Lecture Notes: Action Potential and Muscle Contraction

Introduction

• Focus on how skeletal muscle contraction occurs at the molecular level.
• Skeletal muscle structures: fascicles → muscle fibers → myofibrils → myofilaments → sarcomeres.

Sarcomere Structure

• Bands and Zones:
  • A Bands: Darker regions; contain myosin.
  • I Bands: Lighter regions; contain actin.
  • H Zone: Lighter region within A Bands; divided by M line.
  • M Line: Contains protein myomicin.
  • Z Disc: Divides I bands.
• Functional Unit: Sarcomere from one Z disc to next.

Myofilament Structure

• Thick Filaments:
  • Made of myosin; have ATP and actin binding sites.
  • Myosin heads form cross bridges.
• Thin Filaments:
  • Made mainly of actin; have active sites for myosin binding.
  • Tropomyosin blocks binding sites in relaxed muscle.
  • Troponin binds actin, tropomyosin, and calcium.
• Elastic Filaments: Made of titin; provide structural core.

Sarcoplasmic Reticulum and T-Tubules

• Surround myofibrils; regulate calcium storage and release.
• T-tubules help signals reach all muscle cell regions.

Sliding Filament Model of Contraction

• Myosin heads bind to actin, pull thin filaments towards sarcomere center.
• Z discs pulled towards M line, shortening muscle cell.
• Process repeated for muscle contraction.

Neuromuscular Junction

• Site of nerve-muscle communication.
• Axon Terminal: Releases acetylcholine into synaptic cleft.
• Synaptic Cleft: Space containing acetylcholine receptors.
• Acetylcholine binding opens ion channels, sodium enters cell.

Action Potential

• Triggered by depolarization; sodium influx.
• Travels along sarcolemma, down T-tubules.
• Triggers calcium release from sarcoplasmic reticulum.

Excitation-Contraction Coupling

• Calcium binds to troponin, moves tropomyosin, exposes myosin-binding sites.
• Myosin-actin cross-bridge cycling begins, fueled by ATP.

Summary

• Signal at neuromuscular junction releases acetylcholine.
• Acetylcholine causes local depolarization and action potential.
• Calcium released, muscle contraction starts.
• Concept of electric potential and charge distribution is crucial.

Conclusion

• Fundamental understanding of skeletal muscle contraction.
• Future topics: graded responses, differences in muscle types.

These notes summarize the key concepts of muscle contraction as explained in the lecture, focusing on the molecular and cellular processes involved.