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

Lecture 4: Muscle Physiology

Unit 1.6: Physiology Foundations

Learning Objectives

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

1. Compare the structure and function of skeletal, cardiac, and smooth muscle
2. Describe the molecular mechanism of muscle contraction (sliding filament theory)
3. Explain excitation-contraction coupling in skeletal muscle
4. Describe the mechanics of muscle contraction (twitch, tetanus, length-tension)
5. Explain the metabolism of muscle during exercise
6. Apply muscle physiology concepts to clinical conditions

Overview of Muscle Types

The three types of muscle tissue share the fundamental property of converting chemical energy into mechanical force but differ in structure, control mechanisms, and functional properties reflecting their specialized roles.

Skeletal muscle attaches to bones and produces voluntary movement under conscious control. Its characteristic striated appearance under microscopy results from the highly organized arrangement of contractile proteins into sarcomeres. Skeletal muscle cells (fibers) are remarkable for their size, often extending the entire length of the muscle, and for their multinucleated organization resulting from fusion of myoblasts during development. The sarcoplasmic reticulum is extensively developed, providing the calcium storage essential for rapid, powerful contractions. T-tubules penetrate the fiber at regular intervals, forming triads with the sarcoplasmic reticulum to enable rapid excitation-contraction coupling.

Cardiac muscle pumps blood continuously through the circulatory system without voluntary control. Like skeletal muscle, cardiac muscle is striated, but its fibers are shorter, branched, and connected by intercalated discs containing gap junctions that allow electrical coupling between cells. This arrangement enables the heart to function as a syncytium, with action potentials spreading throughout the myocardium to produce coordinated contraction. Cardiac muscle has intrinsic automaticity—pacemaker cells generate spontaneous action potentials that determine heart rate without neural input, though autonomic innervation modulates this rate.

Smooth muscle lines blood vessels, airways, the gastrointestinal tract, and other hollow organs, controlling functions from blood pressure regulation to peristalsis. Its lack of striations reflects a less organized contractile apparatus with dense bodies serving the anchoring function of Z-lines. Smooth muscle cells are small, spindle-shaped, and contain a single central nucleus. Contraction is slower but more sustained than skeletal muscle, appropriate for maintaining vascular tone and propelling contents through tubes. Control mechanisms include autonomic innervation, circulating hormones, and local factors, with significant variation among different smooth muscle types.

<image>Panel A: Skeletal muscle structure showing long cylindrical fibers with peripheral nuclei, cross-striations, and T-tubule triads with sarcoplasmic reticulum. Panel B: Cardiac muscle with shorter branched fibers, intercalated discs with gap junctions, and central nuclei. Panel C: Smooth muscle with small fusiform cells, single central nuclei, no striations, and dense bodies for contraction. Panel D: Characteristic contractile response traces comparing fast skeletal twitches, rhythmic cardiac contractions, and slow sustained smooth muscle responses.</image>

Skeletal Muscle Structure

Skeletal muscle organization follows a hierarchical pattern from the whole muscle to the molecular level. The entire muscle is wrapped in epimysium, dense connective tissue that continues into tendons attaching to bone. Within this covering, fascicles (bundles of muscle fibers) are wrapped in perimysium, and individual muscle fibers are surrounded by endomysium. This connective tissue framework transmits contractile force to the skeleton and carries blood vessels and nerves into the muscle.

The muscle fiber (cell) is a syncytium formed by fusion of many myoblasts, resulting in a single cell containing hundreds of peripherally located nuclei. The sarcolemma (plasma membrane) propagates action potentials over the fiber surface and into the interior via T-tubules. The sarcoplasm (cytoplasm) contains abundant myoglobin for oxygen storage, mitochondria for ATP production, and the extensive sarcoplasmic reticulum that stores and releases calcium.

Myofibrils are the contractile units within each muscle fiber, arranged in parallel arrays that give skeletal muscle its striated appearance. Each myofibril consists of repeating units called sarcomeres, the fundamental contractile units bounded by Z-lines. Within the sarcomere, the A-band represents the length of the thick (myosin) filaments and remains constant during contraction. The I-band contains only thin (actin) filaments and shortens as thin filaments slide toward the center. The H-zone in the center of the A-band contains only thick filaments and also shortens during contraction. The M-line at the center of the sarcomere anchors the thick filaments.

