Quiz sur le système musculaire

Introduction to Muscle Physiology

• Muscle contraction is the basis of all movement
• Movement requires the cooperation of multiple body systems
• Muscle cells (muscle fibers) produce and use ATP as energy for muscle contraction and force production

Types of Muscle Tissues

• Skeletal Muscle: Primarily responsible for voluntary movements. Attaches to bones via tendons.
• Cardiac Muscle: Found only in the heart. Responsible for blood circulation.
• Smooth Muscle: Present in internal organs and tubes. Controls involuntary movements.

Muscle Tissue Structure

• Myofilaments: The contractile structures within muscle cells, primarily made of proteins.
• Skeletal Muscle Qualities: Exhibit diverse qualities and are classified based on their actions. Voluntary muscles are responsible for conscious movement. Striated muscles have a repetitive pattern giving them a striped appearance.

Functions of Skeletal Muscle

• Primary Function: Movement (locomotion, manipulation)
• Posture Maintenance: Maintaining the body's position.
• Circulatory Support: Assisting in returning venous blood to the heart.
• Heat Production: Thermoregulation.
• Protein Reserve: Skeletal muscles contain a significant amount of body protein.
• Energy Conversion: Converting chemical energy (ATP) into mechanical and heat energy.

Characteristics of Muscle Tissue

• Electrical to Mechanical Conversion: Converting electrical signals into mechanical muscle contraction.
• Stimulus Response: Responsive to stimuli.
• Contractility: Capacity to shorten.
• Extensibility: Capacity to lengthen.
• Elasticity: Capacity to return to its resting length.

Organization of Skeletal Muscle

• Attachment: Tendons attach muscles to bones, facilitating movement.
• Organization: Muscles composed of muscle fibers, fascicles, and individual muscle cells.
• Fascia: Connective tissue surrounding muscles for support and structure.
• Ligaments: Connect bones to other bones.

Internal Structure of Skeletal Muscle

• Epimysium: Outer layer of connective tissue surrounding the entire muscle.
• Perimysium: Connective tissue that surrounds bundles (fascicles) of muscle fibers.
• Endomysium: Connective tissue that surrounds each individual muscle fiber.
• T Tubules: Inward extensions of the sarcolemma that allow signals to penetrate deep into the muscle fiber.
• Sarcoplasmic Reticulum: A network of tubules that stores and releases calcium ions, regulating muscle contraction.
• Myofibrils: Long cylindrical structures within a muscle cell; composed of repeating units called sarcomeres.
• Sarcolemma: The cell membrane of a muscle fiber (cell).
• Sarcoplasm: The cytoplasm of a muscle fiber.

Microscopic Structure of Muscle Fibers

• Myofibrils: Components within muscle fibers that consist of repeating units called sarcomeres.
• Sarcomeres: The functional units of muscle contraction; composed of overlapping protein filaments (actin and myosin).
• Myofilaments: The proteins actin and myosin, which slide past each other during contraction.

Organization of Muscle Fibers

• Sarcomere Banding Patterns: Alternating light (I bands) and dark (A bands) regions give the muscle fiber its striated appearance. The basic contractile unit of striated muscle is called a sarcomere. A sarcomere contains overlapping myofilaments of actin and myosin.

Sarcomere Structure

• Z-discs: Boundaries of the sarcomere, anchoring the actin filaments.
• Actin filaments: Thin filaments that bind with myosin during contraction.
• Myosin filaments: Thick filaments with heads that bind to actin.
• I Bands: Regions containing only actin filaments.
• A Bands: Regions containing both actin and myosin filaments.
• H Zone: Region containing only myosin filaments; shortens during contraction.
• M Line: Middle of the sarcomere, anchoring the myosin filaments.

Auxiliary Proteins in the Sarcomere

• Structural proteins: Proteins that maintain the structure of the sarcomere.
• Regulatory proteins: Proteins that control the interaction between actin and myosin.
• Titin: A large protein that stabilizes the sarcomere and prevents excessive stretching.
• Nebulin: A protein that regulates the length of actin filaments.

Thick Filaments

• Myosin Molecule: Composed of a long tail and two globular heads.
• Head Structure: Heads are responsible for binding to actin and undergoing conformational changes during muscle contraction.
• ATP Binding: ATP is necessary for the binding of myosin to actin.

