뒤로Skeletal Muscle Tissue: Structure, Function, and Comparison
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Skeletal Muscle Tissue
Introduction to Muscle Tissue
Muscle tissue is a fundamental component of the human body, responsible for movement, posture, and heat generation. It is classified into three types: skeletal, cardiac, and smooth muscle, each with distinct structural and functional characteristics.
Skeletal muscle: Attached to bones, responsible for voluntary movements.
Cardiac muscle: Found only in the heart, responsible for pumping blood.
Smooth muscle: Located in walls of hollow organs, responsible for involuntary movements.
Properties of Muscle Tissue
Muscle tissue exhibits four key properties:
Contractility: Ability to shorten and generate force, due to actin and myosin myofilaments.
Excitability: Responds to nerve signals, generating electrical impulses along the sarcolemma.
Extensibility: Can be stretched by opposing muscle or internal substances.
Elasticity: Recoils after being stretched.
Terminology Specific to Muscle Tissue
Myo and mys: Prefixes meaning "muscle".
Sarco: Prefix meaning "flesh".
Sarcolemma: Plasma membrane of muscle cells.
Sarcoplasm: Cytoplasm of muscle cells.
Functions of Muscle Tissue
Muscle tissue serves several essential functions:
Produce movement: Skeletal muscle moves bones; smooth muscle moves substances through organs.
Open and close body passageways: Sphincters act as valves.
Maintain posture and stabilize joints: Muscle tone stabilizes synovial joints.
Heat generation: Muscle contractions produce heat, maintaining body temperature.
Structure and Organization of Skeletal Muscle
Gross Anatomy of Skeletal Muscle
Skeletal muscle is an organ composed of muscle tissue, connective tissue, blood vessels, and nerves. Connective tissue sheaths bind muscle fibers together and are continuous with tendons.
Epimysium: Surrounds entire muscle.
Perimysium: Surrounds each fascicle (group of muscle fibers).
Endomysium: Wraps each individual muscle cell.
Muscle Attachments
Most skeletal muscles attach from one bone to another:
Origin: Less movable attachment.
Insertion: More movable attachment.
Attachments may be direct (fleshy) or indirect (via tendons or aponeuroses).
Microscopic Anatomy of Skeletal Muscle
Skeletal muscle fibers are long, cylindrical, multinucleate cells formed by fusion of embryonic cells. Nuclei are located peripherally. 
Myofibrils and Sarcomeres
Myofibrils are specialized contractile organelles composed of repeating units called sarcomeres, which are the functional units of muscle contraction.
Z disc (Z line): Boundary of each sarcomere.
Thin (actin) filaments: Extend from Z disc toward center.
Thick (myosin) filaments: Located in center, overlap with actin.
A band: Full length of thick filament.
I band: Region with only thin filaments.
H zone: Center of A band, no thin filaments.
M line: Center of H zone, holds thick filaments together.
Titin and Other Myofibrils
Titin is a springlike molecule that resists overstretching, holds thick filaments in place, and unfolds when muscle is stretched.
Sarcoplasmic Reticulum and T Tubules
The sarcoplasmic reticulum (SR) is a specialized smooth ER that surrounds each myofibril and stores calcium ions. T tubules are deep invaginations of the sarcolemma, forming a triad with terminal cisterns of the SR.
Calcium ions released from SR trigger muscle contraction.
Triad: T tubule flanked by two terminal cisterns.
Mechanism of Muscle Contraction
Types of Contraction
Concentric contraction: Muscle shortens to do work.
Eccentric contraction: Muscle generates force as it lengthens.
Sliding Filament Mechanism
The sliding filament mechanism explains concentric contraction:
Myosin heads attach to actin in thin filaments and pull them toward the center of the sarcomere.
Thin and thick filaments do not shorten; sarcomere shortens.
Initiated by calcium release from SR and powered by ATP.
Changes in Striation Pattern During Contraction
Fully relaxed: Thin filaments partially overlap thick filaments.
Contracted: Z discs move closer, sarcomere shortens, I bands shorten, H zone disappears, A band remains same length.
