Unit 9

1. Muscle Fiber Anatomy (The Sarcomere)

The sarcomere is the actual unit that does the work. To identify it from a picture, look for these:

  • Actin: The "Thin" filament.

  • Myosin: The "Thick" filament (it has the heads that look like golf clubs).

  • Z-line: The boundaries of one sarcomere (it looks like a zigzag line).

  • Troponin & Tropomyosin: The "guards" on the actin that prevent contraction until Calcium arrives.

2. The Sliding Filament Model (How it moves)

Think of this like a rowing team.

  1. Stimulus: A motor neuron sends an electrical signal.

  2. Calcium Release: Calcium floods the muscle cell.

  3. The Guard Drops: Calcium binds to Troponin, which moves Tropomyosin out of the way.

  4. Power Stroke: Myosin heads grab the Actin and pull it toward the center.

  5. ATP: A fresh molecule of ATP is needed to make the Myosin "let go" so it can reset and pull again.

3. Muscle Energy Sources

Your muscles need a constant supply of energy (ATP). They get it in three ways:

  1. Creatine Phosphate: A "battery" that lasts about 10–15 seconds (good for a sprint).

  2. Anaerobic Respiration (Glycolysis): Breaks down sugar without oxygen. Fast, but produces Lactic Acid, which causes that "burning" feeling and fatigue.

  3. Aerobic Respiration: Uses oxygen to make a ton of ATP. Slow to start, but lasts for hours (good for a marathon).

4. Key Definitions to Know

  • Rigor Mortis: After death, ATP is no longer produced. Since ATP is needed for Myosin to "let go" of Actin, the muscles stay locked in a contracted state.

  • Atrophy vs. Hypertrophy: Atrophy is muscle wasting away (use it or lose it); Hypertrophy is muscles getting bigger (from exercise).

  • Peristalsis: The rhythmic, wave-like contraction of Smooth Muscle that moves food through your digestive tract.

  • Origin vs. Insertion: The Origin is the end of the muscle attached to the bone that doesn't move; the Insertion is attached to the bone that does move.

5. The Three Muscle Types

  • Skeletal: Attached to bones; Voluntary; Striated (striped); Multi-nucleated.

  • Cardiac: Found only in the heart; Involuntary; Striated; Contains Intercalated Discs (to sync the heartbeat).

  • Smooth: Found in walls of hollow organs (stomach, vessels); Involuntary; Non-striated (no stripes).

6. Muscle Anatomy (Large to Small)

A muscle is like a bundle of cables. You must know the layers and their protective "mysium" coverings:

  1. Whole Muscle: Surrounded by the Epimysium.

  2. Fascicle: A bundle of muscle cells; surrounded by Perimysium.

  3. Muscle Fiber (Cell): Individual cell; surrounded by Endomysium. Its membrane is called the Sarcolemma.

  4. Myofibril: Rod-like structures inside the cell.

  5. Myofilaments: The proteins Actin (thin) and Myosin (thick).

7. The Sarcomere & Sliding Filament Theory

The Sarcomere is the smallest functional unit of a muscle (from one Z-line to the next).

  • The Players: Myosin (thick filament with "heads") and Actin (thin filament).

  • The Guard: Tropomyosin blocks the myosin from touching the actin.

  • The Key: Calcium is released when a nerve impulse hits. Calcium binds to Troponin, which pulls the Tropomyosin away, exposing the actin.

  • The Movement: Myosin heads grab actin and pull. Important: The filaments themselves do not shorten; they simply slide past each other.

8. Energy Sources for Contraction

Muscles need ATP to contract and—crucially—to relax (to make the myosin "let go").

  1. Creatine Phosphate: Instant energy for very short bursts (approx. 15 seconds).

  2. Anaerobic Respiration (Glycolysis): No oxygen needed; fast but produces Lactic Acid, leading to muscle fatigue.

  3. Aerobic Respiration: Requires oxygen; produces the most ATP; used for long-term endurance.

9. Muscle Fiber Types

  • Slow-Twitch (Type I): Red in color; high endurance; lots of mitochondria (e.g., marathon runners).

  • Fast-Twitch (Type II): White in color; powerful and fast but fatigue quickly (e.g., sprinters).

10. Quiz "Quick-Hits" (Common Questions)

  • All-or-None Law: A single muscle fiber will either contract 100% or not at all; there is no "partial" contraction of a fiber.

  • Origin vs. Insertion: The Origin is the fixed end of a muscle; the Insertion is the end that moves the bone.

  • Hypertrophy: Muscles getting larger due to use/exercise.

  • Atrophy: Muscles wasting away due to lack of use.

  • Rigor Mortis: After death, a lack of ATP prevents myosin from releasing actin, locking muscles in place.