Lecture Notes: Structure and Function of Skeletal Muscle

Overview

• Continuation from previous class discussion on skeletal muscle structure.
• Focus on cellular details and organelles within skeletal muscle cells.

Key Concepts

Myofibrils

• Major organelle in skeletal muscle cells.
• Take up most of the cell's internal volume.
• The number of myofibrils depends on:
  • Cell type
  • Maturity
  • Usage
  • Muscle fiber type
• Organelles like mitochondria are arranged around myofibrils.

Excitation-Contraction Coupling

• Sarcoplasmic Reticulum (SR):
  • Stores and releases calcium.
  • Proximity to myofibrils is crucial for function.
• Calcium's Role:
  • Directly interacts with actin myofilaments.
  • Essential for muscle contraction.
• Action Potential:
  • Electrical event leading to mechanical contraction.
  • Calcium release and resequestering are part of this process.

Muscle Cell Structure

Sarcomere

• Contractile unit within myofibrils.
• Structure impacts biochemical processes.
• Boundaries marked by Z-lines.

Transverse Tubules (T-Tubules)

• Invaginations of the sarcolemma.
• Facilitate rapid transmission of action potentials into the muscle cell depth.
• Surround each myofibril at the Z-lines.
• Allow extracellular fluid to flow through the cell.

Hierarchy of Vocabulary

• Myocyte: Muscle cell
• Myofibril: Organelle within myocyte
• Myofilaments: Protein components (actin and myosin)

Sarcoplasmic Reticulum (SR) and Triad

Structure

• Terminal Cisternae:
  • Ends of the SR, store calcium.
• Longitudinal Tubules:
  • Connect terminal cisternae.
• Triad:
  • Composed of two terminal cisternae and one T-tubule.

Function

• Calcium release into cytoplasm triggers muscle contraction.
• Structural arrangement minimizes diffusion distance for efficient calcium delivery to actin.

Calcium's Role in Contraction

Calcium Release Process

• Action Potential: Initiated at neuromuscular junction.
• L-Type Calcium Channels (DHPR):
  • Voltage-gated, opened by action potential.
  • High-five interaction with ryanodine receptors (RYR).
• Ryanodine Receptors (RYR):
  • Major calcium release channels on SR.
• Released calcium binds to troponin C on actin.

Troponin Complex

• Troponin C: Binds calcium.
• Conformational change in troponin complex exposes myosin binding sites on actin.

Conclusion

• Importance of calcium in muscle contraction: magnitude of release determines force.
• Structural and functional integration in muscle cells ensures precise contraction and relaxation cycles.
• Upcoming focus on biochemical processes involved with muscle contraction.