Muscle System Function Quiz

Musculoskeletal System (Part 2)

• Learning Outcomes: Students will be able to list muscle types, characterize each muscle, understand action potential mechanisms, identify neuromuscular junction components and events, and understand muscle tissue contraction and relaxation.

Functions of Muscular System

• Producing body movements
• Stabilizing body positions
• Moving substances within the body
• Producing heat
• Supporting soft tissue
• Guarding body entrances and exits
• Providing nutrient reserves

Types of Muscle Tissues

• Skeletal muscle tissue: Primarily attached to bones; striated and voluntary.
• Cardiac muscle tissue: Forms the heart wall; striated and involuntary.
• Smooth muscle tissue: Located in internal organs; non-striated and involuntary.

Skeletal Muscle Tissue

• Most abundant tissue in the human body.
• Under voluntary control.
• Contains skeletal muscle cells, connective tissues, blood vessels, and nerves.
• Skeletal muscle cells are called skeletal muscle fibers.
• Each skeletal muscle fiber is long, cylindrical, and contains many nuclei and striations (alternating light and dark bands).
• Skeletal muscle fibers organize into bundles, which further combine to form muscles.

Skeletal Muscle Tissue Components

• Epimysium: Surrounds the entire muscle.
• Perimysium: Sheathes bundles of muscle fibers (fascicles).
• Endomysium: Covers individual muscle fibers.
• Tendons: Attach muscle to bone or another muscle.

Skeletal Muscle Fibers

• Sarcolemma: Cell membrane of the muscle fiber
• Sarcoplasm: Cytoplasm of the muscle fiber
• Sarcoplasmic Reticulum: A modified endoplasmic reticulum with a high concentration of calcium ions
• T-tubules: Allow for rapid transmission of action potentials deep into the muscle fiber.
• Myofibrils: Contain protein filaments, actin, and myosin for muscle contraction..
• Sarcomeres: The repeating units of myofibrils are sarcomeres.

Cardiac Muscle Tissue

• Found only in the heart.
• Composed of cells called cardiocytes.
• Striated, and involuntary.
• Usually contains centrally located single nucleus.
• A bundle of specialized cells (pacemaker) in the upper part of the heart creates electrical signals that rhythmically contract the heart and pumps blood.

Smooth Muscle Tissue

• Found in many internal organs (abdomen, intestines, blood vessels).
• Spindle-shaped cells with a single nucleus.
• Non-striated and involuntary.
• Enables movement of food through the digestive tract, controls blood flow, and regulates pupil size.

Muscle Fiber Contraction

• Action potential: starts at the neuromuscular junction and travels along the sarcolemma causing Ca2+ release from the sarcoplasmic reticulum.
• Thick/thin filament interaction: Calcium ions bind to troponin, causing tropomyosin to expose the active binding sites on actin. Myosin heads attach to actin, undergo a power stroke, and contract. ATP is needed to detach myosin heads. This repeats, shortening the sarcomere.
• Muscle fiber contraction: The contraction of multiple sarcomeres reduces the length of the entire muscle, causing movement.

Sliding Filament Theory

• Thick and thin filaments slide past each other during muscle contraction.
• The process begins with calcium release from the sarcoplasmic reticulum.
• Calcium binds to troponin, causing tropomyosin to move and expose actin binding sites.
• Myosin heads attach to actin, form cross-bridges, and pull thin filaments towards the center of the sarcomere, shortening the sarcomere. ATP is used to allow release of the cross-bridges.

Muscle Relaxation

• Acetylcholinesterase breaks down ACh, stopping nerve impulse propagation, and reducing the Ca2+ concentration in the sarcoplasm.
• Troponin and tropomyosin return to their original positions covering the actin binding sites, preventing further cross-bridge formation.
• The muscle fibers lengthen.

Tension Production

• The amount of tension produced depends on the number of cross-bridges formed.
• Skeletal muscle contracts most forcefully over a range of resting lengths.
• A twitch is a single contraction and relaxation cycle in response to a stimulus.

Summation

• Repeated stimulation before relaxation leads to wave summation (one twitch added to another).
• Incomplete tetanus results when the muscle does not completely relax between stimuli
• Complete tetanus occurs when the relaxation phase is eliminated.

Motor Units and Recruitment

• A motor unit consists of a motor neuron and the muscle fibers it stimulates.
• A single motor neuron may innervate 100-3000 muscle fibers to perform movements.
• Precise control requires smaller motor units with fewer muscle fibers.
• The strength of a contraction depends on the number and size of the motor units activated.

Fatigue

• Muscle fibers use ATP faster than they produce it, resulting in weaker contractions.
• ATP is necessary to detach myosin heads from actin.
• Muscle cramps and rigor mortis are examples of fatigue.

Types of Muscle Contractions

• Isometric contractions: Tension increases, but muscle length does not change.
• Isotonic contractions: Tension increases, and muscle length changes as the muscle shortens during concentric contractions or lengthens during eccentric contractions. These contractions always involve resistance and speed of contraction that are inversely related..
  • Concentric: Muscle shortens as it contracts
  • Eccentric: Muscle lengthens while it contracts

Muscle Performance

• Fast fibers (type II myosin): Large in diameter, high glycogen reserves, powerful contractions, but fatigue quickly. Called "white muscles"
• Slow fibers (type I myosin): Half the diameter of fast fibers, take longer to contract, have high, mitochondria, high myoglobin concentrations, resist fatigue, longer contractions. Called "red muscles"

Additional Information (Animations)

• YouTube links for animations on muscle contraction and related topics are included

Additional Information (Summary)

• The text covers different parts and functionalities of the human musculoskeletal system, giving insights into various muscles and how they perform.