Learning Objectives
- Differentiate between the cellular mechanisms of Atrophy and Hypertrophy.
- Identify the Ubiquitin-Proteasome System as the primary driver of myofibril removal during muscle wasting.
- Explain the role of Satellite Cells in skeletal muscle repair and their clinical absence in cardiac tissue.
- Describe the structural change of adding sarcomeres in parallel during muscle growth.
1. Muscle Adaptation
Skeletal muscle mass is determined by the dynamic balance between protein synthesis and protein degradation. This plasticity allows the body to conserve resources during periods of disuse or increase power in response to mechanical load.
| Feature | Atrophy (Wasting) | Hypertrophy (Growth) |
|---|---|---|
| Myofibrils | Decreased (↓): Proteins are removed via the Ubiquitin-Proteasome System. | Increased (↑): Growth occurs via the addition of sarcomeres in parallel. |
| Myonuclei | Decreased (↓): Reduction occurs through selective apoptosis of nuclei. | Increased (↑): New nuclei are added through the fusion of satellite cells. |
2. The Pathways of Change
The Atrophy Pathway (Degradation)
When a muscle is immobilized, or during systemic illness (cachexia), the cell activates its degradation machinery to recycle structural proteins.
- Mechanism: The cell tags myofibrillar proteins with Ubiquitin molecules.
- The Proteasome: This tagged protein is recognized and shredded by the Proteasome, a barrel-shaped complex that breaks proteins into peptides.
The Hypertrophy Pathway (Regeneration)
Resistance training causes micro-trauma, which signals for “backup” from specialized stem cells to reinforce and thicken the fiber.
- Satellite Cells: These are quiescent myogenic stem cells located between the sarcolemma and basal lamina.
- The Action: Upon activation, these cells proliferate and fuse with existing myofibrils, donating their nuclei to support increased protein synthesis.
Clinical Notes & Step 1 Pearls:
- Cardiac Contrast: Cardiac muscle lacks satellite cells. After a myocardial infarction, the heart cannot regenerate muscle; it heals via fibrosis (scarring).
- Parallel vs. Series: Adding sarcomeres in parallel increases fiber diameter/force. Adding them in series increases fiber length (seen during bone growth).
Activity: Adaptive Mechanics Quiz
Quick Mnemonics:
Ubiquitin: “U-be-quittin’” on your muscles (Atrophy).
Satellite: They stay on the outside (peripheral) until called in to help.