Muscle Contraction: Myofibrils and Sarcomeres

• Muscles facilitate daily activities like breathing, blood circulation, and movement.
• Cardiac and smooth muscle tissues operate involuntarily.
• Skeletal muscle functions under voluntary control.
• Skeletal muscles consist of bundles of muscle fibers.
• Muscle fibers are elongated, cylindrical cells containing multiple nuclei.
• Muscles contract or relax in response to signals from the nervous system.
• A neuromuscular junction is where the synaptic bulb of an axon terminal connects with a muscle fiber, facilitating signal exchange.

Myofibrils and Sarcomeres

• Muscle fibers are composed of myofibrils.
• Myofibrils contain sarcomeres, which are contractile units.
• Sarcomeres are arranged adjacently along the myofibril.
• Sarcomeres consist of alternating thick and thin protein filaments, giving skeletal muscle a striated appearance.

Protein Filaments

• Muscle contraction occurs when the thick and thin filaments slide past each other.
• Thick filaments are made of myosin, anchored at the M line in the center of the sarcomere.
• Thin filaments are composed of actin, anchored to the Z lines at the outer edges of the sarcomere.
• Sarcomeres shorten from both sides when actin filaments slide along myosin filaments.

Sliding Filament Mechanism

• Myosin filaments pull actin along their length during contraction.
• Cross-bridges of myosin filaments attach to actin filaments and exert force to move them.
• The sliding filament mechanism describes how sarcomeres shorten without altering the length of the thick or thin filaments.

Contraction Process

• Contraction begins when ATP is hydrolyzed to ADP and inorganic phosphate.
• Hydrolysis of ATP causes the myosin head to extend and attach to a binding site on actin, forming a cross-bridge.
• The power stroke is triggered, pulling the actin filament toward the M line and shortening the sarcomere.
• ADP and inorganic phosphate are released during the power stroke.
• Myosin remains attached to actin until a new ATP molecule binds, freeing myosin for another cycle or relaxation.

Regulation by Calcium

• Muscle contractions are regulated by calcium ions.
• Thin actin filaments are associated with troponin and tropomyosin.
• Tropomyosin blocks cross-bridge binding sites on actin when the muscle is relaxed.
• Calcium ions bind to troponin, displacing tropomyosin and exposing myosin binding sites on actin when calcium levels are high enough and ATP is present.
• Calcium ions are stored in the sarcoplasmic reticulum and released in response to nervous system signals.

Neural Signals and Muscle Contraction

• Neurotransmitter molecules released from a neuron bind to receptors, depolarizing the muscle fiber membrane.
• Electrical impulses travel down T tubules and open calcium stores.
• Calcium ions flow to myofibrils, triggering muscle contraction.
• Sarcomeres shorten as Z lines draw closer to the M line as actin and myosin slide along each other
• Muscle fibers contract in unison to produce enough force to move the body.