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Experimental Physiology Jan 2024Muscle spindles encode mechanosensory information by mechanisms that remain only partially understood. Their complexity is expressed in mounting evidence of various... (Review)
Review
Muscle spindles encode mechanosensory information by mechanisms that remain only partially understood. Their complexity is expressed in mounting evidence of various molecular mechanisms that play essential roles in muscle mechanics, mechanotransduction and intrinsic modulation of muscle spindle firing behaviour. Biophysical modelling provides a tractable approach to achieve more comprehensive mechanistic understanding of such complex systems that would be difficult/impossible by more traditional, reductionist means. Our objective here was to construct the first integrative biophysical model of muscle spindle firing. We leveraged current knowledge of muscle spindle neuroanatomy and in vivo electrophysiology to develop and validate a biophysical model that reproduces key in vivo muscle spindle encoding characteristics. Crucially, to our knowledge, this is the first computational model of mammalian muscle spindle that integrates the asymmetric distribution of known voltage-gated ion channels (VGCs) with neuronal architecture to generate realistic firing profiles, both of which seem likely to be of great biophysical importance. Results predict that particular features of neuronal architecture regulate specific characteristics of Ia encoding. Computational simulations also predict that the asymmetric distribution and ratios of VGCs is a complementary and, in some instances, orthogonal means to regulate Ia encoding. These results generate testable hypotheses and highlight the integral role of peripheral neuronal structure and ion channel composition and distribution in somatosensory signalling.
Topics: Animals; Muscle Spindles; Mechanotransduction, Cellular; Neurons; Ion Channels; Electrophysiological Phenomena; Mammals
PubMed: 36966478
DOI: 10.1113/EP091099 -
Experimental Brain Research Oct 2019This is an account of experiments carried out in my laboratory over more than 20 years, exploring the influence of exercise on human limb position sense. It is known... (Review)
Review
This is an account of experiments carried out in my laboratory over more than 20 years, exploring the influence of exercise on human limb position sense. It is known that after intense exercise we are clumsy in the execution of skilled movements. The first question we posed concerned eccentric exercise, where the contracting muscle is forcibly lengthened. Such exercise produces muscle damage, and the damage might extend to the muscle's proprioceptors, the muscle spindles, producing a disturbance of limb position sense. However, provided the exercise was sufficiently severe (20-30% fall in muscle force), comparing eccentric exercise with concentric exercise, where no damage ensues, there was no difference in the effects on position sense. After exercise of elbow muscles, the forearm was always perceived as more extended than its actual position. It led to a new hypothesis: after exercise, did the extra effort required to lift the fatigued arm provide a position signal? Findings based on spindles' thixotropic behaviour did not support such a proposition for the elbow joint, although at the wrist an effort signal may contribute. Spindle thixotropy has also been proposed to explain the poor proprioception experienced under conditions of weightlessness. After exercise of elbow extensors or flexors, the position errors were always in the direction of forearm extension. At the knee, after exercise the lower leg was always perceived as more flexed. These findings led to the conclusion that disturbances to position sense, post-exercise, did not involve peripheral receptors, and that the effect arose within the brain.
Topics: Exercise; Fatigue; Humans; Muscle Contraction; Muscle Spindles; Muscle, Skeletal; Proprioception; Retrospective Studies
PubMed: 31471677
DOI: 10.1007/s00221-019-05634-8 -
BMJ Neurology Open 2023Muscle membranes have a sensation of pain, but within the muscle tissue, the origin of pain is unclear. We present a hypothesis that the pain receptors of the muscle...
BACKGROUND
Muscle membranes have a sensation of pain, but within the muscle tissue, the origin of pain is unclear. We present a hypothesis that the pain receptors of the muscle tissue are situated principally in the muscle spindles. A recent report reintroduced that 'end plate spikes' in needle electromyography (EMG) are fusimotor unit potentials of the intrafusal muscle fibres, and thus represent a marker of muscle spindles.
METHODS
We studied four relaxed muscles with 50 EMG needle insertions in each and mapped the appearance of pain and spontaneous EMG activity.
RESULTS
Only 4.0% of the needle insertions in muscle tissue elicited pain. However, needle insertions in local active points showing 'end plate spikes' and, thus, fusimotor unit potentials of the muscle spindles elicited pain in 86% of the insertions, whereas needle insertions in points without 'end plate spikes' elicited pain in only 1.0% of the insertions (p<0.001).
CONCLUSIONS
Muscle spindles have pain receptors. The extrafusal muscle tissue is practically pain-free for the needle insertions. This demonstrates a scarcity of extrafusal pain receptors. How this observation is put into perspective with the muscle pain syndromes was discussed.
