Muscle Contraction

Learning Objectives

Describe the molecular mechanisms behind skeletal muscle contraction.
Understand how muscle structure leads to a rise in calcium
Understand that role of ATP hydrolysis in the powerstoke.
Describe the major types of muscle pathophysiology with particular focus on dysfunctional ion channels, nicotinic acetylcholine receptors and molecular stablisers.
Appreciate the role of the myelin sheath in neuromuscular contraction.

Structure of the Sarcomere:
- Sarcomere: The basic contractile unit of skeletal muscle, containing:
  - Thick filaments: Composed of myosin.
  - Thin filaments: Composed of actin, tropomyosin, and troponin.
- Titin: Acts as a molecular spring, restoring the sarcomere to its resting state after contraction.
Sliding Filament Model:
- Thick myosin filaments slide over thin actin filaments, shortening the sarcomere.
- Changes During Contraction:
  - The I-band shortens.
  - The Z-discs move closer together.
  - The A-band remains the same length.
Initiation of Contraction:
- Motor neuron action potentials release acetylcholine (Ach) at the neuromuscular junction (NMJ).
- Ach binds to nicotinic receptors on the sarcolemma, opening ion channels and causing membrane depolarization.
- The action potential travels down T-tubules, depolarizing the sarcoplasmic reticulum (SR).
- Calcium ions are released from the SR into the cytoplasm.

Troponin-C Binding:
- Calcium binds to troponin-C, causing a conformational change.
- This shifts tropomyosin, exposing myosin-binding sites on actin.
Cross-Bridge Formation:
- Myosin heads bind to actin, forming cross-bridges.

Binding of ATP:
- ATP binds to the myosin head, causing it to detach from actin.
ATP Hydrolysis:
- ATP is hydrolyzed to ADP and Pi, energizing the myosin head ("cocked" position).
Power Stroke:
- Myosin releases Pi and pulls the actin filament inward, shortening the sarcomere.
Recycling:
- ADP is released, and ATP binds to repeat the cycle.

Cause:
- Mutation in the dystrophin gene.
- Dystrophin stabilizes the sarcolemma during contraction.
Effect:
- Muscle fibers are damaged during contraction.
- Intracellular calcium dysregulation leads to protein degradation and cell death.
Symptoms:
- Progressive muscle weakness.
- Loss of ambulation by ~12 years; death by ~20-30 years due to cardiac or respiratory failure.

Cause:
- Autoimmune destruction of nicotinic acetylcholine receptors at the NMJ.
Effect:
- Impaired neuromuscular transmission.
- Muscle weakness that worsens with activity but improves with rest.
Treatment:
- Acetylcholinesterase inhibitors: Increase Ach availability.
- Immunosuppressants: Reduce antibody production.

Cause:
- Degeneration of motor neurons (e.g., ALS).
- Genetic and environmental factors.
Effect:
- Progressive loss of voluntary muscle function.
- Death often results from respiratory failure.

The myelin sheath, produced by Schwann cells in the peripheral nervous system, insulates axons and:
- Increases conduction speed of action potentials via saltatory conduction.
- Ensures rapid and efficient signal transmission to skeletal muscles.
- Damage to myelin (e.g., multiple sclerosis) disrupts action potential propagation, impairing muscle contraction.

Contraction Overview:
- Calcium release initiates contraction by exposing actin-binding sites for myosin.
- ATP provides the energy for cross-bridge cycling and sarcomere shortening.
Nicotinic Acetylcholine Receptors:
- Ligand-gated ion channels at the NMJ mediate rapid depolarization and muscle contraction.
- Dysfunction leads to conditions like MG.
Myelin’s Role:
- Critical for action potential propagation and efficient neuromuscular signaling.

Reading: Campbell Biology, Chapter 50 (pages 1187-1193).
Practice Questions:
- Explain how the sliding filament model shortens the sarcomere.
- Compare the pathophysiological mechanisms of DMD, MG, and MND.
- Discuss how ATP hydrolysis facilitates the power stroke.

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