Sliding Filament Theory Quiz

Sliding Filament Theory

• The sliding filament theory is the most accepted explanation for muscle contraction
• Muscles shorten or lengthen because myofibrillar filaments slide past each other without changing length
• Myosin cross-bridges are the molecular motors driving the shortening process
• They bind, rotate, and detach from actin filaments, needing ATP hydrolysis for energy

Molecular Mechanisms of Muscle Contraction

• In the relaxed state, calcium ion concentration is low
• Actin and myosin filaments lie parallel
• Myosin heads are in a high-energy state, "cocked up," bound to ADP and inorganic phosphate
• Active sites on G-actin molecules are covered by the troponin-tropomyosin complex

Sliding Filament Theory Diagrams

• Visual representations show sarcomeres in relaxed, partially contracted, and maximally contracted states
• Diagram shows overlapping thick (myosin) and thin (actin) filaments
• The diagram displays areas including the H zone, I band, A band, and Z disc in relation to contraction
• Figures demonstrate how filaments slide past each other during contraction

Muscle Contraction Process Steps

• Action potentials in the T tubule release calcium ions from the sarcoplasmic reticulum into the cytosol
• Calcium binds to troponin, causing a conformational change reducing the troponin-actin bond
• This exposes actin's active sites, allowing the "cocked" myosin to bind (cross-bridge)
• Myosin head undergoes a conformational (ratchet) change pulling the actin filament towards the centre of the sarcomere, releasing ADP and inorganic phosphate ("power stroke")
• ATP binds to the myosin head, decreasing its affinity for actin. The myosin head detaches from the actin filament.
• ATP is hydrolyzed, recocking the myosin head and resetting it for another cycle.
• Cycling continues until calcium levels remain high
• Myosin molecules do not move simultaneously; they move sequentially, sliding along the actin filament

Energy for Muscle Contraction

• The energy source for muscle contraction is ATP
• Myosin ATPase hydrolyzes ATP, energizing cross-bridges prior to cycling
• ATP binding to myosin dissociates cross-bridges from actin, allowing the next cycle of activity
• Calcium ATPase actively transports calcium into the sarcoplasmic reticulum, ending contraction and allowing relaxation

Rigor Mortis

• This is a condition of muscles after death
• ATP is unavailable, arresting the cycle at the actomyosin complex formation
• Formation of "permanent actomyosin" complexes leads to a state of rigor
• This condition ends with denaturation of proteins

Test your understanding of the sliding filament theory and the molecular mechanisms underlying muscle contraction. This quiz covers essential concepts such as filament interaction, ATP requirements, and the roles of calcium ions. Perfect for biology students looking to solidify their knowledge.