Extracellular matrix and transmembrane linkages at the termination of intrafusal fibers and the outer capsule in chicken muscle spindles

From Muscle to the Myofascial Unit: Current Evidence and Future Perspectives

The "motor unit" or the "muscle" has long been considered the quantal element in the control of movement. However, in recent years new research has proved the strong interaction between muscle fibers and intramuscular connective tissue, and between muscles and fasciae, suggesting that the muscles can no longer be considered the only elements that organize movement. In addition, innervation and vascularization of muscle is strongly connected with intramuscular connective tissue. This awareness induced Luigi Stecco, in 2002, to create a new term, the "myofascial unit", to describe the bilateral dependent relationship, both anatomical and functional, that occurs between fascia, muscle and accessory elements. The aim of this narrative review is to understand the scientific support for this new term, and whether it is actually correct to consider the myofascial unit the physiological basic element for peripheral motor control.

Delayed Onset Muscle Soreness and Critical Neural Microdamage-Derived Neuroinflammation

Piezo2 transmembrane excitatory mechanosensitive ion channels were identified as the principal mechanotransduction channels for proprioception. Recently, it was postulated that Piezo2 channels could be acutely microdamaged on an autologous basis at proprioceptive Type Ia terminals in a cognitive demand-induced acute stress response time window when unaccustomed or strenuous eccentric contractions are executed. One consequence of this proposed transient Piezo2 microinjury could be a VGLUT1/Ia synaptic disconnection on motoneurons, as we can learn from platinum-analogue chemotherapy. A secondary, harsher injury phase with the involvement of polymodal Aδ and nociceptive C-fibers could follow the primary impairment of proprioception of delayed onset muscle soreness. Repetitive reinjury of these channels in the form of repeated bout effects is proposed to be the tertiary injury phase. Notably, the use of proprioception is associated with motor learning and memory. The impairment of the monosynaptic static phase firing sensory encoding of the affected stretch reflex could be the immediate consequence of the proposed Piezo2 microdamage leading to impaired proprioception, exaggerated contractions and reduced range of motion. These transient Piezo2 channelopathies in the primary afferent terminals could constitute the critical gateway to the pathophysiology of delayed onset muscle soreness. Correspondingly, fatiguing eccentric contraction-based pathological hyperexcitation of the Type Ia afferents induces reactive oxygen species production-associated neuroinflammation and neuronal activation in the spinal cord of delayed onset muscle soreness.

Age-Related Alterations of Hyaluronan and Collagen in Extracellular Matrix of the Muscle Spindles

Background: Muscle spindles (MSs) play a crucial role in proprioception and locomotor coordination. Although the elasticity and viscosity of the extracellular matrix (ECM) within which MSs are embedded may play a key role in MS function, the impact of aging on ECM components is unclear. The aim of the current study was to investigate the age-related physiological changes of the ECM and to verify if these could be due to alterations of the environment directly surrounding MSs. Methods: Hematoxylin Eosin and picrosirius-red staining was carried out; collagen types I (COLI) and III (COLIII) were assessed, and biotinylated hyaluronan binding protein (HABP) immunohistochemical analysis was undertaken to evaluate alterations of the ECM in the intramuscular connective tissue (IMCT) of the hindlimbs of C57BL/6J male mice. Assessments were carried out on 6-week-old (Group A), 8-month-old (Group B), and 2-year-old (Group C) laboratory mice. Results: The capsule’s outer layer became progressively thicker with aging (it was 3.02 ± 0.26 μm in Group A, 3.64 ± 0.31 μm in Group B, and 5.81 ± 0.85 μm in Group C). The collagen in IMCT around and within the MSs was significantly higher in Group C, but there were no significant differences between Groups A and B. The MS capsules and continuous IMCT were primarily made up of COLI and COLIII. The average optical density (AOD) values of COLI in IMCT surrounding MS were significantly higher after aging (p < 0.05), but there were no significant differences in COLIII in the three groups (p > 0.05). HA was present in IMCT and filled the MSs capsule. The AOD of HABP of MS showed that there were lower HA levels in Group C with respect to Group A (p = 0.022); no significant differences were noted neither between Groups A and B nor between Groups B and C (p > 0.05). Conclusion: Age-related collagen accumulation and lower HA in the ECM in which the MSs were embedded may probably cause more stiffness in the ECM in vivo, which could help to partly explain the peripheral mechanisms underlying the age-related decline in functional changes related to MSs.

Generating intrafusal skeletal muscle fibres in vitro: Current state of the art and future challenges

Intrafusal fibres are a specialised cell population in skeletal muscle, found within the muscle spindle. These fibres have a mechano-sensory capacity, forming part of the monosynaptic stretch-reflex arc, a key component responsible for proprioceptive function. Impairment of proprioception and associated dysfunction of the muscle spindle is linked with many neuromuscular diseases. Research to-date has largely been undertaken in vivo or using ex vivo preparations. These studies have provided a foundation for our understanding of muscle spindle physiology, however, the cellular and molecular mechanisms which underpin physiological changes are yet to be fully elucidated. Therefrom, the use of in vitro models has been proposed, whereby intrafusal fibres can be generated de novo. Although there has been progress, it is predominantly a developing and evolving area of research. This narrative review presents the current state of art in this area and proposes the direction of future work, with the aim of providing novel pre-clinical and clinical applications.

Proportions of slow myosin heavy chain-positive fibers in muscle spindles and adjoining extrafusal fascicles, and the positioning of spindles relative to these fascicles

Chicken leg muscles were examined to calculate the percentages of slow myosin heavy chain (MHC)-positive fibers in spindles and in adjacent extrafusal fascicles, and to clarify how the encapsulated portions of muscle spindles are positioned relative to these fascicles. Unlike mammals, in chicken leg muscles slow-twitch MHC and slow-tonic MHC are expressed in intrafusal fibers and in extrafusal fibers, suggesting a close developmental connection between the two fiber populations. In 8-week-old muscles the proportions of slow MHC-positive extrafusal fibers that ringed muscle spindles ranged from 0-100%. In contrast, proportions of slow MHC-positive intrafusal fibers in spindles ranged from 0-57%. Similar proportions in fiber type composition between intrafusal fibers and surrounding extrafusal fibers were apparent at embryonic days 15 and 16, demonstrating early divergence of extrafusal and intrafusal fibers. Muscle spindles were rarely located within single fascicles. Instead, they were commonly placed where several fascicles converged. The frequent extrafascicular location of spindles suggests migration of intrafusal myoblasts from developing clusters of extrafusal fibers toward the interstitium, perhaps along a neurotrophic gradient established by sensory axons that are advancing in the connective tissue matrix that separates adjoining fascicles.