Medical School · Year 1 · Histology · includes a quiz and discussion video

Lecture 7: Muscle Tissue - Skeletal

Unit 1.2: Histology and Basic Tissues

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the hierarchical organization of skeletal muscle from whole muscle to sarcomere
2. Explain the structure of the sarcomere and identify its components (A band, I band, Z line, H zone, M line)
3. Describe the ultrastructure of skeletal muscle including T-tubules and sarcoplasmic reticulum
4. Explain the sliding filament mechanism of muscle contraction
5. Identify the structure and function of the neuromuscular junction
6. Recognize skeletal muscle in histological sections and electron micrographs

Introduction to Muscle Tissue

Muscle tissue is a specialized tissue designed for contraction, converting chemical energy stored in ATP into mechanical force and movement. This fundamental ability depends on the organized arrangement of contractile proteins—principally actin and myosin—that slide past one another to shorten the cell. Muscle tissue represents approximately 40% of body weight, making it the most abundant tissue type.

Three types of muscle tissue exist in the body, each with distinct structural and functional characteristics. Skeletal muscle attaches primarily to the skeleton and is responsible for voluntary movements under somatic nervous system control. Cardiac muscle forms the contractile wall of the heart and generates the pumping force for circulation. Smooth muscle is found in the walls of hollow organs and blood vessels, controlling involuntary functions such as digestion and blood flow.

Skeletal muscle possesses several defining characteristics. It is striated, displaying a distinctive pattern of cross-bands visible in both light and electron microscopy, resulting from the precise alignment of contractile proteins. Skeletal muscle cells are uniquely multinucleated, formed by the fusion of multiple precursor cells during development, and may reach lengths of up to 30 centimeters. These cells are under voluntary control and can produce rapid, powerful contractions, though they fatigue more readily than cardiac or smooth muscle.

<image>Panel A: Skeletal muscle at 400x showing long cylindrical fibers with obvious cross-striations of alternating dark A bands and light I bands with multiple peripheral nuclei beneath sarcolemma and schematic of single long fiber. Panel B: Cardiac muscle showing shorter branching fibers with less pronounced striations, central nuclei, and prominent dark intercalated discs at cell junctions with schematic of branching fibers. Panel C: Smooth muscle showing spindle-shaped cells without striations arranged in sheets with single elongated central nuclei that may be corkscrew-shaped when contracted with schematic of overlapping cells. Panel D: Comparison table listing striation yes-yes-no, nuclei multiple peripheral versus single central versus single central, intercalated discs no-yes-no, branching no-yes-no, and control voluntary-involuntary-involuntary at 50 micrometer scale.</image>

Organization of Skeletal Muscle

Skeletal muscle exhibits a remarkable hierarchical organization, with each level of structure wrapped in connective tissue that provides support, carries blood vessels and nerves, and transmits contractile force to tendons and bones.

Hierarchical Structure

The whole muscle—such as the biceps brachii or gastrocnemius—is an organ composed of muscle tissue along with connective tissue, blood vessels, and nerves. The entire muscle is surrounded by a dense irregular connective tissue layer called the epimysium, which is continuous with the tendons at either end of the muscle.

Within each muscle, the tissue is organized into bundles of muscle fibers called fascicles, clearly visible when muscle is cut in cross-section. Each fascicle is surrounded by the perimysium, a connective tissue layer that carries the blood vessels and nerves that supply the muscle fibers within.

Individual muscle fibers (cells) are the structural units of muscle tissue. Each fiber is an extremely elongated, multinucleated cell that may extend the entire length of the muscle, reaching lengths of up to 30 centimeters in some muscles. Fibers are cylindrical and range from 10 to 100 micrometers in diameter depending on the muscle and the individual's training state. A delicate layer of connective tissue called the endomysium surrounds each individual fiber, consisting primarily of reticular fibers and basal lamina material.

Within each muscle fiber, the cytoplasm (called sarcoplasm in muscle cells) is filled with long cylindrical structures called myofibrils, each approximately 1-2 micrometers in diameter. The myofibrils are the contractile elements, composed of precisely arranged myofilaments—the thin (actin-containing) and thick (myosin-containing) filaments whose interaction produces contraction.

Muscle Fiber Characteristics

Skeletal muscle fibers are remarkable cells with features adapted to their contractile function. The cell membrane, called the sarcolemma, surrounds the fiber and includes specialized infoldings (T-tubules) that allow action potentials to penetrate deep into the cell. The numerous nuclei in each fiber lie just beneath the sarcolemma at the cell periphery—a key identifying feature distinguishing skeletal muscle from cardiac muscle (where nuclei are central).

