7 Skeletal Muscle I

Skeletal Muscle I

Presented by Gianfranco Orta Calafiore, M.S., D.C.

Muscle Tissue Types

• Cardiac Muscle:

  • Controlled involuntarily, found only in the heart; responsible for pumping blood throughout the body.

  • Striated appearance and includes intercalated discs that facilitate synchronized contraction.

• Skeletal Muscle:

  • Under voluntary control, allows for conscious movement; primarily responsible for body movements and posture.

  • Striated appearance due to the organized arrangement of muscle fibers.

• Smooth Muscle:

  • Involuntary control, found in the walls of hollow organs (e.g., stomach, intestines, blood vessels); facilitates movement of substances through these systems.

  • Non-striated and controlled autonomously by the autonomic nervous system.

Muscle Tissue Properties

• Excitability: Ability of muscle cells to respond to stimuli, such as nerve impulses or hormonal signals.

• Contractility: Capacity of muscle fibers to shorten and generate force when stimulated.

• Extensibility: Ability of muscle cells to stretch without damage, allowing for a full range of motion.

• Elasticity: Ability of muscle tissue to recoil back to its original length after being stretched or contracted, which helps maintain muscle tone and posture.

Functions of Skeletal Muscle

1. Movement: Facilitates voluntary body movements, including locomotion and manipulation.

2. Posture/Position: Maintains appropriate posture and body position through sustained contractions.

3. Soft-tissue support: Supports surrounding soft tissues, protecting them and aiding in their function.

4. Protection: Acts as a barrier to underlying structures and organs, providing mechanical protection.

5. Maintain body temperature: Generates heat during muscle contraction, essential for thermoregulation.

6. Nutrient storage: Stores energy in the form of glycogen, which can be utilized during periods of intense activity.

Skeletal Muscle Organization

• Three layers of connective tissue:

  • Epimysium:

    • The outermost layer of dense irregular connective tissue that encases the entire muscle, providing structural support and protection.

  • Perimysium:

    • Divides the muscle into smaller bundles called fascicles; contains blood vessels and nerves serving the muscle fibers within these bundles.

  • Endomysium:

    • Surrounds each individual skeletal muscle fiber (cell); provides a supportive environment and contains a network of capillaries and myosatellite cells (stem cells).

Anatomy of Muscle

• Epimysium: The complete outer covering of the muscle, ensuring protection against mechanical injuries.

• Tendon: Connects muscle to bone, transmitting force generated by the muscle to facilitate movement.

• Perimysium: The tissue that surrounds fascicles, contributing to the overall structure of the muscle.

• Fascicle: A bundle of skeletal muscle fibers, surrounded by perimysium; the arrangement allows for organized contraction.

• Endomysium: Internal membrane that creates a nourishing environment for muscle fibers.

• Blood vessel: Essential for supplying nutrients and oxygen, removing wastes, and delivering hormones.

Tendons and Aponeurosis

• Tendon: Formed by the merging of all three connective tissue layers (epimysium, perimysium, and endomysium), facilitating the connection between muscle and bone.

• Aponeurosis: A broad, flat sheet of connective tissue that serves a similar function to tendons but can attach muscles to multiple sites.

Muscle Fibers Structure

• Multi-nucleated: Skeletal muscle fibers contain multiple nuclei, which allow for increased protein synthesis and muscle repair.

• Sarcolemma: Plasma membrane surrounding each muscle fiber, crucial for conducting electrical impulses (action potentials).

• Myofibrils: Bundles within muscle fibers that contract to produce muscle tension.

• Sarcoplasmic reticulum (SR): Membranous structure that stores calcium ions, crucial for muscle contraction.

• Terminal cisternae: Enlargements of sarcoplasmic reticulum adjacent to T tubules; store calcium ions used during contraction.

• Transverse tubule (T-tubule): Extensions of sarcolemma that penetrate into the muscle cell, facilitating the transmission of signals for contraction.