Structure and Organizational Levels of Skeletal Muscle
Table: Structure and Organizational Levels
Structure and Organizational Level | Description | Connective Tissue Wrapping |
|---|---|---|
Muscle (organ) | A muscle consists of hundreds to thousands of muscle cells, plus connective tissue wrappings, blood vessels, and nerve fibers. | Covered by epimysium |
Fascicle (portion of the muscle) | A fascicle is a discrete bundle of muscle cells, segregated from the rest of the muscle by a connective tissue sheath. | Surrounded by perimysium |
Structure and Organizational Level | Description | Connective Tissue Wrapping |
|---|---|---|
Muscle Fiber (cell) | A muscle fiber is an elongated multinucleate cell; it has a banded (striated) appearance. | Surrounded by endomysium |
Myofibril (complex organelle containing myofilaments) | Myofibrils are rodlike contractile organelles that occupy most of the cell volume. Composed of sarcomeres arranged end to end, they appear banded, and bands of adjacent myofibrils are aligned. | None |
Structure and Organizational Level | Description | Connective Tissue Wrapping |
|---|---|---|
Sarcomere (a segment of a myofibril) | A sarcomere is the contractile unit, composed of myofilaments made up of contractile proteins. | None |
Myofilament or Filament | Contractile myofilaments are of two types—thick and thin. The thick filaments contain bundled myosin molecules; the thin filaments contain actin molecules (plus the troponin and tropomyosin). Sliding of the thin filaments past the thick filaments produces muscle shortening. The elastic filaments (titin) maintain the organization of the A band and provide elastic recoil when muscle contraction ends. | None |
Innervation of Skeletal Muscle
Neuromuscular Junction
Motor neurons innervate skeletal muscle tissue at the neuromuscular junction, where nerve endings meet muscle fibers.
Terminal boutons: Store neurotransmitters.
Synaptic cleft: Space between axon terminal and sarcolemma.
Acetylcholine (ACh) is released, stimulating muscle contraction.
Motor Units
A motor unit consists of a motor neuron and all the muscle fibers it innervates. Axons branch to form neuromuscular junctions with muscle fibers. 
Types of Skeletal Muscle Fibers
Classification of Muscle Fibers
Skeletal muscle fibers are categorized by their method of ATP production and contraction speed:
Slow oxidative fibers: Red, aerobic, slow contraction, fatigue resistant, small diameter.
Fast glycolytic fibers: White, anaerobic, rapid contraction, tire quickly, large diameter.
Fast oxidative fibers: Intermediate diameter, quick contraction, oxygen dependent, somewhat fatigue resistant.
Comparison of Skeletal, Cardiac, and Smooth Muscle
Table: Comparison of Muscle Types
Characteristic | Skeletal | Cardiac | Smooth |
|---|---|---|---|
Body location | Attached to bones or skin | Walls of the heart | Walls of hollow organs |
Characteristic | Skeletal | Cardiac | Smooth |
|---|---|---|---|
Cell shape and appearance | Long cylindrical, multinucleate, striated | Branching chains, uni- or binucleate, striated | Single, fusiform, uninucleate, no striations |
Characteristic | Skeletal | Cardiac | Smooth |
|---|---|---|---|
Connective tissue components | Epimysium, perimysium, endomysium | Endomysium attached to heart | Endomysium |
Characteristic | Skeletal | Cardiac | Smooth |
|---|---|---|---|
Presence of myofibrils and sarcomeres | Yes | Yes, irregular thickness | No, actin and myosin filaments present |
Presence of T tubules and site of invagination | Yes, at A-I junctions | Yes, at Z discs | No T tubules |
Characteristic | Skeletal | Cardiac | Smooth |
|---|---|---|---|
Sarcoplasmic reticulum | Elaborate | Less elaborate | Equivalent to cardiac muscle |
Source of Ca2+ for contraction | Sarcoplasmic reticulum | SR and extracellular fluid | SR and extracellular fluid |
Presence of gap junctions | No | Yes, at intercalated discs | Yes, in single-unit muscle |
Cells exhibit individual neuromuscular junctions | Yes | No | No, except in multi-unit smooth muscle |
Characteristic | Skeletal | Cardiac | Smooth |
|---|---|---|---|
Regulation of contraction | Voluntary via somatic nervous system | Involuntary, intrinsic system, autonomic regulation | Involuntary, autonomic nerves, hormones, local chemicals |
Energetics | Aerobic and anaerobic | Aerobic | Mainly aerobic |
Disorders of Muscle Tissue
Muscular Dystrophy
Muscular dystrophy is a group of inherited muscle-destroying diseases characterized by muscle enlargement due to fat and connective tissue, followed by muscle degeneration. Types include Duchenne and myotonic dystrophy.
Myofascial Pain Syndrome and Fibromyalgia
Myofascial pain syndrome: Caused by tightened bands of muscle fibers.
Fibromyalgia: Chronic pain syndrome, mostly affecting women, with symptoms of fatigue, sleep abnormalities, pain, and headache.
Muscle Tissue Throughout Life
Development and Aging
Muscle tissue develops from myoblasts, which fuse to form skeletal muscle fibers.
Cardiac muscle begins pumping blood three weeks after fertilization.
Satellite cells surround skeletal muscle fibers and aid in growth.
Muscle mass and strength decrease with age; sarcopenia is muscle wasting.
Sex Differences | Females: ~36% body mass; Males: ~42% body mass (due to androgens) |
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Additional info: The notes have been expanded with academic context and structured tables for clarity. All included images directly reinforce the adjacent explanations.