Muscle fibers arise during development from the fusion of many precursor cells called myoblasts. This fusion creates the multinucleated structure and explains why skeletal muscle has limited regenerative capacity—mature muscle fibers cannot divide. However, a population of stem cells called satellite cells persists between the fiber and its basal lamina. These quiescent cells can be activated following injury to proliferate, fuse, and regenerate damaged muscle fibers to a limited extent.

<image>Panel A: Whole biceps muscle in anatomical context surrounded by blue epimysium as dense irregular connective tissue at several centimeter scale with magnified fascicle segment showing multiple fascicles surrounded by green perimysium with blood vessels and nerves. Panel B: Individual muscle fibers within fascicle at 10-100 micrometer diameter each surrounded by yellow endomysium showing striations and peripheral nuclei with satellite cell indicated between fiber and basal lamina. Panel C: Cutaway of single fiber revealing multiple myofibrils at 1-2 micrometer diameter filling sarcoplasm with characteristic banding pattern. Panel D: Highest magnification showing thin and thick filament arrangement within sarcomere with actin at approximately 7 nm and myosin at approximately 15 nm diameter with arrows indicating magnification progression between levels.</image>

Skeletal Muscle Histology

In light microscopy, skeletal muscle has a distinctive appearance that allows ready identification.

Longitudinal Section

In longitudinal section, skeletal muscle fibers appear as long, cylindrical structures running parallel to one another. The most striking feature is the cross-striation pattern—alternating dark and light bands perpendicular to the fiber's long axis. The dark bands are called A bands (A for anisotropic, meaning they appear dark in polarized light), and the light bands are called I bands (I for isotropic, appearing light in polarized light). With careful observation, a thin dark line called the Z line (from the German Zwischenscheibe, meaning "between disc") can be seen bisecting each I band. This banding pattern results from the highly ordered arrangement of myofilaments, with all myofibrils within a fiber aligned in register so that their bands coincide.

The nuclei of skeletal muscle fibers are located at the periphery of the cell, just beneath the sarcolemma. They appear as elongated, dark-staining structures, and their peripheral location is a crucial feature distinguishing skeletal muscle from cardiac muscle (which has central nuclei). Multiple nuclei are typically visible along the length of each fiber.

Cross-Section

In cross-section, skeletal muscle fibers appear as polygonal profiles packed together. The peripheral nuclei appear as small dark dots at the edge of each fiber. With careful staining, the myofibrils within the fiber may be visible as small dots (Cohnheim's fields), sometimes with clear spaces between groups due to differential shrinkage during preparation.

Satellite cells, the muscle stem cells, are located between the sarcolemma and the basal lamina of the fiber. They are difficult to identify definitively in routine preparations but may appear as small cells closely associated with the fiber surface. Immunohistochemical staining for specific markers (such as Pax7) is required for reliable identification.

<image>Panel A: Longitudinal section at 200x showing multiple horizontal skeletal muscle fibers with clearly visible cross-striations of dark A bands alternating with light I bands and peripheral nuclei as dark elongated structures beneath sarcolemma with aligned striations across fibers. Panel B: Higher magnification inset at 400x showing banding detail with Z line as thin dark line bisecting I band and H zone as lighter region within A band with thin pink endomysium between fibers. Panel C: Cross section at 400x showing polygonal fiber profiles with peripheral nuclei as small dark dots at margins and fine stippling in fiber centers representing cut myofibrils with endomysium surrounding each fiber. Panel D: Satellite cell indicated as small cell with dark nucleus between sarcolemma and basal lamina plus modified Gomori trichrome inset showing different fiber types with different staining intensities at 50 micrometer scale.</image>

The Sarcomere

The sarcomere is the fundamental contractile unit of striated muscle, defined as the region between two adjacent Z lines. Sarcomeres measure approximately 2.5 micrometers in length at rest in mammalian muscle. The precise, repeating arrangement of sarcomeres produces the striated appearance visible in light microscopy.

Components of the Sarcomere

The Z line (Z disc) forms the boundary of each sarcomere. It appears as a thin dark line in electron microscopy and contains alpha-actinin, which anchors the thin filaments. The Z line is not simply a line but a complex structure where thin filaments from adjacent sarcomeres interdigitate and are held in a square lattice arrangement.

