Skeletal muscle
Skeletal muscle is one of the three major types of muscle tissue in vertebrates, distinguished by its striated, multinucleated fibers that enable voluntary control and attachment to the skeleton via tendons or aponeuroses.[1][2] These muscles, which comprise approximately 40% of body weight in adults,[3] are responsible for all deliberate movements, including locomotion, posture maintenance, and facial expressions.[1] Unlike cardiac or smooth muscle, skeletal muscle operates under conscious neural control from the somatic nervous system.[4] Structurally, skeletal muscle is organized hierarchically for efficient force generation and transmission. At the macroscopic level, an entire muscle is encased in epimysium, a dense connective tissue sheath, while bundles of fibers (fascicles) are wrapped in perimysium, and individual fibers are surrounded by delicate endomysium; these layers converge into tendons that anchor the muscle to bone.[5] Microscopically, each fiber is a syncytium containing numerous myofibrils aligned parallel to the long axis, with repeating sarcomeres—the fundamental contractile units—composed of overlapping thin (actin) and thick (myosin) filaments that produce the characteristic striations visible under light microscopy.[1][4] This architecture allows for precise length-tension relationships during contraction, supported by an abundant vascular supply of arteries, veins, and capillaries that deliver oxygen and nutrients essential for sustained activity.[5] Functionally, skeletal muscle converts chemical energy from ATP into mechanical work through excitation-contraction coupling, where motor neuron impulses at the neuromuscular junction release acetylcholine, depolarizing the fiber membrane (sarcolemma) and triggering calcium release from the sarcoplasmic reticulum to initiate actin-myosin cross-bridging.[6] Fibers are classified into types based on myosin isoforms and metabolic properties: slow-twitch type I (oxidative, fatigue-resistant for endurance activities like posture), fast-twitch type IIa (oxidative-glycolytic for moderate-intensity efforts), and type IIx (glycolytic for rapid, powerful bursts like sprinting).[6][7] Beyond movement, these muscles stabilize joints, generate heat via thermogenesis, store glycogen and proteins as metabolic reserves, and contribute to overall basal metabolic rate.[6][1] Impairments in skeletal muscle, such as those from injury, atrophy, or neuromuscular disorders, can profoundly affect mobility, respiration, and metabolic homeostasis.[1]Anatomy
Gross Anatomy
Skeletal muscle is a type of striated muscle tissue that is under voluntary control and primarily functions to produce movement by contracting and relaxing.[6] It is attached to bones via tendons, which are dense, fibrous connective tissues that transmit the force generated by muscle contraction to the skeletal system, enabling locomotion and posture maintenance.[8][1] These muscles are distributed throughout the body, with over 600 named skeletal muscles in humans, collectively accounting for approximately 40% of total body weight.[6] The gross structure of skeletal muscle is organized into hierarchical layers of connective tissue that provide support, protection, and pathways for blood vessels and nerves. The entire muscle is enveloped by the epimysium, a dense sheath of connective tissue that surrounds the muscle as a whole and extends to form tendons at the ends.[1] Within the epimysium, bundles of muscle fibers known as fascicles are wrapped by the perimysium, another layer of connective tissue that divides the muscle into compartments and allows for compartmentalized contraction.[1] Individual muscle fibers within each fascicle are surrounded by the delicate endomysium, which directly invests each fiber and facilitates nutrient exchange while maintaining structural integrity.[1] These connective tissue layers collectively contribute to the muscle's tensile strength and ability to withstand mechanical stress during contraction. Skeletal muscles exhibit various arrangements of muscle fibers within fascicles, which influence their mechanical properties such as force production and range of motion. In parallel arrangements, fibers run longitudinally along the muscle's axis, allowing for a greater excursion and range of motion but relatively lower force output compared to other patterns; examples include strap-like muscles such as the sartorius.[9] Fusiform muscles, a subtype of parallel arrangement, taper at the ends for smoother attachment to tendons and provide balanced force with moderate range, as seen in the biceps brachii.[9] Pennate arrangements, where fibers attach obliquely to a central tendon, enable higher force generation by packing more fibers into a given cross-sectional area, though at the cost of reduced shortening distance; unipennate, bipennate, and multipennate subtypes exist, exemplified by the rectus femoris (bipennate).[9][10] Skeletal muscles are named according to standardized conventions that reflect their anatomical and functional characteristics. Names may indicate location (e.g., tibialis anterior for the anterior tibia), shape (e.g., deltoid for triangular form), size (e.g., gluteus maximus for the largest buttock muscle), number of origins (e.g., biceps brachii for two heads), points of origin and insertion (e.g., sternocleidomastoid originating from sternum and clavicle), or primary action (e.g., flexor carpi radialis for wrist flexion).[11][12] Gross features such as muscle length and cross-sectional area vary widely; for instance, longer muscles like the sartorius span multiple joints, while thicker ones like the quadriceps have larger cross-sectional areas to generate substantial force.[9]Microscopic Anatomy
