muscle spindles are skeletal muscle sensory organs that contribute to fine motor control and provide axial and limb position information to the [[Introduction to Neuroanatomy|central nervous system]]. They are involved in the sensation of position and movement of the body, also known as proprioception <ref name="idiopathic">Grünewald R.A., Yoneda Y., Shipman J.M., Sagar H.J. Idiopathic focal dystonia: a disorder of muscle spindle afferent processing. Brain. 1997;120:2179-2185. Level of evidence: A2</ref><ref name="senses">Proske U., Candevia S.C. The kineasthetic senses. Journal of Physiology. 2009;17:4139-4146. Level of evidence: A1</ref>.  
* Many [[Neuromuscular Disorders|neuromuscular diseases]] affect muscle spindle function contributing, among others, to an unstable gait, frequent [[falls]] and [[Ataxia|ataxic]] behavior in the affected patients.<ref name=":2">Kröger S, Watkins B. [https://skeletalmusclejournal.biomedcentral.com/articles/10.1186/s13395-020-00258-x Muscle spindle function in healthy and diseased muscle.] Skeletal Muscle. 2021 Dec;11(1):1-3.


'''Proprioceptors'''


Proprioceptors are specialised sensory receptors that are located within joints, muscles, and tendons. As these receptors are sensitive to both tension and pressure pressure, they play a role in relaying information concerning muscle dynamics to the conscious and subconscious parts of the central nervous system. They provide the brain with information concerning kinesthetic sense, or conscious appreciation of the position of body parts with respect to gravity. Most of this proprioceptive information is processed at a subconscious level, therefore we do not have to dedicate any conscious activity toward tasks such as maintaining posture or position of body parts<ref name=":0" />. Golgi tendon organs (GTOs) are an example of proprioceptors. They are located in tendons near the myotendinous junction and are in series, that is, attached end to end, with extrafusal muscle fibers<ref name=":0" />.
</ref>
[[File:Spindle- comp.jpg|thumb|370x370px|A mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in spinal cord (middle) and expanded schematic (right).<ref>https://upload.wikimedia.org/wikipedia/commons/thumb/2/28/Muscle_spindle_model.jpg/450px-Muscle_spindle_model.jpg</ref>]]


== Composition  ==
=='''Role in Proprioception'''==
[[File:303px-MuscleSpindle.svg.png|thumb|303x303px]]
[[File:Balance-board-benefits.jpg|right|frameless]]
Muscle spindles are small sensory organs with an elongated shape. They are proprioceptors that consist of several modified muscle fibers enclosed in a sheath of connective tissue<ref name=":0" />.  These modified fibers are called intrafusal fibers. These&nbsp;fibers&nbsp;are oriented parallel to the regular, power-producing extrafusal muscle fibers. Intrafusal muscle fibers are at both ends connected to either tendinous ligaments or extrafusal fibers, namely contractile proteins <ref name="senses" />.&nbsp;So, intrafusal fibers are stretched or shortened correspondingly, when extrafusal fibres change length. The central part of the muscle spindle is covered with a capsule of connective tissue. The sensory dendrites of the muscle spindle afferent wrap the central region. The muscle spindle is stretched when the muscle lengthens increase, this opens mechanically-gated ion channels in the sensory dendrites. This leads to a receptor potential that triggers action potentials in the muscle spindle afferent <ref name="Unusual">Soukup T., Thernell L.-E. Unusual intrafusal fibres in human muscle spindles. Physiol. Res. 1999;48:519-523. Level of evidence: B2</ref><ref name="spasticity">Mukherjee A., Chakravarty Ambar. Spasticity mechanisms for the clinician. Frontiers in Neurology. 2010;1:149:1-10. Level of evidence: A1</ref>.   
Although Golgi tendon organs, joint receptors and other [[Sensation|sensory systems]] also contribute to [[proprioception]], muscle spindles are the most important proprioceptors.   


