Skeletal Muscle Tissue

Types of Muscle Tissue

There are three main types of muscle tissue in the human body, each with distinct structural and functional characteristics:

• Cardiac Muscle: Involuntary, found only in the heart, branched and striated with intercalated discs for synchronized contraction.

• Smooth Muscle: Involuntary, found in walls of hollow organs and blood vessels, spindle-shaped, uninucleate, and non-striated.

• Skeletal Muscle: Voluntary, attached to bones, cylindrical fibers with multiple nuclei and striations, responsible for movement by acting on the skeleton as levers.

Functions of Skeletal Muscle Tissue

• Produce skeletal movement

• Maintain posture and body position

• Support soft tissues

• Guard entrances and exits (e.g., sphincters)

• Maintain body temperature (thermogenesis)

• Store nutrient reserves (glycogen)

Structure of Skeletal Muscle

Skeletal muscle is composed of muscle tissue, connective tissue, nerves, and blood vessels. The organization of these components is essential for muscle function.

• Epimysium: Dense layer of collagen fibers surrounding the entire muscle.

• Perimysium: Collagen and elastic fibers dividing muscle into fascicles (bundles of muscle fibers).

• Endomysium: Areolar connective tissue wrapping each muscle fiber (cell).

• Each muscle fiber contains myofibrils (bundles of protein filaments).

Development of Skeletal Muscle

Skeletal muscle fibers develop from embryonic cells called myoblasts:

• Myoblasts fuse to form large, multinucleate muscle fibers.

• Some myoblasts remain as myosatellite cells in the endomysium, aiding in muscle repair and regeneration.

• Mature muscle fibers can be up to 30 cm (12 inches) long and contain many nuclei, allowing rapid protein synthesis for contraction.

Key Terms in Muscle Cell Structure

• Muscle fiber: Another term for a muscle cell.

• Sarcolemma: The cell membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber.

• T-tubules (Transverse tubules): Extensions of the sarcolemma that penetrate into the cell, allowing rapid transmission of action potentials.

• Sarcoplasmic Reticulum (SR): Specialized smooth endoplasmic reticulum that stores calcium ions, essential for muscle contraction.

• Myofibrils: Bundles of myofilaments (thin and thick filaments) responsible for muscle contraction.

Sarcomere Structure

The sarcomere is the basic contractile unit of skeletal muscle, composed of repeating arrangements of thin (actin) and thick (myosin) filaments. The organization of these filaments gives skeletal muscle its striated appearance.

• Actin: Thin filament

• Myosin: Thick filament

• Sarcomeres are separated by Z lines.

• Smooth muscle lacks sarcomeres and is non-striated.

Thick and Thin Filament Structure

• Thick Filament (Myosin): Composed of ~300 myosin molecules, each with a tail and two heads. The core contains titin, which connects to Z-lines and provides elasticity.

• Thin Filament (Actin): Anchored to the Z-line, composed of F-actin (two strands of G-actin), nebulin (holds F-actin together), tropomyosin (covers myosin binding sites), and troponin (binds Ca2+ and moves tropomyosin).

The Sliding Filament Theory

The sliding filament theory explains how muscles contract:

• Thin filaments slide past thick filaments, shortening the sarcomere.

• H and I bands decrease in size, zones of overlap increase, and Z lines move closer together.

• The A band remains the same length.

• All sarcomeres in a myofibril contract simultaneously, shortening the entire muscle fiber.

Muscle Cell Excitability and Action Potentials

Skeletal muscle fibers are excitable cells with a resting membrane potential (~-80 mV). When stimulated, they generate action potentials that trigger contraction.

• Na+/K+ Pump: Maintains resting potential by pumping 3 Na+ out and 2 K+ in.

• Large negatively charged proteins inside the cell contribute to the negative charge.

• Depolarization occurs when ligand-gated Na+ channels open in response to acetylcholine (ACh), allowing Na+ influx.

• Voltage-gated Na+ and K+ channels propagate the action potential.

Neuromuscular Junction (NMJ) and Excitation-Contraction Coupling

The neuromuscular junction is the synapse between a motor neuron and a skeletal muscle fiber. It is essential for translating neural signals into muscle contraction.

• Motor neuron axon terminal: Releases ACh into the synaptic cleft.