The I band is the light band that spans the Z line, extending from the edge of one A band to the edge of the A band in the adjacent sarcomere. The I band contains only thin (actin) filaments, without thick (myosin) filaments, which is why it appears light. During contraction, the I band shortens as thin filaments slide toward the center of the sarcomere.

The A band is the dark band that occupies the central portion of the sarcomere. It contains the entire length of the thick (myosin) filaments, with thin filaments overlapping at its lateral edges. The A band appears dark because of the greater protein density in this region. Importantly, the length of the A band remains constant during contraction because it corresponds to the unchanging length of the thick filaments.

The H zone is a lighter region within the center of the A band. It represents the area where only thick filaments are present, without overlap by thin filaments. Like the I band, the H zone shortens during contraction as thin filaments slide further into the A band.

The M line is a thin dark line at the center of the H zone (and thus the center of the sarcomere). It contains proteins such as myomesin that link adjacent thick filaments, maintaining their precise hexagonal arrangement, as well as creatine kinase for local ATP regeneration.

<image>Panel A: Electron micrograph appearance of two complete sarcomeres with dark Z lines defining boundaries at approximately 2.5 micrometer distance, dark central A band, lighter I band bisected by Z line, and lighter H zone with thin dark M line at midpoint. Panel B: Schematic interpretation showing thin blue actin filaments anchored at Z lines extending toward center and thick red myosin filaments with projecting cross-bridge heads in A band region interdigitating at overlap zones. Panel C: H zone showing only thick filaments and M line with proteins connecting adjacent thick filaments plus arrows indicating I band and H zone shortening while A band length remains constant during contraction. Panel D: Cross-sectional view inset at different levels showing thin filaments only in hexagonal array through I band, thick filaments each surrounded by six thin filaments through overlap zone, and thick filaments only through H zone.</image>

Myofilaments

The contractile machinery of muscle depends on two types of myofilaments: thin filaments composed primarily of actin, and thick filaments composed of myosin. Regulatory proteins on the thin filaments control when contraction can occur, while accessory proteins maintain the structural organization.

Thin Filaments

Thin filaments are approximately 7 nanometers in diameter and extend from the Z line toward the center of the sarcomere. Their backbone is filamentous actin (F-actin), formed by the polymerization of globular actin (G-actin) monomers into two intertwined helical strands resembling two strings of pearls twisted around each other. Each actin monomer contains a binding site for myosin heads.

Two regulatory proteins associate with actin to control contraction. Tropomyosin is a rod-shaped protein that lies in the groove between the two actin strands. In the resting state, tropomyosin blocks the myosin-binding sites on actin, preventing cross-bridge formation. The troponin complex is located at regular intervals along the thin filament and consists of three subunits: troponin C (TnC) binds calcium ions and is the sensor for the signal to contract; troponin T (TnT) binds to tropomyosin, anchoring the complex; and troponin I (TnI) binds to actin and has an inhibitory function, holding the tropomyosin-troponin complex in the "off" position.

Thick Filaments

Thick filaments are approximately 15 nanometers in diameter and 1.6 micrometers in length. They are composed of approximately 300 myosin II molecules, each consisting of two heavy chains and four light chains.

Each myosin heavy chain has a long alpha-helical tail and a globular head. The tails of many myosin molecules intertwine to form the backbone of the thick filament, with the heads projecting outward to form cross-bridges that can interact with actin. The myosin head contains two critical functional sites: an actin-binding site and an ATPase site that hydrolyzes ATP to provide energy for the power stroke.

The light chains (two per head) are associated with the neck region connecting the head to the tail. They include essential light chains (required for structural integrity) and regulatory light chains (involved in modulating contraction).

Accessory Proteins

Several accessory proteins maintain the structural organization of the sarcomere. Titin (connectin) is one of the largest known proteins, spanning from the Z line to the M line. Within the I band, titin acts as a molecular spring, providing passive elasticity that returns the sarcomere to its resting length after contraction and preventing overstretch. Within the A band, titin associates with thick filaments and may help center them in the sarcomere. Nebulin extends along the length of thin filaments and may regulate their length by acting as a molecular ruler. Dystrophin links the internal cytoskeleton to the extracellular matrix via a complex of membrane-associated proteins (the dystrophin-associated glycoprotein complex), transmitting force from sarcomeres to the endomysium and ultimately to tendons.

