Muscle Structure and Function
💡 Core Concept: Skeletal muscle is designed to generate force. The Sarcomere is the fundamental contractile unit. Force production depends on the length of the muscle (Length-Tension Relationship) and the speed of contraction (Force-Velocity Relationship).
1. Elements of Muscle Structure
A. Connective Tissue Layers
- Epimysium: Surrounds the entire muscle belly.
- Perimysium: Surrounds fascicles (bundles of fibers). Contains blood vessels and nerves.
- Endomysium: Surrounds individual muscle fibers.
B. The Contractile Unit (Sarcomere)
The basic unit of muscle contraction, defined as the area between two Z-discs.
- Actin (Thin Filament): Contains Troponin and Tropomyosin.
- Myosin (Thick Filament): Contains heads that form cross-bridges.
- Titin: A giant protein responsible for Passive Tension (resting stiffness).
2. The Motor Unit
Defined as: One Alpha Motor Neuron and all the muscle fibers it innervates.
🏆 "Golden Points"
- Henneman’s Size Principle: Motor units are recruited from Small to Large (Type I → Type IIa → Type IIx).
- Small motor units (Type I) have low thresholds and are fatigue-resistant (Postural muscles).
- Large motor units (Type II) generate high force but fatigue quickly.
3. Muscle Function: Biomechanics
A. Length-Tension Relationship
The force a muscle can generate depends on its length.
- Active Tension: Generated by Actin-Myosin cross-bridges. Maximal at Resting Length (optimal overlap).
- Passive Tension: Generated by the stretch of connective tissues (Titin, Epimysium). Increases as muscle is lengthened.
- Total Tension: Active + Passive.
B. Force-Velocity Relationship
- Concentric: As speed increases, force Decreases. (You can't lift a heavy weight fast).
- Eccentric: As speed increases, force Increases initially. Eccentric contractions generate the highest total force.
C. Insufficiency (Multi-joint Muscles)
| Type | Definition | Example (Hamstrings) |
|---|---|---|
| Active Insufficiency | Muscle is too short to generate force. | Full Hip Extension + Knee Flexion (Cramp). |
| Passive Insufficiency | Muscle is too tight/long to allow full ROM. | Hip Flexion with Knee Extended (SLR limited). |
4. Clinical Implications
A. Immobilization
- Shortened Position: Leads to rapid atrophy, loss of sarcomeres, and increased connective tissue (stiffness).
- Lengthened Position: Less atrophy, may actually add sarcomeres in series.
B. Aging (Sarcopenia)
Loss of muscle mass and function associated with aging.
- Selective loss of Type II (Fast Twitch) fibers.
- Infiltration of fat (marbling) into muscle.
- Decrease in motor unit firing rates.
📝 20 High-Yield MCQs
Test your knowledge for Academic & Clinical Exams.
Q1. The fundamental functional unit of a skeletal muscle fiber is the:
Rationale: The Sarcomere is the contractile unit found between two Z-discs.
Q2. Which connective tissue layer surrounds an individual muscle fiber?
Rationale: Endomysium is the deepest layer, wrapping each single fiber. Perimysium wraps fascicles. Epimysium wraps the whole muscle.
Q3. A Motor Unit consists of:
Rationale: This is the standard definition. It is the smallest unit of neuromuscular control.
Q4. According to Henneman's Size Principle, which fibers are recruited first?
Rationale: The body recruits small, efficient, fatigue-resistant motor units (Type I) first, then recruits larger ones as force demand increases.
Q5. Passive Tension in a muscle is primarily generated by:
Rationale: Passive tension occurs when a muscle is stretched, caused by the resistance of non-contractile tissues like Titin and the extracellular matrix.
Q6. Active Tension is maximal at which muscle length?
Rationale: At resting length, the overlap between actin and myosin heads is optimal, allowing the maximum number of cross-bridges to form.
Q7. "Active Insufficiency" occurs when a two-joint muscle:
Rationale: When shortened across both joints (e.g., wrist flexion + finger flexion), the sarcomeres are too overlapped to generate force (grip is weak).
Q8. Which type of contraction generates the highest amount of force?
Rationale: Eccentric contractions utilize both active cross-bridges and the passive resistance of the stretched connective tissue, creating maximal force.
Q9. Sarcopenia refers to:
Rationale: Sarcopenia is the involuntary loss of skeletal muscle mass and strength as a result of aging.
Q10. Which protein covers the binding sites on Actin in a resting muscle?
Rationale: Tropomyosin blocks the active sites. Calcium binds to Troponin, which moves Tropomyosin out of the way.
Q11. Immobilizing a muscle in a shortened position results in:
Rationale: To adapt to the new short length, the muscle removes sarcomeres in series, making the muscle physically shorter and stiffer.
Q12. The Force-Velocity relationship states that during a CONCENTRIC contraction:
Rationale: Cross-bridges need time to attach and detach. At high speeds, fewer cross-bridges can form, reducing force.
Q13. "Passive Insufficiency" acts as a limit to:
Rationale: Passive insufficiency occurs when the antagonist muscle is stretched to its limit, physically preventing further range of motion at the joint.
Q14. Which fiber type is characterized by high mitochondrial density and fatigue resistance?
Rationale: Type I (Slow Oxidative) fibers are designed for endurance (aerobic metabolism) and resist fatigue.
Q15. A Pennate muscle architecture (like the Rectus Femoris) is designed for:
Rationale: Pennate muscles pack more fibers into a given area (Physiological Cross-Sectional Area), allowing for greater force generation at the expense of speed/ROM.
Q16. Delayed Onset Muscle Soreness (DOMS) is most commonly associated with:
Rationale: Eccentric contractions cause the most micro-trauma to the Z-discs and connective tissue, leading to DOMS.
Q17. The role of Titin is to:
Rationale: Titin acts as a molecular spring, keeping the myosin centered and providing resistance when the muscle is stretched.
Q18. The Golgi Tendon Organ (GTO) monitors:
Rationale: GTOs are located in the tendon and sense tension (force). Muscle Spindles sense length.
Q19. Calcium ions ($Ca^{2+}$) needed for contraction are stored in the:
Rationale: The Sarcoplasmic Reticulum releases Calcium when an action potential arrives.
Q20. A fusiform muscle (parallel fibers) is biomechanically designed for:
Rationale: Parallel fibers (like the Sartorius) are long, allowing for greater shortening distance and velocity, but less force than pennate muscles.
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