Lecture Notes: Muscular System (Chapter 10)

Overview

• Second lecture on the muscular system
• Focus on the physiology of muscle contraction
• Review of previous lecture on muscle anatomy

Warm-up Questions

1. Where is calcium stored in a cell?

   • Stored in the smooth endoplasmic reticulum, specifically called the sarcoplasmic reticulum (SR) in muscle cells.

2. Why is calcium stored in a cell?

   • Calcium binds to various molecules, which is usually undesirable, so it's stored to prevent unwanted binding.

3. What is a sarcomere?

   • The functional contractile unit of skeletal and cardiac muscle.

4. Primary proteins in a sarcomere:

   • Actin and myosin.

5. How is an action potential passed between neurons?

   • Across a synaptic cleft using neurotransmitters.

Muscle Structure Recap

• Muscle cells contain myofibrils, which consist of repeating units called sarcomeres.
• Sarcomeres run from one Z disk to another and are the sites of contraction.

Sliding Filament Theory

• Key Points:
  • Proteins (actin and myosin) do not shorten; instead, they slide past each other.
  • Actin filaments are pulled toward the M line, shortening the I band and potentially eliminating the H zone.
  • The overall length of the sarcomere shortens during contraction.

Neuromuscular Junction

• Components:
  • Axon terminals, synaptic cleft, and muscle cell membrane.
• Process:
  • Action potential arrives at axon terminal, releasing neurotransmitters (e.g., acetylcholine).
  • Acetylcholine binds to ligated cation channels on the muscle cell, triggering an action potential.

Excitation-Contraction Coupling

• Action potential travels along the sarcolemma and into T-tubules.
• T-tubules interact with sarcoplasmic reticulum to release calcium.
• Calcium binds to troponin, causing tropomyosin to move and expose active sites on actin.
• Myosin heads bind to actin, forming a cross-bridge and beginning the contraction cycle.

Cross-Bridge Cycle

• Steps:
  1. Formation: Myosin heads bind to actin; inorganic phosphate is released.
  2. Power Stroke: ADP is released, and myosin head pivots, sliding actin.
  3. Detachment: New ATP binds to myosin, breaking the cross-bridge.
  4. Reactivation: ATP is hydrolyzed to ADP and inorganic phosphate, re-cocking the myosin head.
• Cycles repeat as long as calcium is present.

Muscle Relaxation

• Calcium is pumped back into the sarcoplasmic reticulum via calquestrin.
• Without calcium binding, troponin and tropomyosin return to their original positions, covering actin binding sites.
• Titin helps restore the muscle to its resting length.

Special Case: Rigor Mortis

• After death, calcium leaks into muscle cells causing contraction (rigor mortis).
• Muscles stay contracted until ATP is depleted.
• Eventually, muscle proteins degrade, leading to relaxation.

Conclusion

• Understanding of muscle contraction and the sliding filament theory is crucial.
• Concepts apply to cardiac muscle as well.
• Next lecture will cover factors influencing muscle contraction.

Note: This lecture is crucial for understanding the basics of muscle physiology and will be foundational for further studies in courses like BIO139.