<image>Panel A: Thin filament showing two helical F-actin strands of pink G-actin spheres with blue rod-shaped tropomyosin in groove and multicolored troponin complexes at regular intervals every 7 actin monomers. Panel B: Troponin complex magnified inset showing yellow TnT extending along tropomyosin, green dumbbell-shaped TnC with calcium-binding sites, and red TnI contacting actin. Panel C: Thick filament showing bare zone at center corresponding to M line region with myosin heads projecting outward in helical pattern plus myosin II molecule inset showing two heavy chains with intertwined tails, two globular heads with ATPase and actin-binding sites, and four light chains wrapping neck region. Panel D: Accessory proteins showing titin spanning Z line to M line with elastic PEVK region in I band, nebulin along thin filament, and dystrophin at sarcolemma connecting to dystrophin-associated glycoprotein complex and extracellular matrix.</image>

Sarcoplasmic Reticulum and T-Tubules

Muscle contraction requires rapid, coordinated calcium release throughout the fiber, achieved through specialized membrane systems that work together to couple excitation (the action potential) to contraction (cross-bridge cycling).

Sarcoplasmic Reticulum

The sarcoplasmic reticulum (SR) is a specialized smooth endoplasmic reticulum that forms an elaborate network surrounding each myofibril. Its primary function is to sequester calcium ions, maintaining very low calcium concentrations in the sarcoplasm at rest (approximately 10⁻⁷ M), and to release calcium rapidly upon stimulation, raising concentrations to approximately 10⁻⁵ M to trigger contraction.

The SR forms a sleeve-like network around each myofibril, with longitudinal tubules running parallel to the filaments. At the A-I junction (in mammalian skeletal muscle), the SR expands into enlarged sacs called terminal cisternae (or lateral sacs). These terminal cisternae are the principal calcium storage sites, containing the calcium-binding protein calsequestrin that allows high concentrations of calcium to be stored without precipitation.

The SR membrane contains two key proteins for calcium handling. Ryanodine receptors (RyR1 in skeletal muscle) are large calcium release channels in the terminal cisternae membrane that open in response to signals from T-tubules. The sarco/endoplasmic reticulum calcium ATPase (SERCA) is the pump that actively transports calcium back into the SR, using ATP hydrolysis to move calcium against its concentration gradient, causing relaxation.

T-Tubules

T-tubules (transverse tubules) are invaginations of the sarcolemma that penetrate deep into the muscle fiber at regular intervals. In mammalian skeletal muscle, T-tubules are located at the A-I junction, resulting in two T-tubules per sarcomere. T-tubules allow action potentials to propagate rapidly from the surface to the interior of the fiber, ensuring that all myofibrils contract simultaneously.

The T-tubule membrane contains voltage-sensitive proteins called dihydropyridine receptors (DHPR), which are L-type calcium channels. In skeletal muscle, DHPR function primarily as voltage sensors rather than as functional calcium channels—they detect depolarization and mechanically activate the adjacent ryanodine receptors in the SR membrane.

The Triad

The triad is a characteristic structural complex consisting of one T-tubule flanked by two terminal cisternae of the SR. Two triads are present per sarcomere, positioned at the A-I junctions. The close apposition of T-tubule and SR membranes in the triad allows the direct mechanical coupling between DHPR (in the T-tubule) and RyR1 (in the SR) that is essential for excitation-contraction coupling.

<image>Panel A: Myofibril segment with visible A and I bands surrounded by light blue interconnected SR mesh running parallel covering A band region with SR expanding into darker blue terminal cisternae at A-I junctions. Panel B: Yellow-gold T-tubules invaginating from sarcolemma at top passing between myofibrils at A-I junction level forming triad structure with one T-tubule flanked by two terminal cisternae. Panel C: Triad molecular detail inset showing red DHPR voltage sensor tetrads in T-tubule membrane physically contacting RyR1 calcium release channels as large foot structures in SR membrane with green calsequestrin dots filling terminal cisternae lumen. Panel D: SR longitudinal tubules with orange SERCA pumps studding membrane for calcium reuptake plus arrows indicating action potential propagation down T-tubule, calcium release from SR into sarcoplasm, and calcium reuptake during relaxation.</image>

Mechanism of Contraction

Sliding Filament Theory

The sliding filament theory, proposed by Hugh Huxley and Andrew Huxley in the 1950s, explains how muscles shorten without the filaments themselves changing length. During contraction, the thin filaments slide past the thick filaments toward the center of the sarcomere, pulling the Z lines closer together.

