Chapter 40: Protection, Support, & Movement

Muscle Structure and Function

Basic Contractile Proteins

  • Actin – Found in all eukaryotic cells capable of movement (e.g., amoeboid movement, cilia, flagella).

  • Myosin – Works with actin in muscle contraction.

  • Associated Proteins – Tropinin & Tropomyosin regulate actin’s interaction with myosin.

Muscle Organization

  • Muscle fibers contain bundles of myofibrils, which are made of myofilaments.

  • Myofibrils are enclosed by the sarcolemma and contain a sarcoplasmic reticulum.

  • Sarcomeres (contractile units) consist of A and I bands, where:

    • Actin filaments are in the I band.

    • Myosin filaments are in the A band.

Neuromuscular Junction & Muscle Contraction

Neuromuscular Junction

  • Acetylcholine (ACh) is released from neurons, attaching to receptors on the sarcolemma.

  • Sodium ions (Na⁺) enter the muscle cell, triggering the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum, initiating contraction.

Muscle Contraction Cycle

  1. ATP binds to myosin, detaching it from actin.

  2. Nerve impulse releases Ca²⁺ from the sarcoplasmic reticulum.

  3. Calcium binds to troponin, shifting tropomyosin to expose actin binding sites.

  4. Myosin splits ATP to ADP + Pi, storing energy.

  5. Myosin head attaches to actin.

  6. Pi is released, triggering the power stroke (myosin pulls actin, contracting the muscle).

  7. ADP is released, and the cycle repeats with new ATP.

Muscle Tone

  • A constant partial contraction of muscles for posture & readiness.

Muscle Types

  • Skeletal Muscle:

    • Slow-twitch (Type I, red fibers) – Used for endurance activities.

    • Fast-twitch (Type II, white fibers):

      • Type IIx (common in sedentary people, can convert to Type IIa with training).

      • Type IIa (more fatigue-resistant than IIx).

  • Smooth Muscle – Found in organs (e.g., small intestines, arterioles).

  • Cardiac Muscle – Found in the heart.

White vs. Dark Meat in Poultry

  • White meat (breast, wings): More fast-twitch fibers for short bursts of energy.

  • Dark meat (legs, thighs): More slow-twitch fibers, better for sustained activity.

Energy for Muscle Activity

Phosphagen System (Immediate Energy) 

  • Stored ATP is used first.

  • Phosphocreatine is broken down to regenerate ATP.

Glycolysis (Short-Term Energy)

  • Breaks down glycogen into glucose.

  • Anaerobic glycolysis (without oxygen) produces lactic acid, causing muscle burn.

  • Why can’t we sprint long distances? The body switches to aerobic respiration, which is slower but produces more ATP.

Aerobic Respiration (Long-Term Energy)

  • Occurs in mitochondria, requires oxygen.

  • Produces 36 ATP per glucose (much more than anaerobic fermentation).

  • Fuels endurance activities by using carbohydrates, fats, and proteins.

Glucose Breakdown Summary 

  1. Glycolysis – Glucose (6C) → 2 Pyruvate (3C each) + 2 ATP.

  2. With Oxygen (Aerobic Respiration): Pyruvate enters mitochondria → 36 ATP produced.

  3. Without Oxygen (Fermentation): Pyruvate → Lactic acid, producing only 2 ATP.

Fermentation in Yeast

  • Pyruvate → Ethanol + CO₂ (used in baking and brewing).