There are two types of sensory endings found in muscle spindles: the primary and secondary endings of spindles, which are located in the middle of the spindle. The primary endings respond to its speed and the size of a muscle length change. They belong to the fastest axons as they are myelinated. They contribute both to movement and the sense of limb position. Secondary endings are only sensitive to length and not to velocity, so they contribute only to the sense of the position. These endings have smaller axons and thus slower conduction speed. Both endings in muscle spindles are very sensitive to low-amplitude changes in muscle length, especially if these changes occur at a high frequency. A spindle ending is located at the end of a neuron or an axon whose body is in the spinal ganglion <ref name="senses" /><ref name="changes" />.  
* Muscle spindles are the most frequently found sense organs in skeletal [[Muscle Cells (Myocyte)|muscles]] and present in almost every muscle.
* The density of muscle spindles within the large muscle mass, however, is low so that they are rather difficult to detect. Rough estimates have suggested approximately 50,000 muscle spindles in the entire human body (interestingly, in humans, muscle spindles are mostly absent in facial muscles).<ref name=":2" />
Two important proprioceptors that play a role in flexibility are the muscle spindle and the golgi tendon organ (GTO), together reflexively work to regulate muscle stiffness.  


<br>
The function of the GTO can be considered opposite of the muscle spindle, which serves to produce muscle contraction. When a GTO is stimulated, it causes its associated muscle to relax by interrupting its contraction.<ref name=":3">Golgi Tendon Organs and Muscle Spindles Explained. Available from:https://www.acefitness.org/fitness-certifications/ace-answers/exam-preparation-blog/5336/golgi-tendon-organs-and-muscle-spindles-explained/ (Accessed, 17.5.2021).</ref>
[[File:StretchRelfexKnee.jpg|thumb|Sequence of events of Stretch reflex in the knee<ref>http://www.dragonsociety.com/the-stretch-reflex/</ref>]]


== Function  ==
== Composition ==
[[File:Spindle- comp.jpg|A mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in spinal cord (middle) and expanded schematic (right).|alt=|right|frameless|429x429px]]Muscle spindles are small sensory organs with an elongated shape, involved in proprioception.   


Muscle spindles stimulate reflexively a muscle contraction to prevent overstretching and muscle fiber damage, this is known as the stretch or myotatic reflex <ref name="idiopathic" />. While stretching the muscle spindle, an impulse is immediately sent to the spinal cord and a response to contract the muscle is received, for protecting it from being pulled forcefully or beyond a normal range. It is a very quick impulse, because the impulse only has to go to the spinal cord and back <ref name="spasticity" />. The stretch reflex has two components. The static component lasts as long as the muscle is being stretched. The dynamic component lasts for only a moment in response to the initial sudden increase in muscle length. The purpose of muscle spindles and the stretch reflex is to protect your body from injury caused by overstretching and to maintain muscle tone <ref name="changes">Hospod V., Aimonetti J.-M., Roll J.-P., Ribot-Ciscar E. Changes in human muscle spindle sensitivity during proprioceptive attention task. The Journal of Neuroscience.2001;27:19:5172-5178. Level of evidence: A2</ref>. This is a process that inhibits the stretch reflex in antagonistic pairs of muscles. An impulse is sent from the stretched muscle spindle when the stretch reflex is activated, the motor neuron is split so that the signal to contract can be sent to the stretched muscle, while a signal to relax can be sent to the antagonist muscles <ref>Kakuda N., Nagaoka M. Dynamic response of human muscle spindle afferents to stretch during voluntary contraction. Journal of Physiology. 1998;513:2:621-628. Level of evidence: A2</ref>.  
Image 2: Mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in [[Spinal cord anatomy|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.   
 
