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1. What are the 3 types of muscle?

  • Skeletal muscle – voluntary, attached to bones, produces movement

  • Cardiac muscle – heart muscle, involuntary

  • Smooth muscle – organs & blood vessels, involuntary

2. Fascia coverings

  • Epimysium – covers entire muscle

  • Perimysium – surrounds fascicles (bundles of fibers)

  • Endomysium – surrounds individual muscle fibers

3. Why does the plasmalemma have shallow folds?

The folds increase surface area and help distribute force across the membrane during contraction. They reduce stress and prevent tearing.

4. What does the plasmalemma fuse with at the end of the muscle fiber?

It fuses with the tendon, helping transmit force from muscle to bone.

5. Satellite cells

  • Located between the sarcolemma (plasmalemma) and basal lamina

  • They are muscle stem cells

  • Help with muscle repair, growth, and regeneration

6. Extensions of the plasmalemma

Called T-tubules (transverse tubules)
They:

  • Carry the action potential deep into the muscle fiber

  • Trigger calcium release from the sarcoplasmic reticulum

7. What are myofibrils made up of?

Myofibrils are made of repeating units called sarcomeres, which contain:

  • Actin (thin filaments)

  • Myosin (thick filaments)

8. Sarcomere Structures


Image


Image


Image


Image

Structure

Location

Z-disks

Boundary of each sarcomere; anchor actin filaments

H-zone

Middle of sarcomere; only myosin present

M-line

Center of sarcomere; holds myosin filaments together

Titin

Runs from Z-disk to myosin; stabilizes & provides elasticity

9. Functions of skeletal muscle

  • Produce movement

  • Maintain posture

  • Stabilize joints

  • Generate heat (thermogenesis)

  • Store nutrients (glycogen, amino acids)

10. Role of calcium in contraction

  • Calcium is released from the sarcoplasmic reticulum

  • Binds to troponin

  • Moves tropomyosin off actin binding sites

  • Allows myosin to bind actin → cross-bridge formation → contraction

No calcium = no contraction.

11. What does nebulin do?

  • Stabilizes actin filaments

  • Helps regulate thin filament length

12. Excitation–Contraction Coupling (quick summary)

  1. Motor neuron releases acetylcholine

  2. Action potential travels along sarcolemma

  3. Down T-tubules

  4. Sarcoplasmic reticulum releases calcium

  5. Cross-bridge cycling begins

13. Role of ATP in contraction

ATP:

  • Detaches myosin from actin

  • Provides energy for the power stroke

  • Powers calcium pumps to return Ca²⁺ to SR

No ATP = rigor (like rigor mortis).

14. Type 1 vs Type 2 Fibers


Type 1

Type 2

Myosin ATPase

Slow

Fast

Contractile speed

Slow

Fast

Fatigue resistance

High

Low

Motor unit strength

Small

Large

Size of motor unit

Small

Large

Sarcoplasmic reticulum

Less developed

Highly developed

Force (same diameter)

Same

Same

Power (same diameter)

Lower

Higher

(Remember: force per cross-sectional area is the same; power differs because speed differs.)

15. Principle explaining orderly recruitment

Size Principle

Small motor units (Type 1) are recruited first → larger units (Type 2) added as force demand increases.

16. How is muscle force graded?

Two ways:

  1. Motor unit recruitment

  2. Rate coding (frequency of stimulation)

17. Two mechanisms explaining muscle memory

  1. Myonuclear addition – nuclei gained during hypertrophy remain even after detraining

  2. Epigenetic changes – DNA modifications that make retraining faster