Muscle Structure, Function, and Physiology: Study Notes for Anatomy & Physiology

Muscle Structure and Terminology

Muscle Tissue Organization

Muscle tissue is organized into hierarchical structures that facilitate contraction and force generation. Understanding the terminology is essential for studying muscle physiology.

• Epimysium: Surrounds the entire muscle.

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

• Endomysium: Surrounds individual muscle fibers (cells).

• Tendon: Connective tissue attaching muscle to bone.

• Sarcolemma: Muscle cell plasma membrane.

• Sarcoplasm: Muscle cell cytoplasm.

• Transverse tubules (T-tubules): Invaginations of sarcolemma that transmit electrical signals.

• Sarcoplasmic reticulum (SR): Smooth ER of muscle cell, stores and releases Ca2+.

Multinucleated muscle fibers result from the fusion of many cells during development.

Muscle Contraction: Structure and Function

Sarcomere Structure

The sarcomere is the basic contractile unit of muscle, composed of organized protein filaments.

• Actin: Thin filaments; contain myosin binding sites.

• Myosin: Thick filaments; heads hydrolyze ATP and bind actin.

• Tropomyosin: Covers myosin binding sites on actin in absence of Ca2+.

• Troponin: Anchors tropomyosin; binds Ca2+ to expose myosin binding sites.

• Z line: Defines boundaries of sarcomere.

• M line: Center of sarcomere.

• I band: Area between Z line and end of myosin (actin only); shortens with contraction.

• A band: Myosin length; does not change during contraction.

Sliding Filament Model

Muscle contraction occurs as myosin heads bind to actin and pull, causing filaments to slide past each other.

• ATP powers myosin cross-bridge cycling.

• Myosin heads bind actin, release, and rebind for each ATP hydrolyzed.

The Cross-Bridge Cycle

The cross-bridge cycle describes the repeated interaction between myosin and actin during contraction.

• Myosin heads bind actin, release after ATP hydrolysis.

• ADP and Pi are released, causing the power stroke.

• ATP binding causes myosin to detach from actin.

Excitation-Contraction Coupling

Excitation-contraction coupling links the action potential in the muscle cell to contraction.

• Action potential travels along sarcolemma and T-tubules.

• Triggers Ca2+ release from sarcoplasmic reticulum.

• Ca2+ binds troponin, moving tropomyosin and exposing myosin binding sites.

• Cross-bridge cycling begins.

• Ca2+ is pumped back into SR to end contraction.

Muscle Twitch and Summation

The Twitch

A muscle twitch is a single contraction in response to one action potential.

• Latent period: Time between stimulus and contraction.

• Contraction phase: Cross-bridge cycling occurs.

• Relaxation phase: Ca2+ reuptake, tension decreases.

Summation and Factors Affecting Force

Summation occurs when multiple action potentials increase contraction strength and duration.

• More Ca2+ released, more cross-bridges form.

• Increased stimulus frequency increases contraction force.

Motor Unit Recruitment

Motor units are groups of muscle fibers innervated by a single motor neuron. Recruitment increases force.

• Small motor units: Fine movements (e.g., eyes).

• Large motor units: Powerful movements (e.g., limbs).

• Recruitment: More motor units activated for greater force.

Skeletal Muscle Metabolism

Sources of ATP

Muscle contraction requires ATP, which is generated by several metabolic pathways.

• Phosphorylation of ADP by Creatine Phosphate: Substrate-level phosphorylation.

• Oxidative phosphorylation: In mitochondria, uses oxygen.

• Anaerobic glycolysis: In cytoplasm, does not require oxygen.

$\text{ADP} + \text{Creatine Phosphate} \rightarrow \text{ATP} + \text{Creatine}$

Muscle Fiber Types

Classification of Muscle Fibers

Muscle fibers are classified by contraction speed and metabolic properties.

• Slow oxidative: Slow contraction, high endurance, uses oxidative phosphorylation.

• Fast oxidative: Faster contraction, intermediate endurance.

• Fast glycolytic: Fast contraction, low endurance, uses glycolysis.

Fiber Type	Contraction Speed	Metabolism	Endurance
Slow oxidative	Slow	Oxidative phosphorylation	High
Fast oxidative	Fast	Oxidative phosphorylation	Intermediate
Fast glycolytic	Fast	Glycolysis	Low

Muscle Spindles and Golgi Tendon Organs

Muscle Spindles

Muscle spindles detect changes in muscle length and initiate reflexes to prevent overstretching.

• Intrafusal fibers wrapped in sensory neurons.

• Gamma motor neurons adjust spindle sensitivity.

Golgi Tendon Organs

Golgi tendon organs sense tension in tendons and protect muscles from excessive force.

• Increased tension activates sensory neurons, causing muscle relaxation.

Smooth Muscle Structure and Function

Smooth Muscle Characteristics

Smooth muscle is found in internal organs and blood vessels, controlled by the autonomic nervous system.

• No striations; contracts more slowly than skeletal muscle.

• Single-unit: Cells connected by gap junctions, contract as a unit.

• Multi-unit: Each cell responds independently, more precise control.

Summary Table: Muscle Fiber Types

Type	Contraction Speed	ATPase Activity	Metabolism	Endurance
Slow oxidative	Slow	Low	Oxidative	High
Fast oxidative	Fast	High	Oxidative	Intermediate
Fast glycolytic	Fast	High	Glycolytic	Low

Additional info:

• Some terminology and details were expanded for clarity and completeness.

• Tables were recreated and summarized for comparison of muscle fiber types.