Several observations support this mechanism. During contraction, the sarcomere shortens, but the lengths of individual thin and thick filaments remain constant. The I band shortens because thin filaments slide deeper into the A band. The H zone shortens (and may disappear in maximal contraction) as thin filaments approach or meet at the center. The A band length remains constant because it corresponds to the unchanging length of the thick filaments. The Z lines move closer together as each sarcomere shortens.

The Cross-Bridge Cycle

The molecular mechanism underlying the sliding of filaments is the cross-bridge cycle, a repeating sequence of interactions between myosin heads and actin, powered by ATP hydrolysis. The cycle consists of four main steps.

Attachment occurs when the myosin head, already "cocked" with bound ADP and inorganic phosphate (from previous ATP hydrolysis), binds to an exposed site on actin. This can only occur when calcium has caused the tropomyosin to move away from the myosin-binding site.

The power stroke follows attachment. Upon binding actin, the myosin head releases the inorganic phosphate, triggering a conformational change that causes the head to pivot, pulling the thin filament toward the center of the sarcomere. ADP is also released during this step. The head moves approximately 10 nanometers and generates a force of about 3-4 piconewtons.

Detachment requires ATP. A new ATP molecule binds to the myosin head, causing a conformational change that releases the head from actin. Without ATP, the myosin head remains locked to actin—this is the basis of rigor mortis.

Cocking (recovery) completes the cycle. The myosin head hydrolyzes the bound ATP to ADP and inorganic phosphate, using the released energy to return to its high-energy "cocked" configuration. The head is now ready to bind actin again if calcium is present.

The cycle repeats as long as ATP is available and calcium maintains the exposed myosin-binding sites. Each cycle produces a small increment of filament sliding; the rapid repetition of cycles (perhaps 5 per second during an isometric contraction, faster during rapid shortening) produces the smooth contraction we observe.

Excitation-Contraction Coupling

Excitation-contraction coupling is the process by which an electrical signal (action potential) leads to a mechanical response (contraction). The sequence begins when an action potential arrives at the axon terminal of a motor neuron, causing release of acetylcholine at the neuromuscular junction. Acetylcholine binds to nicotinic receptors on the motor end plate, triggering an end plate potential that reaches threshold and generates an action potential in the sarcolemma.

The action potential propagates along the sarcolemma in both directions from the motor end plate and travels down the T-tubules into the interior of the fiber. When the action potential reaches the triad, the voltage change is detected by DHPR in the T-tubule membrane. Through direct mechanical coupling, DHPR activation causes opening of RyR1 channels in the SR membrane. Calcium ions rush out of the SR terminal cisternae into the sarcoplasm, raising calcium concentration from approximately 10⁻⁷ M to 10⁻⁵ M.

Calcium binds to troponin C, causing a conformational change that is transmitted through the troponin complex to tropomyosin. Tropomyosin shifts deeper into the groove of the actin helix, uncovering the myosin-binding sites on actin. Cross-bridge cycling can now proceed, and the muscle contracts.

Relaxation

Relaxation occurs when the sarcoplasmic calcium concentration falls. SERCA pumps actively transport calcium back into the SR, using ATP (two calcium ions pumped per ATP hydrolyzed). As calcium dissociates from troponin C, tropomyosin returns to its resting position, covering the myosin-binding sites. Cross-bridge cycling ceases because new attachments cannot form. The muscle relaxes and returns passively to its resting length, aided by the elastic properties of titin and the connective tissue framework.

<image>Panel A: Cross-bridge cycle steps 1-2 showing red myosin head in cocked position with ADP and Pi binding to exposed blue actin site after green tropomyosin shifts away then Pi release causing 45-degree pivot moving thin filament 10 nm toward sarcomere center with ADP release to rigor configuration. Panel B: Cross-bridge cycle steps 3-4 showing ATP binding causing conformational change releasing head from actin then ATP hydrolysis to ADP plus Pi returning head to cocked position ready for next cycle with arrows connecting steps. Panel C: Excitation-contraction coupling flowchart showing action potential propagating down T-tubule, DHPR activating, RyR1 opening in triad detail, calcium release from SR, calcium binding troponin C, tropomyosin shifting to expose binding sites, and cross-bridge cycling beginning. Panel D: Relaxation parallel pathway showing SERCA pumping calcium back into SR, calcium dissociating from troponin, tropomyosin covering binding sites, and muscle relaxation occurring.</image>

Neuromuscular Junction

The neuromuscular junction (NMJ) is the synapse between a motor neuron and a skeletal muscle fiber, where the electrical signal in the nerve is transmitted to the muscle to initiate contraction.