Skeletal muscle fibers, also known as myofibers, are elongated, multinucleated cells that form the fundamental contractile units of skeletal muscle tissue. These fibers typically range from 10 to 100 micrometers in diameter and can extend up to several centimeters in length, exhibiting a striated appearance under light microscopy due to their organized internal components. The plasma membrane of each myofiber, termed the sarcolemma, encloses the sarcoplasm, which is the specialized cytoplasm rich in mitochondria, glycogen, and other organelles. Embedded within the sarcoplasm is an extensive network called the sarcoplasmic reticulum, a modified endoplasmic reticulum that stores and releases calcium ions essential for muscle contraction.[6] The internal architecture of myofibers is dominated by bundles of myofibrils, which are cylindrical structures composed of repeating units known as sarcomeres, the basic functional segments of contraction. Each sarcomere is delimited by Z-lines (or Z-disks), thin protein structures that anchor actin filaments, and spans from one Z-line to the next, measuring approximately 2 to 3 micrometers in length at rest. The sarcomere exhibits distinct bands: the A-band, a dark central region corresponding to the length of thick myosin filaments; the I-band, a lighter region on either side of the Z-line containing only thin actin filaments; and the H-zone, a lighter area within the A-band where actin filaments do not overlap with myosin. Thin actin filaments, approximately 7 nm in diameter, interdigitate with thicker myosin filaments, about 15 nm in diameter, forming the sliding filament array that enables muscle shortening.[1][13][14] Satellite cells, mononucleated stem cells residing between the sarcolemma and the basal lamina of myofibers, play a critical role in maintaining muscle architecture by contributing to repair and regeneration. These cells, comprising 2-10% of myonuclei in adult muscle, remain quiescent under normal conditions but activate in response to injury or stress to fuse with existing fibers or form new ones. The extracellular matrix (ECM) surrounding myofibers, including the endomysium, perimysium, and epimysium layers, provides structural support, transmits force, and facilitates signal transduction; it consists primarily of collagen types I and III, laminin, and fibronectin, integrating with the sarcolemma via proteins like dystrophin.[15][16] At the microscopic level, skeletal muscle receives dense vascular and neural innervation to support its metabolic demands and contractile function. Capillaries, embedded within the endomysium, form a rich network around individual myofibers, with each fiber typically contacted by 4-6 capillaries to ensure efficient oxygen and nutrient delivery; these vessels originate from arterioles branching within the perimysium.[6][17] Neural supply occurs via motor end plates, specialized synaptic junctions where alpha motor neuron axons terminate on the sarcolemma, forming a complex of prejunctional nerve terminals, postsynaptic folds, and synaptic cleft filled with acetylcholine receptors. These end plates, visible under electron microscopy as convoluted junctional folds increasing surface area for neurotransmitter binding, are distributed along the fiber length, often in a banded pattern.[6][18] Histological examination of skeletal muscle relies on staining techniques to visualize its microscopic features. Hematoxylin and eosin (H&E) staining is commonly used, where hematoxylin binds to nuclei and acidic structures for a blue-purple hue, and eosin stains the cytoplasm and ECM pink, highlighting the striated pattern of myofibrils and distinguishing connective tissue layers. Other methods, such as Masson's trichrome, accentuate collagen in the ECM, while electron microscopy provides ultrastructural details of sarcomeres and motor end plates not resolvable by light microscopy.[13]Muscle Fiber Types
Skeletal muscle fibers are classified into distinct types based on their myosin heavy chain (MHC) isoforms, contractile properties, and metabolic characteristics. The primary types in humans are Type I (slow-twitch, oxidative), Type IIa (fast-twitch, oxidative-glycolytic), and Type IIx (fast-twitch, glycolytic), with Type IIx serving as the human equivalent of Type IIb found in some rodents.[19] These classifications arise from differences in the expression of MHC genes, where Type I fibers express MYH7, Type IIa express MYH2, and Type IIx express MYH1.[19] Type I fibers are characterized by slow contraction speeds and high resistance to fatigue, owing to their reliance on oxidative metabolism supported by abundant mitochondria and myoglobin, which imparts a red color to these fibers.[20] In contrast, Type IIa fibers exhibit intermediate properties, with faster twitch speeds than Type I but greater fatigue resistance than Type IIx due to a mix of oxidative and glycolytic capacities, resulting in a pinkish-red appearance from moderate myoglobin and mitochondrial density.[20] Type IIx fibers are pale or white, lacking significant myoglobin and having low mitochondrial density, which enables rapid contractions but leads to quick fatigue through predominant glycolytic metabolism. Regarding force-velocity relationships, Type I fibers generate lower maximum velocities but sustain force over time, while Type IIx fibers achieve higher shortening velocities for brief, powerful actions, with Type IIa falling in between.[19] The distribution of fiber types varies across human muscles to match functional demands. For instance, the soleus muscle, involved in sustained postural activities, is predominantly composed of Type I fibers (approximately 70-90%), whereas the gastrocnemius, which supports more dynamic movements, shows a mixed composition with about 50% Type I and the remainder split between Type IIa and Type IIx.[21][21] Fiber typing methods enable precise identification of these characteristics. Histochemical staining, particularly for myofibrillar ATPase activity at varying pH levels (e.g., preincubation at pH 4.6), distinguishes fiber types based on staining intensity: Type I fibers stain lightly, Type IIa darkly, and Type IIx intermediately after acid preincubation.[22] Immunohistochemical techniques use antibodies against specific MHC isoforms (e.g., BA-D5 for Type I, SC-71 for Type IIa, MY-32 for Type IIx) on muscle cross-sections to quantify pure and hybrid fibers via fluorescence microscopy. Physiological methods, such as electromyography (EMG), assess fiber type indirectly by measuring twitch contraction times or motor unit firing rates, where slower twitch times correlate with Type I dominance and faster rates with Type II enrichment.[23]| Fiber Type | Twitch Speed | Fatigue Resistance | Metabolic Profile | Color (due to myoglobin/mitochondria) | Example MHC Isoform |
|---|---|---|---|---|---|
| Type I | Slow | High | Oxidative | Red | MYH7 |
| Type IIa | Fast | Moderate | Oxidative-glycolytic | Pink-red | MYH2 |
| Type IIx | Very fast | Low | Glycolytic | White | MYH1 |