• Synaptic cleft: Space between neuron and muscle fiber.

• Motor end plate: Region of sarcolemma with ACh receptors and Na+ channels.

• ACh binding opens Na+ channels, depolarizing the muscle fiber and generating an action potential.

• ACh is broken down by acetylcholinesterase, ending the signal.

Excitation-Contraction Coupling

This process links the action potential in the sarcolemma to muscle contraction:

1. Action potential travels along sarcolemma and down T-tubules.

2. Triggers Ca2+ release from the sarcoplasmic reticulum.

3. Ca2+ binds to troponin, moving tropomyosin and exposing myosin binding sites on actin.

4. Myosin heads bind to actin, initiating the contraction cycle.

The Contraction Cycle

The contraction cycle consists of several steps, powered by ATP:

1. Myosin heads are energized (cocked) by ATP hydrolysis.

2. Ca2+ binds to troponin, exposing active sites on actin.

3. Myosin heads form cross-bridges with actin.

4. Myosin heads pivot, pulling actin filaments toward the M-line (power stroke).

5. ADP and Pi are released; a new ATP binds, causing myosin to detach from actin.

6. ATP is hydrolyzed, re-cocking the myosin head for another cycle.

Muscle Tension and Contraction Types

Muscle Tension

Muscle tension is the force generated by muscle contraction. It depends on:

• The amount of overlap between thick and thin filaments (sarcomere length).

• The number of muscle fibers stimulated (motor unit recruitment).

Optimal overlap allows for the greatest number of cross-bridges and maximal tension.

Muscle Twitch and Phases

A muscle twitch is a single contraction-relaxation cycle in a muscle fiber, consisting of three phases:

• Latent Period: Action potential triggers Ca2+ release.

• Contraction Phase: Ca2+ binds troponin, cross-bridge cycling, tension rises.

• Relaxation Phase: Ca2+ returns to SR, cross-bridges detach, tension falls.

Types of Muscle Contractions

• Isotonic Contraction: Muscle changes length (shortens or lengthens) while tension remains constant.

• Concentric: Muscle shortens as tension exceeds load.

• Eccentric: Muscle lengthens as tension is less than load.

• Isometric Contraction: Muscle length does not change; tension never exceeds load (e.g., postural muscles).

Muscle Metabolism and Fiber Types

ATP and Muscle Contraction

Muscle contraction requires ATP, which is generated by:

• Aerobic Cellular Respiration: Glycolysis, citric acid cycle, and electron transport chain (requires oxygen).

• Creatine Phosphate: Stores energy to quickly regenerate ATP from ADP.

• Glycogen: Stored glucose for glycolysis and aerobic respiration.

Muscle Fiber Types

• Fast Fibers: Large, powerful, fatigue quickly, pale in color.

• Slow Fibers: Smaller, slower to contract, resist fatigue, rich in myoglobin, red in color.

• Intermediate Fibers: Characteristics between fast and slow fibers.

Clinical Correlations

Muscle Disorders

• Hypertrophy: Increase in muscle size due to repeated stimulation.

• Atrophy: Decrease in muscle size due to disuse, aging, or nerve damage.

• Muscular Dystrophy: Inherited diseases causing muscle weakness and deterioration (e.g., Duchenne and Becker types).

• Polio: Viral infection attacking motor neurons, causing paralysis.

• Tetanus: Bacterial toxin causing sustained muscle contraction.

• Botulism: Bacterial toxin blocking ACh release, causing paralysis.

• Myasthenia Gravis: Autoimmune disease reducing ACh receptors, leading to muscle weakness.

• Rigor Mortis: Post-mortem muscle contraction due to ATP depletion and sustained Ca2+ presence.

The Muscular System

Organization of Muscles

• Axial Muscles: Support and position the axial skeleton (head, neck, trunk).

• Appendicular Muscles: Support, move, and brace the limbs.

Muscle fiber arrangement (parallel, convergent, pennate, circular) and attachment points (origin and insertion) determine muscle action and leverage.

• Origin: Fixed attachment point.

• Insertion: Movable attachment point.

• Agonist (Prime Mover): Main muscle responsible for movement.

• Synergist: Assists the agonist.

• Antagonist: Opposes the agonist.

Summary Table: Muscle Tissue Types