Structure

The NMJ consists of three components: the presynaptic axon terminal (nerve terminal), the synaptic cleft, and the postsynaptic motor end plate (specialized region of the sarcolemma).

The axon terminal is the enlarged ending of the motor neuron axon. It contains numerous synaptic vesicles filled with the neurotransmitter acetylcholine (ACh), approximately 10,000 molecules per vesicle. The terminal also contains mitochondria (for ATP production) and voltage-gated calcium channels in its membrane.

The synaptic cleft is the 50-70 nanometer gap between the nerve terminal and the muscle fiber membrane. It contains a basal lamina (continuous with the muscle fiber's basal lamina) that anchors the enzyme acetylcholinesterase, which rapidly breaks down ACh after release.

The motor end plate is the specialized postsynaptic region of the sarcolemma beneath the nerve terminal. It features extensive junctional folds—deep infoldings that increase the membrane surface area. The crests of these folds contain a high density of nicotinic acetylcholine receptors (nAChR), positioned directly opposite the active zones of the nerve terminal where vesicles fuse. The troughs of the folds contain voltage-gated sodium channels.

Signal Transmission

When an action potential arrives at the axon terminal, voltage-gated calcium channels open, and calcium enters the terminal. The rise in calcium triggers fusion of synaptic vesicles with the presynaptic membrane and exocytosis of ACh into the synaptic cleft. The released ACh diffuses across the cleft and binds to nicotinic receptors on the motor end plate. These receptors are ligand-gated ion channels; ACh binding opens the channel, allowing sodium influx (and some potassium efflux), producing a local depolarization called the end plate potential (EPP).

The EPP at a healthy NMJ is always suprathreshold—it is large enough to trigger an action potential in the adjacent sarcolemma. This action potential then propagates along the entire muscle fiber, initiating the contraction process. Acetylcholinesterase rapidly hydrolyzes ACh in the synaptic cleft, terminating the signal and allowing the motor end plate to repolarize and respond to the next nerve impulse.

Motor Unit

A motor unit consists of a single motor neuron and all the muscle fibers it innervates. All fibers in a motor unit contract together when their motor neuron fires—this is the functional unit of motor control.

Motor unit size varies dramatically depending on the precision of control required. Muscles requiring fine control, such as those moving the eyes or fingers, have small motor units (as few as 10 fibers per motor neuron), allowing precise, graded movements. Muscles generating large forces for gross movements, such as the gastrocnemius or quadriceps, have large motor units (up to 2,000 fibers per motor neuron), sacrificing fine control for power and efficiency.

<image>Panel A: Low-magnification view of motor neuron axon approaching muscle fiber and branching into terminals at motor end plate region with motor unit inset showing one motor neuron from spinal cord innervating multiple muscle fibers. Panel B: Medium-magnification cross-section of yellow axon terminal with numerous ACh-containing synaptic vesicles clustered at active zones and mitochondria plus 50-70 nm synaptic cleft with basal lamina and motor end plate with deep junctional folds. Panel C: High-magnification molecular detail showing voltage-gated calcium channels opening in response to action potential triggering vesicle fusion and ACh release diffusing across cleft to blue pentameric nicotinic receptors on fold crests. Panel D: Sodium flowing through opened channels producing EPP with brown acetylcholinesterase enzymes in basal lamina breaking down ACh into acetate and choline plus voltage-gated sodium channels in fold troughs indicated.</image>

Muscle Fiber Types

Skeletal muscle fibers are not uniform; they differ in their contractile speed, resistance to fatigue, and predominant metabolic pathway. Three main types are recognized, though a spectrum exists.

Type I fibers (slow oxidative, slow-twitch) contract slowly but are highly resistant to fatigue. They are rich in myoglobin (giving them a red color), have abundant mitochondria and oxidative enzymes, and rely primarily on aerobic metabolism. Type I fibers have small motor units and are recruited first for sustained, low-intensity activities such as maintaining posture.