* Consist of several modified muscle fibers enclosed in a sheath of [[Connective Tissue Disorders|connective tissue]]<ref name=":0" />.    
* The modified fibers are called intrafusal fibers. These&nbsp;fibers&nbsp;are oriented parallel to the regular, power-producing extrafusal muscle fibers.   
* Multiply innervated and named according to the arrangement of their nuclei as nuclear bag or nuclear chain fibers. 
* Intrafusal muscle fibers are up to 8-mm long in humans. Each muscle spindle contains on average 8–20 (human) intrafusal fibers. With a diameter of 8 to 25 μm [30], intrafusal muscle fibers are much thinner than extrafusal muscle fibers<ref name=":2" />  
 
== Function ==
[[File:Reflex.jpg|right|frameless|532x532px]]
Imagine a muscle spindle as if it were a thread spiraled (or wrapped around) muscle fibers near the muscle belly; as the muscle lengthens or stretches, it pulls on the spindle causing it to lose its spiral shape and also stretch. This signals the muscle to contract (after which, the spiral regains its shape), in turn protecting the muscle from being overstretched. This process is called the stretch [[Reflexes|reflex]]. 
 
When a muscle spindle’s associated muscle is rapidly stretched, the spindle can cause two things to happen:
 
* Functionally, muscle spindles are stretch detectors, and sense  how much and how fast a muscle is lengthened or shortened. Accordingly, when a muscle is stretched, this change in length is transmitted to the spindles and their intrafusal fibers which are subsequently similarly stretched <ref name=":2" />.  
* It may signal its muscle to contract to prevent it from going too far, too quickly in the stretch. The stimulation of a reflexive  muscle contraction is known as the stretch or myotatic [[Reflexes|reflex]] <ref name="idiopathic" />.
* It can inhibit the opposing muscle, ie the antagonist to the muscle being stretched, to prevent it from contracting so that it can’t contribute to any further stretching (ie [[Stretching|reciprocal inhibition]]) .
* In humans, the sensory innervation of the muscle spindle arises from both group Ia and group II afferent fibers which differ in their axonal conduction velocity. The sensory terminals form irregular coils with branches and varicose swellings.
* The cell bodies of these proprioceptive afferent fibers constitute 5–10% of all neurons in the dorsal root ganglion <ref name=":2" /> .
 
Ultimately, the muscle spindle functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far. These are important concepts in understanding body awareness (also known as proprioception and kinesthetic awareness)<ref name=":3" />.  


Spindles thus indicate the degree to which the muscle must be activated in order to overcome a given resistance. As a load increases, the muscle is stretched to a greater extent, and engagement of muscle spindles results in greater activation of the muscle. Muscles that perform precise movements have many spindles per unit of mass to help ensure exact control of their contractile activity<ref name=":0" />.  
Spindles thus indicate the degree to which the muscle must be activated in order to overcome a given resistance. As a load increases, the muscle is stretched to a greater extent, and engagement of muscle spindles results in greater activation of the muscle. Muscles that perform precise movements have many spindles per unit of mass to help ensure exact control of their contractile activity<ref name=":0" />.  
{{#ev:youtube|442UbX9qeLk|300}}<ref>https://www.youtube.com/watch?v=442UbX9qeLk</ref>


{{#ev:youtube|442UbX9qeLk |300}}<ref > Animated Anatomy. Muscle Spindle - Muscle Stretch Reflex.  Available from: https://www.youtube.com/watch?v=442UbX9qeLk [Accessed, 08/10/2021] </ref>


[[File:Knee-jerk.jpg|thumb|352x352px|Knee-jerk reflex and motor-neuron connection<ref>https://media1.britannica.com/eb-media/62/141362-004-BB0AD15A.jpg</ref>]]


== Example  ==
A simple example of muscle spindle activity is the knee jerk reflex (Patellar reflex), sudden kicking movement of the lower leg in response to a sharp tap on the patellar tendon, which lies just below the kneecap<ref name=":0">Brukner P. Brukner & Khan's clinical sports medicine. North Ryde: McGraw-Hill; 2012.</ref>. Tapping on the tendon of the knee extensor muscle group below the patella stretches the muscle spindle fibers. This causes activation of extrafusal muscle fibers in the same muscle. A knee jerk occurs as these fibers actively shorten. This, in turn, shortens the intrafusal fibers and causes their discharge to cease<ref name=":0" />.


