Chap 12 Study guide:

1. What are the structural components of a skeletal muscle fiber?

A muscle fiber is a single muscle cell. Inside it are bundles of myofibrils, which are made of myofilaments. These myofilaments include:

  • Thin filaments: made of actin, troponin, and tropomyosin

  • Thick filaments: made of myosin

💡 Tip: Think of a muscle like a rope made of smaller ropes (fascicles), and inside each smaller rope are even thinner threads (myofibrils).

2. What is the functional unit of a muscle fiber?

The sarcomere is the functional unit of a muscle fiber. It's the section of a myofibril that shortens during contraction.

💡 Tip: "Sarcomere = segment that slides!"

3. Describe the sliding filament model.

During contraction, actin filaments slide toward the center of the sarcomere, shortening it. Myosin heads form crossbridges with actin and pull the filaments inward using ATP.

4. Describe the structure of the thick and thin filaments.

  • Thick filaments: made of myosin; binds to actin and ATP

  • Thin filaments: made of actin, troponin, and tropomyosin

    • Troponin binds to calcium

    • Tropomyosin blocks actin sites until moved by troponin

5. Describe what happens at the neuromuscular junction (NMJ).

  • Motor neuron releases acetylcholine (ACh)

  • ACh binds to receptors on the motor end plate (part of the sarcolemma)

  • This triggers an action potential (AP) in the muscle fiber

💡 Tip: NMJ = neuron talks to muscle using ACh

6. Explain excitation-contraction coupling.

  • AP travels along the sarcolemma and down T-tubules

  • Triggers release of Ca²⁺ from sarcoplasmic reticulum (SR)

  • Ca²⁺ binds to troponin, moving tropomyosin

  • Myosin binds to actin → crossbridge cycle begins

7. What stimulates the release of Ca²⁺ from the sarcoplasmic reticulum?

The action potential traveling down the T-tubule stimulates the SR to release Ca²⁺.

8. Describe the crossbridge cycle.

  1. Myosin binds to actin (crossbridge)

  2. Power stroke – myosin pulls actin

  3. ATP binds to myosin, breaking the bridge

  4. ATP hydrolysis resets myosin head

💡 Tip: No ATP = myosin stuck → rigor mortis!

9. What causes the "latent period"?

The latent period is the delay between the AP and muscle twitch. It’s the time needed for Ca²⁺ release and crossbridge setup.

10. How are muscle fibers classified?

  • Based on speed (fast or slow)

  • Based on ATP source:

    • Oxidative: uses oxygen, long-lasting

    • Glycolytic: fast, short bursts

11. What are the 3 types of skeletal muscle fibers?

  1. Slow-twitch oxidative – endurance, fatigue-resistant

  2. Fast-twitch oxidative – mix of strength + endurance

  3. Fast-twitch glycolytic – strong, quick fatigue

💡 Tip: Think: SO, FO, FG (slow, fast, fast)

12. What is a motor unit?

A motor unit is one motor neuron and all the muscle fibers it activates.

13. How are motor units recruited?

  • Small units are used first (for light tasks)

  • Large units are added as more force is needed

💡 Tip: Body adds just enough units = efficient!

14. What’s the difference between isotonic and isometric contractions?

  • Isotonic: force > load → muscle shortens and moves load

  • Isometric: load > force → tension, but muscle does not shorten

15. How does summation lead to tetanus?

Rapid APs overlap → twitches stack → fused tetanus (sustained contraction)

16. What ends a muscle contraction?

  1. ACh is removed from synapse

  2. Ca²⁺ is pumped back into SR

17. Contrast skeletal, smooth, and cardiac muscle.

Feature

Skeletal

Smooth

Cardiac

Control

Voluntary

Involuntary

Involuntary

Striated?

Yes

No

Yes

Nuclei

Multinucleated

Single nucleus

Single nucleus

Speed

Fast

Slow

Moderate

Connections

None

Gap junctions

Gap junctions

Tetanus possible?

Yes

Yes

No (due to long APs)

Additional Terms Recap

  • Acetylcholine – neurotransmitter at NMJ

  • Fascicle – bundle of muscle fibers

  • Myofilament – actin or myosin strand

  • Myofibril – bundle of myofilaments inside fiber

  • Sarcomere – functional unit of contraction

  • Sarcolemma – muscle cell membrane

  • Sarcoplasmic reticulum – stores/release Ca²⁺

  • T-tubule – helps transmit AP into fiber

  • Motor end plate – part of sarcolemma that receives ACh

  • Tendon – connects muscle to bone

  • Recruitment – adding more motor units for more force

  • Force/Tension – muscle strength

  • Troponin & Tropomyosin – regulate actin-myosin binding

  • Oxidative vs. glycolytic – oxygen vs. no oxygen energy sources

  • Fast-twitch vs. Slow-twitch – strength vs. endurance