<image>Panel A: Whole muscle organization showing epimysium covering and fascicles wrapped in perimysium containing muscle fibers. Panel B: Single muscle fiber structure with peripheral nuclei, sarcolemma, and parallel myofibril arrangement. Panel C: Sarcomere organization showing Z-lines at borders, I-bands, A-band, H-zone, and M-line. Panel D: Electron micrograph appearance of sarcomeric banding pattern with corresponding structural components labeled.</image>

Contractile Proteins

Thick filaments are composed primarily of myosin, a motor protein with distinct structural and functional domains. Each myosin molecule contains two heavy chains whose long α-helical tails intertwine to form the backbone of the thick filament. The globular heads (S1 regions) project outward from the filament backbone and contain the functional domains for contraction: an actin-binding site and an ATP-binding site with ATPase activity. Four light chains associate with the neck region of each myosin head, regulating ATPase activity and contributing to the power stroke. In the thick filament, myosin molecules arrange with heads projecting at regular intervals along the filament length, with a bare zone at the center where only tails are present.

Thin filaments consist of actin as the core structural component, wrapped by regulatory proteins tropomyosin and troponin. G-actin (globular actin) monomers polymerize into F-actin (filamentous actin), forming two intertwined helical strands. Each actin monomer contains a myosin-binding site that, when exposed, allows cross-bridge formation. Tropomyosin is a long, rod-shaped protein that lies in the groove of the actin helix, spanning seven actin monomers. In the resting state, tropomyosin covers the myosin-binding sites on actin, preventing cross-bridge formation.

Troponin is a regulatory complex of three subunits that controls the position of tropomyosin. Troponin T (TnT) binds to tropomyosin, anchoring the troponin complex to the thin filament. Troponin I (TnI) inhibits the actin-myosin interaction by holding tropomyosin in the blocking position. Troponin C (TnC) binds calcium ions; when calcium binds, conformational changes in the troponin complex allow tropomyosin to shift deeper into the actin groove, exposing the myosin-binding sites and permitting contraction.

Accessory proteins maintain sarcomere structure and function. Titin, one of the largest known proteins, extends from the Z-line to the M-line, acting as a molecular spring that centers thick filaments and provides passive elasticity. Nebulin runs along thin filaments, regulating their length. Dystrophin links the intracellular cytoskeleton to the extracellular matrix through the dystrophin-associated protein complex, transmitting force and stabilizing the sarcolemma during contraction.

<image>Panel A: Thick filament structure showing myosin molecules with intertwined tails and globular heads containing ATP and actin binding sites. Panel B: Thin filament with twisted actin strands, tropomyosin in the groove, and troponin complex with TnT, TnI, and TnC subunits. Panel C: Resting versus activated states showing tropomyosin position change upon calcium binding to TnC exposing myosin binding sites. Panel D: Accessory proteins including titin spanning Z-line to M-line and dystrophin linking actin to the sarcolemma.</image>

Sliding Filament Theory

Muscle contraction results from thick and thin filaments sliding past each other, shortening the sarcomere while individual filament lengths remain constant. This sliding filament mechanism, proposed by Huxley and Niedergerke in 1954, explains the changes in sarcomere banding patterns observed during contraction and is powered by the cyclic interaction of myosin heads with actin.

The cross-bridge cycle describes the molecular events underlying the power stroke. In the resting state following the previous cycle, the myosin head is cocked (in a high-energy position) with ADP and inorganic phosphate bound. When calcium exposes actin binding sites, the myosin head binds strongly to actin, forming a cross-bridge. This binding triggers phosphate release, which initiates the power stroke—the myosin head pivots, pulling the actin filament approximately 10 nanometers toward the center of the sarcomere. ADP is then released, completing the power stroke and leaving myosin bound tightly to actin (the rigor state).

ATP binding to the myosin head causes immediate detachment from actin. Without ATP, this detachment cannot occur, explaining the rigid state of rigor mortis after death when ATP is depleted. Once detached, the myosin head hydrolyzes ATP to ADP and phosphate, and this hydrolysis provides energy to cock the myosin head back to its high-energy position, ready for another cycle if calcium remains available.