PubMed: 37337530
DOI: 10.1136/bmjno-2023-000420 -
Current Opinion in Neurology Oct 2018Muscle spindles are encapsulated mechanosensory receptors within skeletal muscle tissue that inform the central nervous system about the contractile status of each... (Review)
Review
PURPOSE OF REVIEW
Muscle spindles are encapsulated mechanosensory receptors within skeletal muscle tissue that inform the central nervous system about the contractile status of each muscle. This information is required for any coordinated movement and for stable posture. This review summarizes recent findings regarding novel functions for proprioceptive feedback information, muscle spindle disease and the molecular basis of mechanoreception.
RECENT FINDINGS
Muscle spindle function is not limited to regulating motor control but is also required for appropriate realignment of fractured bones, successful regeneration of spinal cord axons after injury and spinal alignment. Several proteins responsible for or modulating mechanotransduction in proprioceptive sensory neurons have been identified, including the Piezo2 channel as a candidate for the principal mechanotransduction channel. Many neuromuscular diseases are known to be accompanied by an impaired function of muscle spindles, resulting in a decline of motor performance and coordination in the patients.
SUMMARY
Our knowledge regarding the molecular basis of muscle spindle function is still incomplete. However, increasing our understanding of mechanotransduction in muscle spindles is a prerequisite for finding appropriate strategies to prevent injuries due to unstable gait and frequent falls.
Topics: Animals; Humans; Mechanoreceptors; Muscle Spindles; Muscle, Skeletal; Proprioception
PubMed: 30095484
DOI: 10.1097/WCO.0000000000000590 -
Anatomical Record (Hoboken, N.J. : 2007) Oct 2023Muscle spindles are sensory receptors in skeletal muscle that provide information on muscle length and velocity of contraction. Previous studies noted that facial...
Muscle spindles are sensory receptors in skeletal muscle that provide information on muscle length and velocity of contraction. Previous studies noted that facial muscles lack muscle spindles, but recent reports indicate that the human platysma muscle and "buccal" muscles contain spindles. Mammalian facial muscles are active in social communication, vibrissa movement, and vocalizations, including human speech. Given these functions, we hypothesized that facial muscles contain muscle spindles, and we predicted that humans would have the greatest number, given the role our lips play in speech. We examined previously sectioned and stained (with H&E and trichrome stains) orbicularis oris (upper fibers) and zygomaticus (major) muscles across a broad phylogenetic range of mammalian species, spanning a wide distribution of body size and ecological niche, to assess the presence of muscle spindles. We also stained several sections with Sirius red to highlight the muscle spindle capsule. Our results indicate that mammalian facial muscles contain muscle spindles, supporting our hypothesis. Contrary to our prediction, though, humans (and other primates) had the lowest number of muscle spindles. We instead found that the carnivoran sample and the horse sample had the greatest number of spindles. Larger body size and nocturnality were also associated with a greater number of spindles. These results must be viewed with caution, though, as our sample size was small and there are critical mammalian taxa missing. Future work should use an expanded phylogenetic range of mammalian species to ascertain the role that phylogeny plays in muscle spindle presence and count.
Topics: Humans; Animals; Horses; Facial Muscles; Muscle Spindles; Phylogeny; Muscle, Skeletal; Mammals
PubMed: 36799659
DOI: 10.1002/ar.25172 -
Neurological Sciences : Official... Sep 2021Focal hand dystonia (FHD) is usually adult-onset focal dystonia that can be associated with marked occupational and functional disability leading to reduced quality of... (Review)
Review
BACKGROUND
Focal hand dystonia (FHD) is usually adult-onset focal dystonia that can be associated with marked occupational and functional disability leading to reduced quality of life.
METHODS
Relevant studies on treatment options for FHD, their limitations, and current recommendations were reviewed using the PubMed search until March 31, 2021. Besides, the reference lists of the retrieved publications were manually searched to explore other relevant studies.
RESULTS
and conclusion Currently, botulinum toxin has the best evidence for treatment of FHD, and 20-90% of patients experience symptomatic improvement. However, its benefit is often limited by the reduction of muscle tonus acting on the muscle spindle. Different surgical modalities that have been used to treat focal hand dystonia include lesional surgery, deep brain stimulation, and magnetic resonance-guided focused ultrasound thalamotomy. Recent studies exploring the role of behavioral techniques, sensorimotor training, and neuromodulation for the treatment of focal hand dystonia have reported good outcomes, but larger studies are required before implementing these interventions in practice.