Thin Filaments

• Actin: The primary protein; with active sites that bind to the myosin heads.
• Tropomyosin: A regulatory protein that covers the active sites on actin in a resting muscle.
• Troponin: A regulatory protein that interacts with tropomyosin and calcium ions to control muscle contraction.

Function of Troponin and Tropomyosin

• Muscle Relaxation: Troponin, with calcium, moves tropomyosin off actin's active sites. Myosin cannot bind to actin; muscles relax.
• Muscle Contraction: When calcium binds to troponin, it causes tropomyosin to move. Myosin heads bind to actin; contraction occurs.

Muscle Fiber Contraction

• Excitation-Contraction Coupling: Conversion of a nerve impulse into a muscle contraction.
• Motor Neurone: Carries the signal to contract to the muscle fibre.
• Neurotransmitters: Chemicals released by the nerve ending (synapse) to generate a signal in the muscle fibre.
• Potential Action: An electrical signal triggering a muscle contraction.

Sliding Filament Theory

• Filament Sliding: Myosin heads bind to actin, causing filaments to slide past each other.
• Sarcomere Shortening: The shortening of the Z-discs reduces the distance between them during contraction; this shortens the sarcomere.
• Force Generation: The sliding of the filaments generates force.
• Constant Filament Length: The filaments themselves neither shorten nor lengthen.

Modification of the Sarcomere During Contraction

• A Band: Remains unchanged in length during contraction.
• I Band: Shortens during contraction.
• H Zone: Shortens and may disappear during contraction.
• Z-discs: Come closer together during contraction.

Excitation-Contraction Coupling (continued)

• Steps of Excitation-Contraction: Phases involved in transforming a stimulus to muscle contraction.
• Propagation of Depolarization: The electrical signal travels through the sarcolemma and T tubules.
• Calcium Release: The release of calcium from the sarcoplasmic reticulum.
• Force Generation: Myosin interacts with actin when calcium binds to troponin.

Cycle of Myosin

• Cross-Bridge Formation: Myosin heads attach to actin.
• Power Stroke: Myosin filament pulls the actin filament closer together.
• Detachment: ATP binding causes myosin to detach from actin.
• Re-energization: ATP hydrolysis energizes the myosin head, returning it to its extended position for the next cycle.

Relaxation

• Stopping the Signal: Cessation of the signal from the nervous system.
• Calcium Reuptake: Calcium returns to the sarcoplasmic reticulum.
• Tropomyosin Movement: Tropomyosin moves back to cover the actin binding sites, preventing further myosin interaction.

Motor Units Principle

• Motor Unit: A motor neuron and all of the muscle fibers it controls.
• All or None Principle: A muscle fiber either contracts fully or not at all when stimulated.
• Recruitment: Activating more motor units increases contractile force.

Muscle Fiber Types

• Contractile Properties: Rapid (fast-twitch), Slow (slow-twitch), intermediate twitch properties.
• Metabolic Properties: Glycolytic, oxidative, or a combination.
• Myosin Isozymes: Different forms of myosin allow for categorization.

Properties of Contractile Fiber Types

• Different Types of Motor Neurons: Type I and Type II
• Fiber Characteristics: Type I and Type II fibers have different sizes, metabolic processes, and functional properties (e.g. contraction speed, force production, etc)
• Recruitment Patterns: Fibers are recruited in a specific order (size principle) based on the intensity and duration of the needed contraction.

Metabolic Properties of Muscle Fibers

• Aerobic Metabolism: Utilizes oxygen to generate ATP, supporting sustained activities and endurance.
• Anaerobic Metabolism: Does not require oxygen but is not sustainable for long periods; supports high-intensity activities.
• Fiber Type Dominance: Oxidative or glycolytic pathways predominate depending on the fiber type.

Nomenclature and Characteristics of Muscle Fibers

• Various classifications differentiate muscle fibres in terms of structure and function.

Distribution of Muscle Fiber Types

• Inter-Individual Variation: Individuals exhibit variable proportions of different muscle fiber types.
• Training Response: Consistent training can alter the fiber type distribution to some extent but not fully.