The visible changes in sarcomere banding during contraction reflect the sliding filament mechanism. The A-band length remains constant because it represents the thick filament length, which does not change. The I-band shortens as thin filaments slide over thick filaments toward the center. The H-zone narrows and may disappear as thin filaments advance toward the M-line. Overall sarcomere length decreases, and the cumulative shortening of thousands of sarcomeres in series produces macroscopic muscle contraction.

<image>Panel A: Sarcomere appearance before contraction showing full I-band and H-zone widths with A-band dimensions. Panel B: Contracted sarcomere with narrowed I-band and H-zone as thin filaments slide toward center while A-band remains constant. Panel C: Cross-bridge cycle showing attachment, power stroke with phosphate and ADP release, and ATP-dependent detachment. Panel D: Myosin head cocking through ATP hydrolysis returning to high-energy position ready for the next cycle.</image>

Excitation-Contraction Coupling

Excitation-contraction coupling is the sequence of events that translates the electrical signal of an action potential into the mechanical response of muscle contraction. This process begins at the neuromuscular junction and culminates in calcium binding to troponin.

At the neuromuscular junction, an action potential arriving at the motor nerve terminal triggers acetylcholine release. Acetylcholine diffuses across the synaptic cleft and binds to nicotinic receptors on the motor end plate, opening cation channels. The resulting end-plate potential (EPP) depolarizes the muscle membrane sufficiently to activate voltage-gated sodium channels in the surrounding sarcolemma, generating a muscle action potential that propagates across the fiber surface and into the interior via T-tubules.

The T-tubule system carries the action potential deep into the muscle fiber, ensuring that the signal reaches all myofibrils nearly simultaneously. T-tubules form triads with the terminal cisternae of the sarcoplasmic reticulum. At these triads, dihydropyridine receptors (DHPRs) in the T-tubule membrane function as voltage sensors. When depolarization reaches the DHPR, conformational changes occur that are mechanically transmitted to ryanodine receptors (RyRs) in the adjacent sarcoplasmic reticulum membrane. This mechanical coupling between DHPR and RyR is unique to skeletal muscle.

Activation of ryanodine receptors opens calcium release channels, flooding the sarcoplasm with calcium ions. Cytoplasmic calcium concentration rises from approximately 100 nanomolar at rest to approximately 10 micromolar during activation. Calcium binds to troponin C, initiating conformational changes that shift tropomyosin away from myosin-binding sites on actin. With binding sites exposed, cross-bridge cycling can proceed.

Relaxation occurs when calcium is actively pumped back into the sarcoplasmic reticulum by SERCA (sarco/endoplasmic reticulum calcium ATPase). As cytoplasmic calcium falls, calcium dissociates from troponin C, tropomyosin returns to its blocking position, cross-bridge cycling ceases, and the muscle relaxes. The rapid calcium reuptake by SERCA enables the brief contractions characteristic of skeletal muscle.

<image>Panel A: Neuromuscular junction activation with acetylcholine release and action potential propagation along sarcolemma and T-tubules. Panel B: Triad structure showing DHPR voltage sensors in T-tubules mechanically coupled to ryanodine receptors in sarcoplasmic reticulum. Panel C: Calcium release from SR upon DHPR activation, calcium binding to troponin C, and tropomyosin shift exposing myosin binding sites. Panel D: Relaxation through SERCA pump returning calcium to the sarcoplasmic reticulum restoring resting calcium concentration.</image>

Muscle Mechanics

The motor unit is the functional unit of muscle activation, consisting of a single motor neuron and all the muscle fibers it innervates. When the motor neuron fires, all fibers in the motor unit contract together following the all-or-none principle. Motor unit size varies: small motor units (few fibers per motor neuron) enable fine control in muscles requiring precision, such as extraocular muscles; large motor units (many fibers per motor neuron) generate powerful forces in muscles like the quadriceps. The size principle governs motor unit recruitment: smaller motor units with lower-threshold motor neurons are recruited first, and larger motor units with higher-threshold neurons are added as greater force is needed.

A twitch is the mechanical response to a single action potential, consisting of three phases. The latent period (about 2 milliseconds) represents the time for excitation-contraction coupling. The contraction phase follows as cross-bridges cycle and force develops. The relaxation phase completes the twitch as calcium is sequestered and cross-bridges detach. Twitch duration varies by muscle type: fast-twitch fibers complete a twitch in 10-30 milliseconds, while slow-twitch fibers require 100 milliseconds or more.

