Skeletal Muscle: Structure, Function, and Physiology

Overview of Muscle Tissue

Muscle tissue is essential for movement, posture, and various physiological functions. There are three main types of muscle tissue, each with distinct structural and functional characteristics.

• Skeletal Muscle: Voluntary, striated, elongated cells with peripheral nuclei. Responsible for body movement and posture.

• Cardiac Muscle: Involuntary, striated, branching cells with central nuclei. Found only in the heart.

• Smooth Muscle: Involuntary, non-striated, spindle-shaped cells with a single central nucleus. Found in walls of hollow organs.

Key Properties of Muscle Tissue:

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten and generate force.

• Extensibility: Ability to be stretched without damage.

• Elasticity: Ability to return to resting length after stretching.

Microscopic Structure of Skeletal Muscle Fibers

Skeletal muscle is composed of muscle fibers (cells), nerves, blood vessels, and connective tissues. Each muscle is an organ made up of hundreds to thousands of muscle fibers bundled into fascicles.

• Connective Tissue Layers:

  • Epimysium: Surrounds the entire muscle.

  • Perimysium: Surrounds fascicles (bundles of muscle fibers).

  • Endomysium: Surrounds individual muscle fibers.

• Muscle Fiber: Multinucleated, contains many mitochondria, and is packed with myofibrils (contractile elements).

• Myofibrils: Composed of repeating units called sarcomeres, the basic contractile unit.

• Myofilaments: Thick filaments (myosin) and thin filaments (actin, troponin, tropomyosin).

Organization of the Sarcomere

• Z-disc: Anchors thin filaments and connects adjacent myofibrils.

• A band: Region of thick filaments (myosin); appears dark.

• I band: Region of thin filaments (actin); appears light.

• H zone: Central region of A band with only thick filaments.

• M line: Center of H zone; connects thick filaments.

Excitation-Contraction Coupling

Excitation-contraction coupling is the process by which an action potential in a motor neuron leads to muscle contraction.

• Transverse (T) Tubules: Invaginations of the sarcolemma that conduct action potentials into the muscle fiber's interior.

• Sarcoplasmic Reticulum (SR): Specialized smooth ER that stores and releases Ca2+ ions, essential for contraction.

• Terminal Cisternae: Enlarged areas of the SR that release Ca2+ upon stimulation.

Steps of Excitation-Contraction Coupling

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

2. Voltage-sensitive proteins trigger Ca2+ release from the SR.

3. Ca2+ binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

4. Cross-bridge cycling occurs, leading to muscle contraction.

Sliding Filament Theory

The sliding filament theory explains how muscles contract by the sliding of thin filaments past thick filaments, shortening the sarcomere and thus the muscle.

• During contraction, the I band and H zone decrease in width, while the A band remains constant.

• Thick and thin filaments do not change length; their relative positions change.

Cross-Bridge Cycle

1. ATP Binding: Myosin head (in low-energy state) is detached from actin.

2. ATP Hydrolysis: Myosin head hydrolyzes ATP to ADP + Pi, entering a high-energy state.

3. Cross-Bridge Formation: Energized myosin head binds to actin.

4. Power Stroke: Release of ADP and Pi causes the myosin head to pivot, pulling actin toward the center of the sarcomere.

5. Detachment: Binding of new ATP molecule causes myosin head to detach from actin.

This cycle repeats as long as Ca2+ and ATP are available.

Regulation of Contraction by Calcium (Ca2+)

• Contraction is triggered by Ca2+ release from the SR into the cytoplasm.

• Ca2+ binds to troponin, shifting tropomyosin and exposing binding sites on actin.

• Relaxation occurs when Ca2+ is actively transported back into the SR by Ca2+ ATPases.

Roles of ATP in Muscle Contraction

• Hydrolysis of ATP energizes the myosin head for the power stroke.

• ATP binding to myosin is required for detachment from actin.

• ATP is necessary for active transport of Ca2+ back into the SR during relaxation.

Rigor Mortis

Rigor mortis is the stiffening of muscles after death due to the lack of ATP, which prevents detachment of myosin from actin. It begins several hours after death and lasts about 24 hours.

Summary Table: Comparison of Muscle Types

Additional info:

• Muscle contraction can be isotonic (muscle changes length) or isometric (muscle length remains the same).

• Motor unit recruitment and temporal summation allow for smooth, graded contractions.

• ATP for contraction is produced both aerobically (with oxygen) and anaerobically (without oxygen).