One of the most common positions that a health care professional will ask a patient to take for the test is to sit with knees bent and with one leg crossed over the other so that the upper foot hangs clear of the floor. The sharp tap on the tendon causes a stretch to the quadriceps. In reaction these muscles contract, and the contraction elicits knee extension or straighten the leg in a kicking motion. Exaggeration or absence of the reaction suggests that there may be damage to the central nervous system. The knee jerk can also be helpful in recognising thyroid disease.<ref>https://www.britannica.com/topic/knee-jerk-reflex</ref>
'''Example'''


== Dysfunction in the Muscle Spindle  ==
A simple example of muscle spindle activity is the knee jerk reflex (Patellar reflex), sudden kicking movement of the lower leg in response to a sharp tap on the patellar tendon, which lies just below the kneecap<ref name=":0">Brukner P. Brukner & Khan's clinical sports medicine. North Ryde: McGraw-Hill; 2012.</ref>. Tapping on the tendon of the [[Quadriceps Muscle|knee extensor muscle group]] below the [[patella]] stretches the muscle spindle fibers. This causes activation of extrafusal muscle fibers in the same muscle. A knee jerk occurs as these fibers actively shorten. This, in turn, shortens the intrafusal fibers and causes their discharge to cease<ref name=":0" />.
Problems may arise in the functioning of the muscle spindle. An upper motor neuron lesion can lead to a loss of supraspinal inhibition. A basal ganglia disorder can cause excessive supraspinal activation<ref name=":1">https://kin450-neurophysiology.wikispaces.com/Muscle+Spindle</ref>. If there is a problem with the muscles spindle it may present as abnormal muscle tone such as spasticity (a velocity dependent increase in resistance to passive stretch which causes exaggerated tendon reflexes called hypereflexia)<ref>Young RR. Spasticity: a review. Neurology. 1994 Nov;44(11 Suppl 9):S12-20.
</ref>. There are several causes of spasticity associated with the muscle spindle. First, overactive input from gamma motor neurons or increased excitability at the central synapse can present as spasticity. This usually occurs due cortical damage and a loss of inhibitory impulses. Secondly, spasticity may be caused by problems with the renshaw cells. Renshaw cells are interneurons that are stimulated by the alpha motor neuron and then, by a feedback mechanism, inhibit the alpha motor neuron, causing auto inhibition.<ref>Thomas RC, Wilson VJ. Precise localization of Renshaw cells with a new marking technique. Nature. 1965 Apr 10;206(4980):211-3.</ref> Problems with the renshaw cells lead to a loss of inhibition on the alpha motor neurons so they simply continue firing. Thirdly, spasticity can arise from a pre-synaptic inhibition of Ia afferent. Finally, it can arise from a neuro-related or structural change in the muscle fibers.<ref name=":1" />


It is often believed that dysfunction caused by strokes and movements disorders, such as muscular dystrophy, also affect the function of muscle spindles. For example, strokes are known to cause weakness, loss of dexterity, and exaggerated reflex response to proprioceptive and cutaneous stimuli due to problems with the gamma motor system. However current research shows that the gamma motor system is not responsible for the deficits of the skeletalomotor system. The discharge rate of muscle spindles in stroke patients was found to be similar to normal subjects. There was no difference in the reflex response to peripheral afferent inputs or in the response to supraspinal drive<ref>Wilson LR, Gandevia SC, Inglis JT, Gracies JM, Burk D. Muscle spindle activity in the affected upper limb after a unilateral stroke. ''Brain: A Journal of Neurology''. 1999; 122: 2079-2088.</ref>.
See [[Reflexes]]