Summation occurs when successive stimuli arrive before complete relaxation from the previous contraction. Temporal summation (wave summation) adds the mechanical effects of overlapping twitches, producing greater force than a single twitch. With increasing stimulus frequency, summation becomes more complete until tetanus is achieved. Unfused (incomplete) tetanus produces a rippled plateau as partial relaxation occurs between stimuli. Fused (complete) tetanus produces a smooth plateau at maximum force when stimuli are so frequent that no relaxation occurs between them.

The length-tension relationship describes how muscle force varies with sarcomere length. Maximum active tension develops at optimal length (L₀), where actin-myosin overlap is ideal. At shorter lengths, tension decreases because thin filaments overlap each other and thick filaments contact Z-lines. At longer lengths, tension decreases because reduced actin-myosin overlap limits cross-bridge formation. Passive tension from connective tissue and titin adds to active tension at longer lengths.

The force-velocity relationship describes the inverse relationship between contraction speed and force. Maximum force occurs during isometric contraction (zero velocity). As velocity increases, force decreases because cross-bridges have less time to attach and complete power strokes. Maximum velocity occurs under zero load.

<image>Panel A: Motor unit organization showing a motor neuron innervating multiple muscle fibers that contract together as a functional unit. Panel B: Twitch phases including latent period, contraction, and relaxation comparing fast and slow fiber responses. Panel C: Summation progression from single twitches through unfused tetanus to fused tetanus with increasing stimulus frequency. Panel D: Length-tension relationship showing optimal force at ideal sarcomere overlap with passive tension contribution at longer lengths.</image>

Muscle Fiber Types

Skeletal muscle fibers are classified by their contractile and metabolic properties into types that reflect adaptation to different functional demands.

Type I (slow oxidative) fibers contract slowly but resist fatigue due to their extensive oxidative metabolism. They are rich in mitochondria, capillaries, and myoglobin (giving them a red color), and express the slow myosin heavy chain isoform with low ATPase activity. These fibers predominate in postural muscles like the soleus that must contract continuously without fatigue. Their high oxidative capacity suits them for sustained, aerobic activity.

Type IIa (fast oxidative-glycolytic) fibers combine fast contraction with moderate fatigue resistance. They express fast myosin isoforms, enabling rapid contraction, but also have substantial oxidative capacity with intermediate mitochondrial and capillary density. This fiber type adapts to training and can shift between more oxidative and more glycolytic phenotypes.

Type IIx (fast glycolytic) fibers produce the fastest, most powerful contractions but fatigue rapidly. They have few mitochondria and capillaries, low myoglobin content (appearing white), and rely on glycolytic metabolism for ATP production. This limits their sustained activity but enables explosive power for sprinting or jumping. They predominate in muscles requiring brief, intense effort.

Fiber type composition varies between muscles according to function and can be modified by training. Endurance training increases oxidative capacity and shifts fibers toward more oxidative phenotypes, while resistance training promotes hypertrophy of all fiber types. However, complete interconversion between Type I and Type II fibers is limited.

<image>Panel A: Type I slow oxidative fibers with high mitochondrial content, capillary density, and myoglobin for fatigue resistance. Panel B: Type IIa fast oxidative-glycolytic fibers with intermediate mitochondrial and capillary density combining speed with endurance. Panel C: Type IIx fast glycolytic fibers with low mitochondrial content producing powerful but fatigable contractions. Panel D: Comparative properties including contraction speed, fatigue resistance, and metabolic characteristics across fiber types.</image>

Muscle Metabolism

Muscle contraction requires ATP for cross-bridge cycling (myosin ATPase) and calcium sequestration (SERCA). Because intramuscular ATP stores are limited—sufficient for only 1-2 seconds of maximal contraction—multiple pathways regenerate ATP to sustain activity.

The creatine phosphate (phosphocreatine) system provides the most rapid ATP regeneration. Creatine kinase catalyzes the transfer of a phosphate group from creatine phosphate to ADP, regenerating ATP nearly instantaneously. This system powers the first 10-15 seconds of intense activity before creatine phosphate stores are depleted. It operates without oxygen and produces no fatigue-inducing metabolites.

