Musculoskeletal Disorders. Sean Gallagher

Musculoskeletal Disorders - Sean Gallagher


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fascia around and into individual muscle fibers. The epimysium (deep fascial fibrous connective tissue wrapping around entire muscle), perimysium (around individual fascicles), and endomysium (Endo; around individual muscle fibers/cells) are indicated.

      Interstitial fascia or interstitium has been recently highlighted as a new term in the literature (Stecco & Caro, 2019; Stecco, Macchi, Porzionato, Duparc, & De Caro, 2011). By definition of its name, interstitial fascia is the located “between the cells.” Anatomically, interstitial fascia is the highly vascularized and highly innervated superficial and deep fascial components mentioned earlier.

      Skeletal Muscle Structure

      Cells

      Based on Gillies, A. R., & Lieber, R. L. (2011). Structure and function of the skeletal muscle extracellular matrix. Muscle Nerve 44(3), 318–331. doi:10.1002/mus.22094.

Characteristic Description
Tissue type Contractile
Cells Main cell types: Individual muscle fibers (myofibers), myoblasts, satellite cells, bone marrow–derived stem cellsAdditional cell types: Resident macrophages, endothelial cells associated with blood vessels throughout muscles, fibroblasts in sheaths, peripheral glial cells associated with nerve endings and neuromuscular junction
Subtypes Type I (slow‐twitch/red), Type IIb (fast‐twitch/white), Type IIa (intermediate), Type IIx
ECM Main composition: Collagen type I and glycoproteins in muscle proper, collagen IV in basement membraneAdditional components: Collagen III, collagen V, and elastin in fascial sheaths
Function Contraction and then movement of the endoskeleton to which the muscle is attached (bones and cartilage)
Photo depicts single muscle cells (fiber) showing multinucleated nature and striations.

      From Tortora, G. J., & Derrickson, B. H. (Eds.), (2010). Muscle. In Introduction to the human body, 11th ed., Wiley.

Photo depicts distribution of different myosin heavy chains detected by immunofluorescence in a rat flexor digitorum muscle.

      The connective tissue sheaths of muscles, described further below, contain fibroblasts, endothelial cells associated with blood vessels, nerve axons, and sometimes adipose cells and myofibroblasts. Muscles also contain resident macrophages that secrete various growth factors for the maintenance of the muscle fibers. The sites of neuromuscular junctions are the sites where nerve axons terminate on muscle fibers, and muscle spindles and Golgi tendon apparati are sensory receptors located within muscles and myotendinous junctions, respectively. These sites contain glial cells, the Schwann cells, and glial satellite cells, in additional to nerve axon terminals.

      Metabolic subtypes

      Muscle tissue obtains energy as adenosine triphosphate (ATP) and phosphocreatine kinase from both the aerobic metabolism of fatty acids and glucose and the anaerobic glycolysis. Skeletal muscle cells can be divided into three subtypes based on their metabolic and histochemical characteristics as well as their myosin heavy‐chain subcomponents. Type I fibers, also known as slow‐twitch fibers or “red” fibers, are small in size, contain many mitochondria and large amounts of myoglobin and cytochromes, and have high type I myosin heavy‐chain content (Figure 3.5). Their glycolytic enzyme content is low. Myoglobin is an iron protein that binds O2 and is the feature that makes these fibers appear dark red in color. Type I fibers obtain their energy primarily from aerobic oxidative phosphorylation of fatty acids. As a consequence, they are adapted for slow contractions over prolonged periods. Postural muscles of the back contain many type I fibers.


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