Muscular dystrophy is characterized by the degeneration of skeletal muscle fibers. Research examining the affect of the degeneration on intrafusal fibers have found that the proprioceptive function of muscle spindles is spared in muscular dystrophy. Research confirmed that muscular dystrophy patients perceived passive movements and experienced illusory movements similar to those perceived by healthy subjects in terms of the movement direction and velocity. They also found that muscular dystrophy patients were able to respond with similar spatial and temporal movement characteristics when compared with the normal subjects<ref>Ciscar ER, Trefouret S, Aimonetti JM, Attarian S, Pouget J, Roll JP. Is muscle spindle proprioceptive function spared in muscular dystrophies? A muscle tendon vibration study. ''Muscle and Nerve.'' 2004; 29(6): 861-866.</ref>.  
== Dysfunctions in the Muscle Spindle  ==
Problems may arise in the functioning of the muscle spindle. An upper motor neuron lesion can lead to a loss of supraspinal inhibition. A basal ganglia disorder can cause excessive supraspinal activation<ref name=":1">https://kin450-neurophysiology.wikispaces.com/Muscle+Spindle</ref>. If there is a problem with the muscles spindle it may present as abnormal muscle tone such as spasticity (a velocity dependent increase in resistance to passive stretch which causes exaggerated tendon reflexes called hypereflexia)<ref>Young RR. [https://europepmc.org/article/med/7970006 Spasticity: a review]. Neurology. 1994 Nov;44(11 Suppl 9):S12-20.
</ref>. There are several causes of spasticity associated with the muscle spindle:
* Overactive input from gamma motor neurons or increased excitability at the central synapse can present as spasticity. This usually occurs due to cortical damage and a loss of inhibitory impulses.
* Spasticity may be caused by problems with the renshaw cells. Renshaw cells are interneurons that are stimulated by the alpha motor neuron and then, by a feedback mechanism, inhibit the alpha motor neuron, causing autoinhibition.<ref>Thomas RC, Wilson VJ. [https://www.nature.com/articles/206211b0 Precise localization of Renshaw cells with a new marking technique.] Nature. 1965 Apr 10;206(4980):211-3.</ref> Problems with the renshaw cells lead to a loss of inhibition on the alpha motor neurons so they simply continue firing.
* Spasticity can arise because of the loss of appropriate pre-synaptic inhibition of Ia afferent.<ref>Mukherjee A, Chakravarty A. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3009478/ Spasticity mechanisms–for the clinician.] Frontiers in neurology. 2010;1.</ref>
* It can also arise from a neuro-related or structural change in the muscle fibers.<ref name=":1" />
It is often believed that dysfunction caused by [[Stroke|strokes]] and [https://physio-pedia.com/Parkinson's_Disease_-_Clinical_Presentation movement disorders], such as [[Muscle Disorders|muscular dystrophy]], also affect the function of muscle spindles. For example, strokes are known to cause weakness, loss of dexterity, and exaggerated reflex response to proprioceptive and cutaneous stimuli due to problems with the gamma motor system. However, current research shows that the gamma motor system is not responsible for the deficits of the skeletal-motor system. The discharge rate of muscle spindles in stroke patients was found to be similar to normal subjects. There was no difference in the reflex response to peripheral afferent inputs or in the response to supraspinal drive<ref>Wilson LR, Gandevia SC, Inglis JT, Gracies JM, Burke D. [https://doi.org/10.1093/brain/122.11.2079 Muscle spindle activity in the affected upper limb after a unilateral stroke]. Brain. 1999 Nov 1;122(11):2079-88.</ref>.


== Resources ==
On the structural level, muscle spindles in [[Older People Introduction|aged humans]] possess fewer intrafusal fibers, an increased capsular thickness and some spindles which show signs of denervation. Together with the proprioceptive system in general and with ageing, significant structural and functional changes occur and the changes are consistent with a gradual decline in proprioceptive function in elderly individuals and animals. These changes might contribute to the frequent falls and motor control problems observed in older adults <ref name=":2" />.