Anaerobic glycolysis breaks down glucose to pyruvate, generating ATP rapidly without oxygen. When oxygen is limiting or energy demand exceeds oxidative capacity, pyruvate is converted to lactate, regenerating NAD⁺ to allow glycolysis to continue. This pathway provides ATP for 1-2 minutes of high-intensity activity but produces hydrogen ions that contribute to muscle fatigue. Lactate accumulation was historically blamed for fatigue, but current understanding emphasizes hydrogen ion effects on enzyme function and calcium handling.

Aerobic (oxidative) metabolism provides the most efficient and sustained ATP production, generating approximately 36 ATP per glucose molecule through glycolysis, the citric acid cycle, and oxidative phosphorylation. This pathway requires oxygen delivery and thus depends on cardiovascular fitness. It can utilize carbohydrates, fats, and even proteins as fuel substrates. Aerobic metabolism predominates during prolonged, moderate-intensity exercise and can sustain activity for hours as long as fuel and oxygen are available.

Oxygen debt refers to the elevated oxygen consumption persisting after exercise. This excess post-exercise oxygen consumption (EPOC) reflects several processes: replenishment of ATP and creatine phosphate stores, reconversion of lactate to glucose (gluconeogenesis), restoration of myoglobin oxygen stores, and metabolic effects of elevated catecholamines and body temperature.

<image>Panel A: Immediate energy from stored ATP depleted within seconds and creatine phosphate system providing rapid ATP regeneration for 10-15 seconds. Panel B: Anaerobic glycolysis peaking around 1-2 minutes providing ATP without oxygen but producing lactate. Panel C: Aerobic oxidative metabolism dominating after several minutes with sustained ATP production from carbohydrates and fats. Panel D: Oxygen consumption during and after exercise showing oxygen debt in recovery period for metabolic restoration.</image>

Smooth Muscle

Smooth muscle differs fundamentally from skeletal muscle in its structure, regulation, and functional properties, reflecting its specialized roles in visceral organs and blood vessels.

Structurally, smooth muscle lacks the sarcomeric organization of striated muscle. Dense bodies throughout the cytoplasm and at the cell membrane anchor thin filaments, functioning analogously to Z-lines but allowing the cell to contract in multiple dimensions. Thick and thin filaments are present but not aligned in register, explaining the absence of striations. The sarcoplasmic reticulum is sparse compared to skeletal muscle, and external calcium entry through the plasma membrane contributes significantly to contraction.

Smooth muscle contraction is regulated through the thick filament rather than the thin filament, a fundamental difference from skeletal muscle. Smooth muscle lacks troponin. Instead, contraction is initiated when calcium binds to calmodulin, a ubiquitous calcium-binding protein. The calcium-calmodulin complex activates myosin light chain kinase (MLCK), which phosphorylates the regulatory light chains of myosin. Phosphorylation enables the myosin ATPase activity necessary for cross-bridge cycling. Relaxation occurs when myosin light chain phosphatase (MLCP) dephosphorylates the myosin light chain, and calcium is removed from the cytoplasm by SERCA, the plasma membrane calcium ATPase (PMCA), and the sodium-calcium exchanger (NCX).

Smooth muscle exhibits two functional types. Single-unit (visceral) smooth muscle functions as a syncytium, with gap junctions coupling cells electrically. Pacemaker cells generate spontaneous depolarizations that spread through the tissue, producing coordinated contractions as seen in the gastrointestinal tract and uterus. Multi-unit smooth muscle consists of discrete motor units without electrical coupling, each fiber innervated separately. This arrangement allows graded, precise control, as seen in the iris, large airways, and large blood vessel walls.

<image>Panel A: Smooth muscle structure with dense bodies, non-sarcomeric actin-myosin arrangement, and sparse sarcoplasmic reticulum. Panel B: Smooth muscle calcium-calmodulin-MLCK pathway for myosin light chain phosphorylation enabling cross-bridge cycling. Panel C: Skeletal muscle thin filament regulation through calcium-troponin-tropomyosin mechanism for comparison. Panel D: Comparative table of regulation site, troponin presence, calcium sources, and contraction characteristics between muscle types.</image>

Clinical Applications

Muscular dystrophies result from genetic defects affecting structural proteins that maintain muscle fiber integrity. Duchenne muscular dystrophy (DMD), the most common and severe form, results from mutations in the dystrophin gene that eliminate functional dystrophin protein. Without dystrophin linking the cytoskeleton to the extracellular matrix, repeated contraction causes membrane damage, calcium influx, and progressive muscle degeneration. Onset occurs in early childhood with proximal weakness, pseudohypertrophy of calves (fat and connective tissue replacement), and progression to wheelchair dependence and cardiorespiratory failure. Becker muscular dystrophy results from mutations producing reduced or truncated but partially functional dystrophin, causing a milder, later-onset phenotype.

