Skeletal Muscle

Introduction to Skeletal Muscle

Skeletal muscle is a type of muscle tissue that is attached to bones and is responsible for voluntary movements of the body. It is composed of muscle fibers, nerves, blood vessels, and various connective tissues that work together to produce force and movement.

• Muscle fibers: Long, cylindrical cells that contract to produce movement.

• Nerves: Control muscle contraction through electrical impulses.

• Blood vessels: Supply oxygen and nutrients, remove waste products.

• Connective tissue: Includes tendons, aponeuroses, fascia, and muscle coverings (epimysium, perimysium, endomysium).

Connective Tissue Components

Connective tissues organize and support skeletal muscle structure, transmitting force and protecting muscle fibers.

• Tendons: Connect muscle to bone, transmitting force for movement.

• Aponeuroses: Broad, flat sheets of connective tissue that attach muscles to other tissues.

• Fascia: Surrounds and separates muscles, binding the body together.

• Epimysium: Outermost layer covering the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers called fascicles; contains blood vessels and nerves.

• Endomysium: Surrounds individual muscle fibers within a fascicle.

Muscle Fiber Structure

Each muscle fiber contains specialized organelles and structures that enable contraction.

• Nucleus: Skeletal muscle fibers are multinucleated.

• Mitochondria: Provide energy for contraction via ATP production.

• Sarcolemma: The cell membrane of a muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber.

• Myofibrils: Rod-like units within muscle fibers, composed of repeating contractile units called sarcomeres.

Muscle Structure and Sarcomeres

Sarcomere Organization

The sarcomere is the fundamental contractile unit of skeletal muscle, responsible for muscle contraction.

• Sarcomere: The segment between two Z-discs; contains thick (myosin) and thin (actin) filaments.

• Myofibril: Composed of repeating sarcomeres along its length.

• Filaments: Thick filaments (myosin) and thin filaments (actin) are arranged in a specific pattern.

• Bands: A band (dark, contains thick filaments), I band (light, contains thin filaments), H zone (center of A band, only thick filaments), Z disc (boundary of sarcomere).

Sliding Filament Theory

The sliding filament theory explains how muscles contract at the molecular level.

• Actin and myosin filaments slide past each other, shortening the sarcomere and producing contraction.

• Cross-bridge formation: Myosin heads bind to actin, pulling the thin filaments toward the center of the sarcomere.

• ATP: Required for myosin head detachment and re-cocking.

Equation:

Excitation-Contraction Coupling

Steps in Excitation-Contraction Coupling

Excitation-contraction coupling is the process by which an electrical signal (action potential) leads to muscle contraction.

1. Action potential arrives at the neuromuscular junction.

2. Acetylcholine is released, binding to receptors on the muscle fiber membrane.

3. Depolarization of the sarcolemma triggers an action potential in the muscle fiber.

4. Calcium ions (Ca2+) are released from the sarcoplasmic reticulum.

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

6. Myosin heads bind to actin, forming cross-bridges and initiating contraction.

7. After contraction, Ca2+ is pumped back into the sarcoplasmic reticulum, ending contraction.

Muscle Contraction Cycle

Cross-Bridge Cycle

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

1. Attachment: Myosin head binds to actin.

2. Power stroke: Myosin head pivots, pulling actin filament.

3. Detachment: ATP binds to myosin, causing it to release actin.

4. Re-cocking: ATP is hydrolyzed, re-cocking the myosin head for another cycle.

Equation:

Types of Muscle Contractions

Contraction Types

Muscle contractions can be classified based on changes in muscle length and tension.

• Concentric contraction: Muscle shortens while generating force (e.g., lifting a weight).

• Eccentric contraction: Muscle lengthens while generating force (e.g., lowering a weight).

• Isometric contraction: Muscle generates force without changing length (e.g., holding a weight steady).

Muscle Twitch and Tetanus

Muscle Twitch

A muscle twitch is the response of a muscle to a single action potential.

• Latent period: Time between stimulation and onset of contraction.

• Contraction period: Muscle tension increases.

• Relaxation period: Muscle tension decreases as Ca2+ is reabsorbed.

Tetanus and Summation

When stimuli are delivered in rapid succession, muscle contractions can summate, leading to tetanus.

• Incomplete tetanus: Muscle tension increases with each stimulus but does not reach a steady plateau; some relaxation occurs between contractions.

• Complete tetanus: Muscle tension reaches a maximum and remains steady; no relaxation occurs between contractions.

Staircase Phenomenon (Treppe)

The staircase phenomenon describes the progressive increase in muscle contraction force with repeated stimulation.

• Each successive twitch produces slightly more force than the previous one.

• Caused by increased Ca2+ availability and warming of the muscle.

Table: Types of Muscle Contractions

Additional Info

• Genetic variation: Some animals (e.g., certain cattle breeds and racing dogs) have a higher number of muscle fibers, resulting in greater muscle mass and strength. Additional info: This is due to genetic mutations affecting muscle development, such as the "double muscling" gene in cattle.

• Muscle cramps: Involuntary contractions can occur due to nerve misfiring or electrolyte imbalance.