[http://www.slideshare.net/biotechvictor1950/physiology-of-muscle-contraction-17064052?qid=356470bf-d9b7-4e5a-8528-d8e88cf3ac0c&v=qf1&b=&from_search=7 The Physiology of Muscle Contraction] by St. Xavier's College <div class="researchbox"></div>  
An impaired proprioception, in some cases associated with an altered muscle spindle morphology, has been documented as a secondary effect in many diseases. In any neuromuscular disease, therapeutic strategies should therefore also aim at restoring/maintaining proprioception and muscle spindle function.<ref name=":2" /> For example, [[Muscular Dystrophy|Muscular dystrophy]] is characterized by the degeneration of skeletal muscle fibers. Research examining the effect of the degeneration on intrafusal fibers have found that the proprioceptive function of muscle spindles is spared in muscular dystrophy. Research confirmed that muscular dystrophy patients perceived passive movements and experienced illusory movements similar to those perceived by healthy subjects in terms of the movement direction and velocity. They also found that muscular dystrophy patients were able to respond with similar spatial and temporal movement characteristics when compared with the normal subjects<ref>Ribot‐Ciscar E, Tréfouret S, Aimonetti JM, Attarian S, Pouget J, Roll JP. [https://doi.org/10.1002/mus.20044 Is muscle spindle proprioceptive function spared in muscular dystrophies? A muscle tendon vibration study.] Muscle & Nerve: Official Journal of the American Association of Electrodiagnostic Medicine. 2004 Jun;29(6):861-6.</ref>. Common therapeutic interventions for patients with muscular dystrophy should aim at increasing muscle strength and reducing muscle fatigue and degeneration. Moreover, therapeutic strategies should also aim at restoring/maintaining proprioception and muscle spindle function in any neuromuscular disease <ref name=":2" />.


== References  ==
== References  ==


<references /><br>  
<references /><br>
 
[[Category:Muscles]]
[[Category:Muscles]] [[Category:Anatomy]] [[Category:Neurology]] [[Category:Cerebral_Palsy]] [[Category:Musculoskeletal/Orthopaedics]]
[[Category:Anatomy]]
[[Category:Neurology]]
[[Category:Cerebral Palsy]]
[[Category:Musculoskeletal/Orthopaedics]]
[[Category:Sports Medicine]]

Latest revision as of 12:46, 3 June 2023

Original Editor - Bo Hellinckx

Top Contributors - Tania Appelmans, Adam Vallely Farrell, Kirenga Bamurange Liliane, Lucinda hampton, Laura Ritchie, Vidya Acharya, Oyemi Sillo, Bo Hellinckx, Scott Buxton, Cindy John-Chu, Ahmed M Diab, Rachael Lowe, Evan Thomas, WikiSysop, Kim Jackson and Wanda van Niekerk

Definition[edit | edit source]

Almost every muscle contains muscle spindles. These delicate sensory receptors inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. With this information, the CNS computes the position and movement of our extremities in space, which is a requirement for motor control, for maintaining posture and for a stable gait. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motor neurons via the stretch reflex to resist muscle stretch[1][2].

Image 1: Muscle Spindle (located in muscle belly)

Role in Proprioception[edit | edit source]

Balance-board-benefits.jpg

Although Golgi tendon organs, joint receptors and other sensory systems also contribute to proprioception, muscle spindles are the most important proprioceptors.

  • Muscle spindles are the most frequently found sense organs in skeletal muscles and present in almost every muscle.
  • The density of muscle spindles within the large muscle mass, however, is low so that they are rather difficult to detect. Rough estimates have suggested approximately 50,000 muscle spindles in the entire human body (interestingly, in humans, muscle spindles are mostly absent in facial muscles).[3]

Two important proprioceptors that play a role in flexibility are the muscle spindle and the golgi tendon organ (GTO), together reflexively work to regulate muscle stiffness.

The function of the GTO can be considered opposite of the muscle spindle, which serves to produce muscle contraction. When a GTO is stimulated, it causes its associated muscle to relax by interrupting its contraction.[4]

Composition[edit | edit source]

Muscle spindles are small sensory organs with an elongated shape, involved in proprioception.

Image 2: 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.