The Triad

• Composed of one T tubule and two terminal cisternae of the sarcoplasmic reticulum, essential for regulating calcium ions during muscle contraction and relaxation.

Muscle Fibers Characteristics

• Skeletal muscle is a striated muscle: Exhibits alternating light and dark bands due to the arrangement of contractile proteins.

• Myofibrils consist of:

  • Thin (actin) filaments: Integral in contraction and regulatory processes.

  • Thick (myosin) filaments: Responsible for generating force during contractions.

• Sarcomere: The functional unit of a myofibril, where the interaction of thin and thick filaments occurs, resulting in contraction.

Filament Composition

• A bands: Dark bands predominantly made up of thick myosin filaments and overlapping areas of thin filaments.

• I bands: Light bands representing areas containing only thin actin filaments.

• H zone: Center of the A band with only thick filaments; appears lighter during contraction due to overlapping.

• Z disc: Serves as the anchoring point for thin filaments, marking the boundaries of each sarcomere.

• M line: The midline of the sarcomere where thick filaments are anchored.

Mechanism of Contraction

• Sliding-filament theory: The filaments do not change in length; instead, they slide past each other, resulting in muscle contraction.

Contraction Cycle

1. Contraction Cycle Begins: Triggered by increased calcium ions released from the sarcoplasmic reticulum.

2. Active-Site Exposure: Calcium binds to troponin, causing a conformational change that exposes the active sites on actin filaments.

3. Cross-Bridge Formation: Myosin heads attach to the exposed active sites, forming cross-bridges.

4. Myosin Head Pivoting: The power stroke occurs as myosin heads pivot towards the M line, pulling the thin filaments along.

5. Cross-Bridge Detachment: ATP binds to the myosin head, leading to the release of the cross-bridge.

6. Myosin Reactivation: Hydrolysis of ATP re-energizes the myosin head, preparing it for the next cycle of contraction.

Neuromuscular Junction

• Axon terminals: End of the motor neuron, containing synaptic vesicles filled with acetylcholine (ACh).

• ACh release: Occurs into the synaptic cleft upon nerve impulse arrival, binding to receptors on the muscle fiber.

• Action potential propagation: ACh generation triggers action potentials that travel down the T-tubule, activating the sarcoplasmic reticulum to release calcium ions.

Steps to Initiate Muscle Contraction

1. ACh is released and binds to receptors on the muscle fiber membrane.

2. Action potential travels down T-tubule.

3. Sarcoplasmic reticulum releases calcium ions into the cytoplasm.

4. Active site exposure occurs on actin; cross-bridge formation begins.

5. Contraction process initiates, leading to muscle shortening.

Steps to End a Muscle Contraction

1. ACh removal: Achieved by breakdown via acetylcholinesterase (AChE) in the synaptic cleft.

2. Calcium recapture: Sarcoplasmic reticulum actively reabsorbs calcium ions.

3. Covering of active sites: Prevents cross-bridge formation, halting contraction.

4. Contraction conclusion: Muscle fibers relax, returning to their resting length.

Muscle Control

• Motor unit: Comprises a single motor neuron and all the muscle fibers it innervates, functioning as the basic unit for muscle control.

• Muscle twitch: A single, brief contraction from a motor unit, which serves as a basis for more complex activities.

• All or none principle: Ensures that when a motor unit is activated, all muscle fibers within that unit will contract fully; they either contract or not at all.

Types of Skeletal Muscle

1. Fast twitch (white fibers):

   • Larger diameter; stores glycogen and is adapted for rapid, intense bursts of activity.

   • Easily fatigued and relies primarily on anaerobic metabolism for energy.

   • Less myoglobin content, leading to lighter coloration.

2. Slow twitch (red fibers):

   • Smaller diameter; more durable and fatigue-resistant, designed for endurance activities.

   • Relies on aerobic metabolism and contains myoglobin for increased oxygen binding capacity, resulting in darker coloration.

3. Intermediate fibers:

   • Possess characteristics between fast and slow twitch fibers; can be trained to enhance endurance or power depending on the training regimen.