Skeletal Muscle Review

Skeletal Muscle Review

• Organization: Skeletal muscle is composed of bundles (fascicles) of muscle fibers, each fiber surrounded by endomysium. Perimysium surrounds fascicles and epimysium surrounds the whole muscle.
• Connective Tissue: Tendons connect muscle to bone.
• Muscle Fiber Structure: Muscle fibers have myofibrils, which consist of sarcomeres.
• Sarcomere Anatomy: Light bands (I-bands) contain thin actin filaments, dark bands (A-bands) contain thick myosin filaments. H zone is part of A band, M-line is in the center. Z-lines mark the boundaries of sarcomeres.
• Protein Filaments: Thick filaments are myosin, thin filaments are actin.
• Muscle Contraction: Myosin heads bind to actin, creating cross-bridges. ATP is required for the cycle of movement (power-stroke), detaching, and resetting. Calcium ions are needed to initiate contraction.
• Excitation-Contraction Coupling: Action potential travels down the sarcolemma (muscle cell membrane) and into T-tubules. This triggers the release of calcium from the sarcoplasmic reticulum. Contraction begins as calcium binds to troponin, causing tropomyosin to shift, exposing myosin-binding sites on actin.

Muscle Contraction Details

• ATP's Role: ATP is required for muscle contraction to occur (for resetting after a power-stroke).
• Neurotransmitter (Ach): Motor neurons release acetylcholine (ACh) at the neuromuscular junction, stimulating muscle cells.
• Depolarization: Muscle cell depolarization causes the release of calcium ions from the sarcoplasmic reticulum leading to a contraction.
• Fatigue: Muscle fatigue occurs due to insufficient oxygen supply, leading in some cases to lactic acid buildup, which can cause pain and weakness.
• Neuromuscular Junction: The point where a motor neuron and a muscle fiber meet; there, acetylcholine initiates muscle contraction. Acetylcholinesterase is the enzyme that breaks down acetylcholine. Calcium is needed to initiate muscle contraction.
• Cross-bridges: Myosin heads form cross-bridges with actin filaments, initiating the sliding filament theory of muscle contraction.
• Isometric vs. Isotonic Contractions: Isometric contractions involve no change in muscle length. Isotonic contractions involve the changing of length.
• Muscle Energy Systems: Muscles use various energy pathways (aerobic and anaerobic) to produce ATP, depending on the intensity and duration of the activity.

Muscle Contraction Steps

• A nerve impulse in a motor neuron triggers the release of acetylcholine (ACh) at the neuromuscular junciton.
• ACh binds to receptors on the muscle fiber membrane, causing it to depolarize.
• The action potential travels along the sarcolemma and down the T-tubules.
• The sarcoplasmic reticulum releases calcium ions.
• Calcium ions bind to troponin, causing tropomyosin to shift, exposing myosin-binding sites on actin.
• Myosin heads bind to actin, forming cross-bridges and initiating the power stroke.
• ATP is required for detachment; the cycle repeats, creating the sliding filament mechanism.
• When the nerve impulse stops, calcium is reabsorbed, tropomyosin blocks the binding sites, and the muscle relaxes.

Rigor Mortis

• Stiffening of muscles after death due to the inability of ATP to detach myosin from actin filaments.

This quiz explores the organization, structure, and function of skeletal muscle. Topics include the arrangement of muscle fibers, the anatomy of sarcomeres, and the process of muscle contraction. Test your understanding of muscle physiology and connective tissues involved in movement.