  • Consist of several modified muscle fibers enclosed in a sheath of connective tissue[5]. 
  • The modified fibers are called intrafusal fibers. These fibers are oriented parallel to the regular, power-producing extrafusal muscle fibers.
  • Multiply innervated and named according to the arrangement of their nuclei as nuclear bag or nuclear chain fibers.
  • Intrafusal muscle fibers are up to 8-mm long in humans. Each muscle spindle contains on average 8–20 (human) intrafusal fibers. With a diameter of 8 to 25 μm [30], intrafusal muscle fibers are much thinner than extrafusal muscle fibers[3]

Function[edit | edit source]

Reflex.jpg

Imagine a muscle spindle as if it were a thread spiraled (or wrapped around) muscle fibers near the muscle belly; as the muscle lengthens or stretches, it pulls on the spindle causing it to lose its spiral shape and also stretch. This signals the muscle to contract (after which, the spiral regains its shape), in turn protecting the muscle from being overstretched. This process is called the stretch reflex.

When a muscle spindle’s associated muscle is rapidly stretched, the spindle can cause two things to happen:

  • Functionally, muscle spindles are stretch detectors, and sense how much and how fast a muscle is lengthened or shortened. Accordingly, when a muscle is stretched, this change in length is transmitted to the spindles and their intrafusal fibers which are subsequently similarly stretched [3].
  • It may signal its muscle to contract to prevent it from going too far, too quickly in the stretch. The stimulation of a reflexive muscle contraction is known as the stretch or myotatic reflex [1].
  • It can inhibit the opposing muscle, ie the antagonist to the muscle being stretched, to prevent it from contracting so that it can’t contribute to any further stretching (ie reciprocal inhibition) .
  • In humans, the sensory innervation of the muscle spindle arises from both group Ia and group II afferent fibers which differ in their axonal conduction velocity. The sensory terminals form irregular coils with branches and varicose swellings.
  • The cell bodies of these proprioceptive afferent fibers constitute 5–10% of all neurons in the dorsal root ganglion [3] .

Ultimately, the muscle spindle functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far. These are important concepts in understanding body awareness (also known as proprioception and kinesthetic awareness)[4].

Spindles thus indicate the degree to which the muscle must be activated in order to overcome a given resistance. As a load increases, the muscle is stretched to a greater extent, and engagement of muscle spindles results in greater activation of the muscle. Muscles that perform precise movements have many spindles per unit of mass to help ensure exact control of their contractile activity[5].


[6]


Example

A simple example of muscle spindle activity is the knee jerk reflex (Patellar reflex), sudden kicking movement of the lower leg in response to a sharp tap on the patellar tendon, which lies just below the kneecap[5]. Tapping on the tendon of the knee extensor muscle group below the patella stretches the muscle spindle fibers. This causes activation of extrafusal muscle fibers in the same muscle. A knee jerk occurs as these fibers actively shorten. This, in turn, shortens the intrafusal fibers and causes their discharge to cease[5].

See Reflexes

Dysfunctions in the Muscle Spindle[edit | edit source]

Problems may arise in the functioning of the muscle spindle. An upper motor neuron lesion can lead to a loss of supraspinal inhibition. A basal ganglia disorder can cause excessive supraspinal activation[7]. If there is a problem with the muscles spindle it may present as abnormal muscle tone such as spasticity (a velocity dependent increase in resistance to passive stretch which causes exaggerated tendon reflexes called hypereflexia)[8]. There are several causes of spasticity associated with the muscle spindle:

  • Overactive input from gamma motor neurons or increased excitability at the central synapse can present as spasticity. This usually occurs due to cortical damage and a loss of inhibitory impulses.
  • Spasticity may be caused by problems with the renshaw cells. Renshaw cells are interneurons that are stimulated by the alpha motor neuron and then, by a feedback mechanism, inhibit the alpha motor neuron, causing autoinhibition.[9] Problems with the renshaw cells lead to a loss of inhibition on the alpha motor neurons so they simply continue firing.
  • Spasticity can arise because of the loss of appropriate pre-synaptic inhibition of Ia afferent.[10]
  • It can also arise from a neuro-related or structural change in the muscle fibers.[7]

It is often believed that dysfunction caused by strokes and movement disorders, such as muscular dystrophy, also affect the function of muscle spindles. For example, strokes are known to cause weakness, loss of dexterity, and exaggerated reflex response to proprioceptive and cutaneous stimuli due to problems with the gamma motor system. However, current research shows that the gamma motor system is not responsible for the deficits of the skeletal-motor system. The discharge rate of muscle spindles in stroke patients was found to be similar to normal subjects. There was no difference in the reflex response to peripheral afferent inputs or in the response to supraspinal drive[11].