Myasthenia gravis results from autoimmune destruction of nicotinic acetylcholine receptors at the neuromuscular junction. Reduced receptor density causes weakness that worsens with activity (fatigability) because repetitive stimulation depletes readily releasable acetylcholine faster than reduced receptors can compensate. Treatment includes acetylcholinesterase inhibitors such as pyridostigmine that increase acetylcholine availability, along with immunosuppression.

Malignant hyperthermia is a pharmacogenetic disorder caused by mutations in the ryanodine receptor (RYR1 gene) that produce abnormal calcium release in response to certain anesthetic agents, particularly volatile anesthetics (sevoflurane, isoflurane) and the depolarizing muscle relaxant succinylcholine. Uncontrolled calcium release causes sustained muscle contraction, hypermetabolism, hyperthermia, acidosis, and rhabdomyolysis. Without treatment, mortality is high. Dantrolene, which blocks ryanodine receptors and inhibits calcium release, is the specific antidote.

Cardiac troponins I and T serve as highly sensitive and specific biomarkers for myocardial injury. Because these isoforms differ from skeletal muscle troponins, their elevation in blood indicates cardiac rather than skeletal muscle damage. Serial troponin measurement is central to the diagnosis and risk stratification of acute coronary syndromes.

Neuromuscular blocking agents used during anesthesia act at the neuromuscular junction. Depolarizing agents (succinylcholine) bind nicotinic receptors and cause initial depolarization followed by persistent block as the membrane cannot repolarize. Non-depolarizing agents (rocuronium, vecuronium) competitively block acetylcholine binding without activating the receptor, producing flaccid paralysis reversed by anticholinesterases.

<image>Panel A: Duchenne muscular dystrophy showing absent dystrophin leading to membrane disruption, calcium entry, and progressive fiber degeneration. Panel B: Myasthenia gravis with antibodies reducing acetylcholine receptor density at the neuromuscular junction causing decremental EMG response. Panel C: Malignant hyperthermia from mutant ryanodine receptors causing uncontrolled calcium release with dantrolene treatment mechanism. Panel D: Cardiac troponin release from injured cardiomyocytes showing characteristic rise-and-fall pattern for myocardial infarction diagnosis.</image>

Summary

The three muscle types serve distinct functions: skeletal muscle produces voluntary movement, cardiac muscle pumps blood rhythmically, and smooth muscle controls visceral functions. Skeletal muscle organization progresses from whole muscle through fascicles and fibers to myofibrils composed of sarcomeres, the functional contractile units bounded by Z-lines and containing A-bands, I-bands, and H-zones.

The sliding filament theory explains contraction through the cross-bridge cycle: myosin heads attach to actin, produce the power stroke with ADP and phosphate release, detach when ATP binds, and cock to the high-energy position through ATP hydrolysis. Excitation-contraction coupling links the muscle action potential to contraction through T-tubule conduction, DHPR-RyR interaction, calcium release from the sarcoplasmic reticulum, calcium binding to troponin C, and tropomyosin movement exposing myosin binding sites.

Muscle mechanics include motor unit recruitment following the size principle, summation of twitches to tetanus, optimal force at ideal sarcomere length (length-tension relationship), and inverse force-velocity relationship. Muscle fiber types differ in contraction speed and metabolic properties: Type I (slow oxidative), Type IIa (fast oxidative-glycolytic), and Type IIx (fast glycolytic). Energy sources progress from stored ATP and creatine phosphate to anaerobic glycolysis and ultimately aerobic oxidative metabolism.

Smooth muscle uses calcium-calmodulin-MLCK signaling for myosin-based regulation rather than troponin-based thin filament regulation. Clinical conditions affecting muscle include muscular dystrophies (dystrophin mutations), myasthenia gravis (anti-AChR antibodies), and malignant hyperthermia (RyR1 mutations).

Key Terms

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