Skeletal Muscle

Muscle Cell Structure and Function

Skeletal muscle cells, also known as muscle fibers, are specialized for contraction and movement. Their unique structure supports efficient force generation and energy use.

• Sarcolemma: The plasma membrane of a muscle cell, responsible for conducting electrical signals.

• Sarcoplasm: The cytoplasm of muscle cells, containing organelles and dissolved substances necessary for contraction.

• Sarcoplasmic Reticulum (SR): A specialized endoplasmic reticulum that stores and releases calcium ions, essential for muscle contraction.

• T Tubules: Invaginations of the sarcolemma that transmit action potentials deep into the muscle fiber.

• Myofibrils: Cylindrical structures within muscle fibers composed of repeating units called sarcomeres.

Filaments

• Thin Filament (Actin): Composed primarily of the protein actin, along with regulatory proteins.

• Thick Filament (Myosin): Made of the protein myosin, which interacts with actin to produce contraction.

Accessory Proteins

• Titin: Provides elasticity and stabilizes thick filaments.

• Nebulin: Helps align actin filaments within the sarcomere.

Regulatory Proteins

• Tropomyosin: Blocks myosin binding sites on actin at rest.

• Troponin: Binds calcium ions and moves tropomyosin away from binding sites during contraction.

Glycogen: A storage form of glucose in muscle cells, used for energy during contraction.

Mitochondria: Organelles responsible for producing ATP through aerobic respiration.

Contractile Structures

• I Band: Region containing only thin filaments.

• H Zone: Region containing only thick filaments.

• Sarcomere: The functional unit of muscle contraction, defined as the segment between two Z lines.

Sliding Filament Theory

The sliding filament theory explains how muscles contract by the sliding of actin and myosin filaments past each other, shortening the sarcomere.

• Myosin heads bind to actin, forming cross-bridges.

• ATP hydrolysis powers the movement of myosin heads, pulling actin filaments inward.

• Repeated cycles of cross-bridge formation and detachment result in muscle contraction.

Steps in Contraction

• Action potential arrives at the neuromuscular junction.

• Acetylcholine (ACh) is released from the motor neuron.

• ACh binds to receptors on the sarcolemma, initiating an action potential in the muscle cell.

• Action potential travels along T tubules, triggering calcium release from the sarcoplasmic reticulum.

• Calcium binds to troponin, causing tropomyosin to move and expose myosin binding sites on actin.

• Myosin binds to actin, and contraction begins.

ATP Supply

• Phosphocreatine: Provides a rapid source of ATP by donating a phosphate group to ADP.

• Carbohydrates: Broken down to glucose, which is metabolized for ATP production.

Mechanics of Body Movement

Types of Muscle Contraction

• Isometric Contraction: Muscle generates force without changing length.

• Isotonic Contraction: Muscle changes length while generating force; includes concentric (shortening) and eccentric (lengthening) contractions.

Muscle Types and Comparison

• Smooth Muscle: Found in walls of hollow organs; involuntary, non-striated.

• Cardiac Muscle: Found in the heart; involuntary, striated, with intercalated discs.

• Skeletal Muscle: Voluntary, striated, attached to bones for movement.

Comparison Table: Smooth vs. Skeletal Muscle

Control of Body Movement

Neural Reflexes

Reflexes are automatic responses to stimuli, mediated by neural circuits.

• Autonomic Reflexes: Regulate involuntary functions (e.g., heart rate, digestion).

• Innate Reflexes: Present at birth (e.g., withdrawal reflex).

• Learned Reflexes: Acquired through experience (e.g., conditioned responses).

• Skeletal Muscle Reflexes: Involve muscle contraction in response to stretch or other stimuli.

• Myotatic Unit: Group of muscles and nerves involved in a stretch reflex.

Integrated Control of Body Movement

• Feedforward Control: Anticipatory adjustments made before movement occurs.

• Voluntary Movement: Initiated consciously by the brain.

• Control in Visceral Muscles: Regulation of movement in smooth and cardiac muscle by autonomic nervous system.

Additional Review Questions

• Describe the differences in structure and function of the three types of muscle: Skeletal, cardiac, and smooth muscles differ in location, control, appearance, and contraction mechanisms.

• What is the difference between a muscle origin and its insertion? Origin is the fixed attachment, while insertion moves with contraction.

• How does antagonistic control of muscle groups work? Muscles often work in pairs; one contracts while the other relaxes to produce movement (e.g., biceps and triceps).

• Describe the sliding filament theory starting from calcium release from the sarcoplasmic reticulum: Calcium binds to troponin, tropomyosin shifts, myosin binds to actin, and filaments slide past each other.

• How is muscle relaxation an active process? ATP is required to pump calcium back into the SR and detach myosin from actin.

• Why do we refer to maximal contraction as tetanus? Tetanus is a sustained muscle contraction resulting from rapid, repeated stimulation.

• Describe how reflexes are organized: Reflexes involve sensory input, integration in the spinal cord or brainstem, and motor output to effectors.

Key Equation:

ATP hydrolysis by myosin:

Example: During a knee-jerk reflex, stretch receptors in the quadriceps muscle send signals to the spinal cord, resulting in contraction of the muscle and extension of the leg.

Additional info: Some details, such as the organization of the myotatic unit and feedforward control, were expanded for academic completeness.