On the structural level, muscle spindles in aged humans possess fewer intrafusal fibers, an increased capsular thickness and some spindles which show signs of denervation. Together with the proprioceptive system in general and with ageing, significant structural and functional changes occur and the changes are consistent with a gradual decline in proprioceptive function in elderly individuals and animals. These changes might contribute to the frequent falls and motor control problems observed in older adults [3].

An impaired proprioception, in some cases associated with an altered muscle spindle morphology, has been documented as a secondary effect in many diseases. In any neuromuscular disease, therapeutic strategies should therefore also aim at restoring/maintaining proprioception and muscle spindle function.[3] For example, Muscular dystrophy is characterized by the degeneration of skeletal muscle fibers. Research examining the effect of the degeneration on intrafusal fibers have found that the proprioceptive function of muscle spindles is spared in muscular dystrophy. Research confirmed that muscular dystrophy patients perceived passive movements and experienced illusory movements similar to those perceived by healthy subjects in terms of the movement direction and velocity. They also found that muscular dystrophy patients were able to respond with similar spatial and temporal movement characteristics when compared with the normal subjects[12]. Common therapeutic interventions for patients with muscular dystrophy should aim at increasing muscle strength and reducing muscle fatigue and degeneration. Moreover, therapeutic strategies should also aim at restoring/maintaining proprioception and muscle spindle function in any neuromuscular disease [3].

References[edit | edit source]

  1. ↑ 1.0 1.1 Kröger S, Watkins B. Muscle spindle function in healthy and diseased muscle. Skeletal muscle. 2021 Dec;11(1):1-3. [1]
  2. ↑ Proske U, Gandevia SC. The kinaesthetic senses. The Journal of physiology. 2009 Sep 1;587(17):4139-46.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Kröger S, Watkins B. Muscle spindle function in healthy and diseased muscle. Skeletal Muscle. 2021 Dec;11(1):1-3.
  4. ↑ 4.0 4.1 Golgi Tendon Organs and Muscle Spindles Explained. Available from:https://www.acefitness.org/fitness-certifications/ace-answers/exam-preparation-blog/5336/golgi-tendon-organs-and-muscle-spindles-explained/ (Accessed, 17.5.2021).
  5. ↑ 5.0 5.1 5.2 5.3 Brukner P. Brukner & Khan's clinical sports medicine. North Ryde: McGraw-Hill; 2012.
  6. ↑ Animated Anatomy. Muscle Spindle - Muscle Stretch Reflex. Available from: https://www.youtube.com/watch?v=442UbX9qeLk [Accessed, 08/10/2021]
  7. ↑ 7.0 7.1 https://kin450-neurophysiology.wikispaces.com/Muscle+Spindle
  8. ↑ Young RR. Spasticity: a review. Neurology. 1994 Nov;44(11 Suppl 9):S12-20.
  9. ↑ Thomas RC, Wilson VJ. Precise localization of Renshaw cells with a new marking technique. Nature. 1965 Apr 10;206(4980):211-3.
  10. ↑ Mukherjee A, Chakravarty A. Spasticity mechanisms–for the clinician. Frontiers in neurology. 2010;1.
  11. ↑ Wilson LR, Gandevia SC, Inglis JT, Gracies JM, Burke D. Muscle spindle activity in the affected upper limb after a unilateral stroke. Brain. 1999 Nov 1;122(11):2079-88.
  12. ↑ Ribot‐Ciscar E, Tréfouret S, Aimonetti JM, Attarian S, Pouget J, Roll JP. Is muscle spindle proprioceptive function spared in muscular dystrophies? A muscle tendon vibration study. Muscle & Nerve: Official Journal of the American Association of Electrodiagnostic Medicine. 2004 Jun;29(6):861-6.


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