Type IIA fibers (fast oxidative-glycolytic) contract rapidly and have moderate fatigue resistance. They combine characteristics of both extremes, with substantial mitochondria and oxidative capacity but also significant glycolytic capacity. They are recruited for activities requiring both speed and endurance, such as middle-distance running.

Type IIB (or IIX in humans) fibers (fast glycolytic, fast-twitch) contract rapidly but fatigue quickly. They have fewer mitochondria, less myoglobin (appearing pale or white), and rely primarily on anaerobic glycolysis for ATP production. They have large motor units and are recruited for brief, powerful movements such as jumping or sprinting.

The proportion of fiber types varies among muscles (postural muscles have more Type I; muscles for rapid movements have more Type II) and among individuals (elite endurance athletes tend to have more Type I fibers; elite sprinters have more Type II). Training can shift fiber characteristics to some degree, particularly converting between Type IIA and IIB.

Clinical Correlations

Several important diseases affect skeletal muscle structure or function.

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder caused by mutations in the dystrophin gene. Without functional dystrophin, the mechanical link between the cytoskeleton and extracellular matrix is broken, and muscle fibers are damaged by the stress of contraction. Boys with DMD show progressive muscle weakness beginning in early childhood, characteristic pseudohypertrophy of the calves (due to replacement of muscle by fat and fibrous tissue), elevated serum creatine kinase (released from damaged fibers), and typically require wheelchair use by age 12 and die of respiratory or cardiac failure in early adulthood. The related Becker muscular dystrophy involves mutations that allow some partially functional dystrophin to be produced, resulting in a milder phenotype.

Myasthenia gravis is an autoimmune disease in which antibodies target nicotinic acetylcholine receptors at the neuromuscular junction. Receptor destruction reduces the number of available receptors, and the EPP becomes smaller. When it falls below threshold, transmission fails and the muscle fiber does not contract. Clinically, patients experience muscle weakness that worsens with repeated activity (fatigue) and improves with rest. Characteristic features include ptosis (drooping eyelids), diplopia (double vision), difficulty chewing and swallowing, and in severe cases respiratory muscle weakness. Diagnosis includes detection of anti-AChR antibodies, electromyographic findings, and improvement with acetylcholinesterase inhibitors.

Malignant hyperthermia is a pharmacogenetic disorder most commonly caused by mutations in the ryanodine receptor (RyR1). Susceptible individuals develop a hypermetabolic crisis when exposed to certain anesthetic agents (particularly volatile anesthetics and succinylcholine). The mutant RyR1 opens abnormally in response to these triggers, causing massive, sustained calcium release. Uncontrolled cross-bridge cycling produces muscle rigidity, extraordinary heat generation (temperature may rise several degrees within minutes), and metabolic acidosis. Without treatment, it is rapidly fatal; treatment includes immediate discontinuation of triggering agents, administration of dantrolene (which blocks calcium release from the SR), and aggressive cooling.

Rigor mortis is the stiffening of muscles that occurs after death. It results from ATP depletion—without ATP, myosin heads cannot detach from actin, locking muscles in a contracted state. Rigor mortis develops several hours after death as ATP stores are exhausted and resolves 24-48 hours later as lysosomal enzymes degrade the myofilaments.

Summary

Skeletal muscle is organized hierarchically: whole muscles contain fascicles, which contain individual muscle fibers (cells), which contain myofibrils, which are composed of myofilaments. Connective tissue layers (epimysium, perimysium, endomysium) surround each level, providing support and transmitting contractile force.

The sarcomere, the region between two Z lines, is the fundamental contractile unit. Its characteristic banding pattern results from the arrangement of thin filaments (actin with tropomyosin and troponin) and thick filaments (myosin). During contraction, thin filaments slide past thick filaments toward the sarcomere center; the I band and H zone shorten while the A band length remains constant.

The sarcoplasmic reticulum and T-tubules form the triad structure essential for excitation-contraction coupling. Action potentials propagate along T-tubules, activating voltage sensors (DHPR) that trigger calcium release through ryanodine receptors in the SR. Calcium binds troponin C, causing tropomyosin to shift and expose myosin-binding sites on actin.

The cross-bridge cycle—attachment, power stroke, detachment, cocking—converts ATP energy into mechanical force. The cycle continues as long as calcium and ATP are present. Relaxation occurs when calcium is pumped back into the SR by SERCA.

The neuromuscular junction transmits signals from motor neurons to muscle fibers via acetylcholine release. One motor neuron plus all fibers it innervates constitutes a motor unit.

Key Terms

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