Anatomy and Physiology: Muscle Contraction and Myosin Cycling

Introduction

• Professor Bob Long delivering a lecture on muscle contraction, specifically focusing on myosin cycling or cross-bridge cycling.
• Part of a series of muscle lectures, available on YouTube and Canvas.

Sarcoplasmic Reticulum and Calcium Ions

• Sarcoplasmic Reticulum (SR): Calcium reservoir.
  • Loaded with calcium ions.
  • Contains calcium ion channels and pumps.

Muscle at Rest

• Actin filaments have active sites covered by tropomyosin.
• Troponin is present, waiting for calcium to bind.
• Myosin heads are positioned as cross-bridges with ADP and phosphate attached.

Myosin Molecule as an Enzyme

• Myosin can hydrolyze ATP, acting as an ATPase.
• ATP structure: Adenosine with three phosphate groups.
  • Breaking ATP into ADP releases energy.

Energy Transfer in Myosin Heads

• Myosin heads hold ADP + phosphate.
• Energy is transferred to the hinge, creating potential energy.
• Myosin has a higher affinity for ATP than for actin or ADP.

Action Potential and Calcium Release

• Action Potential (AP) triggers calcium channels to open.
• Calcium binds to troponin, changing its shape.
  • Tropomyosin is pulled off the active site on actin.

Steps of Myosin Cycling

Active Site Exposure

• Calcium binds to troponin.
• Tropomyosin is moved away from actin's active site.

Cross-Bridge Attachment

• Myosin heads bind to exposed active sites on actin.
• ADP + phosphate is released.

Myosin Pivoting

• Myosin heads pivot or pull, moving actin filaments towards the M line.
• Results in sarcomere contraction.

Cross-Bridge Detachment

• Myosin heads detach when ATP binds.
• ATP is broken down to ADP + phosphate, reactivating myosin.

Reactivation of Myosin

• Myosin reverts to its original conformation with ADP + phosphate.
• Cycle can repeat if active sites remain exposed.

Energy Consumption and Muscle Fatigue

• Myosin cycling consumes a large number of ATP molecules.
• Explains muscle fatigue during prolonged contractions.

Rigor Mortis

• Occurs post-mortem due to lack of ATP.
• Myosin heads remain attached to actin, causing stiffness.
• Over time, proteins are broken down, relieving stiffness.

Conclusion

• Lecture part of online learning due to the coronavirus outbreak.
• Lecture notes follow the textbook on page 56.
• Worksheets and quizzes to be provided online.

• Note: These notes are based on Professor Bob Long's lecture on myosin cycling and muscle contraction.