Topics: Adult; Botulinum Toxins; Dystonic Disorders; Humans; Magnetic Resonance Imaging; Quality of Life
PubMed: 34213695
DOI: 10.1007/s10072-021-05432-7 -
Current Opinion in Neurobiology Jun 2022The muscle spindle (MS) provides essential sensory information for motor control and proprioception. The Group Ia and II MS afferents are low threshold slowly-adapting... (Review)
Review
The muscle spindle (MS) provides essential sensory information for motor control and proprioception. The Group Ia and II MS afferents are low threshold slowly-adapting mechanoreceptors and report both static muscle length and dynamic muscle movement information. The exact molecular mechanism by which MS afferents transduce muscle movement into action potentials is incompletely understood. This short review will discuss recent evidence suggesting that PIEZO2 is an essential mechanically sensitive ion channel in MS afferents and that vesicle-released glutamate contributes to maintaining afferent excitability during the static phase of stretch. Other mechanically gated ion channels, voltage-gated sodium channels, other ion channels, regulatory proteins, and interactions with the intrafusal fibers are also important for MS afferent mechanosensation. Future studies are needed to fully understand mechanosensation in the MS and whether different complements of molecular mediators contribute to the different response properties of Group Ia and II afferents.
Topics: Action Potentials; Ion Channels; Mechanoreceptors; Muscle Spindles; Neurons, Afferent; Proprioception
PubMed: 35430481
DOI: 10.1016/j.conb.2022.102542 -
Journal of Neurophysiology Aug 2018Muscle spindles are ubiquitous encapsulated mechanoreceptors found in most mammalian muscles. There are two types of endings, primary and secondary, and both are... (Review)
Review
Muscle spindles are ubiquitous encapsulated mechanoreceptors found in most mammalian muscles. There are two types of endings, primary and secondary, and both are sensitive to changes in muscle length and velocity, with the primary endings having a greater dynamic sensitivity. Unlike other mechanoreceptors in the somatosensory system, muscle spindles are unique in possessing motor innervation, via γ-motoneurons (fusimotor neurons), that control their sensitivity to stretch. Much of what we know about human muscles spindles comes from studying the behavior of their afferents via intraneural microelectrodes (microneurography) inserted into accessible peripheral nerves. We review the functional properties of human muscle spindles, comparing and contrasting with what we know about the functions of muscle spindles studied in experimental animals. As in the cat, many human muscle spindles possess a background discharge that is related to the degree of muscle stretch, but mean firing rates are much lower (~10 Hz). They can faithfully encode changes in muscle fascicle length in passive conditions, but higher level extraction of information is required by the central nervous system to measure changes in muscle length during muscle contraction. Moreover, although there is some evidence supporting independent control of human muscle spindles via fusimotor neurons, any effects are modest compared with the clearly independent control of fusimotor neurons observed in the cat.
Topics: Action Potentials; Animals; Humans; Motor Neurons, Gamma; Muscle Contraction; Muscle Spindles; Neurons, Afferent; Proprioception
PubMed: 29668385
DOI: 10.1152/jn.00071.2018 -
Experimental Brain Research Nov 2014When a muscle relaxes after a contraction, cross-bridges between actin and myosin in sarcomeres detach, but about 1% spontaneously form new, non-force-generating... (Review)
Review
When a muscle relaxes after a contraction, cross-bridges between actin and myosin in sarcomeres detach, but about 1% spontaneously form new, non-force-generating attachments. These bridges give muscle its thixotropic property. They remain in place for long periods if the muscle is left undisturbed and give the muscle a passive stiffness in response to a stretch. They are detached by stretch, but reform at the new length. If the muscle is then shortened, the presence of these bridges prevents muscle fibres from shortening and they fall slack. So, resting muscle can be in one of two states, where it presents in response to a stretch with a high stiffness, if no slack is present, or with a compliant response in the presence of slack. Intrafusal fibres of muscle spindles show thixotropic behaviour. For spindles, after a conditioning contraction, they are left stretch sensitive, with a high level of background discharge. Alternatively, if after the contraction the muscle is shortened, intrafusal fibres fall slack, leaving spindles with a low level of background activity and insensitivity to stretch. Muscle spindles are receptors involved in the senses of human limb position and movement. The technique of muscle conditioning can be used to help understand the contribution of muscle spindles to these senses and how the brain interprets signals arising in spindles. When, in a two-arm position-matching task, elbow muscles of the two arms are deliberately conditioned in opposite ways, the blindfolded subject makes large position errors of which they are unaware. The evidence suggests that the brain is concerned with the difference signal coming from the antagonists acting at the elbow and with the overall difference in signal from the two arms. Another way of measuring position sense is to use a single arm and indicate its perceived position with a pointer. Here, there is no access to a signal from the other limb, and position sense relies on referral to a central map of the body, the postural schema.
Topics: Extremities; Humans; Movement; Muscle Contraction; Muscle Spindles; Muscle, Skeletal; Proprioception
PubMed: 25200179
DOI: 10.1007/s00221-014-4088-5 -
The Journal of Physiology May 2021
Topics: Humans; Motor Neurons, Gamma; Muscle Spindles; Proprioception
PubMed: 33749841
DOI: 10.1113/JP281595