Human Phys Chapter 9

DMuscle Microanatomy

Q: Define the levels of skeletal muscle microanatomy.
A:

  • Muscle: Bundles of muscle fibers.

  • Muscle fiber: A single muscle cell.

  • Myofibril: Rod-like structures inside muscle fibers, made of sarcomeres.

  • Sarcomere: The functional unit of contraction, composed of actin & myosin filaments.

  • Filaments:

    • Actin: Thin filament; binding site for myosin.

    • Myosin: Thick filament; contains heads that form cross-bridges.

Q: What are the functions of tropomyosin and troponin?
A:

  • Tropomyosin: Blocks myosin-binding sites on actin at rest.

  • Troponin: Binds calcium to move tropomyosin, allowing contraction.

Q: What are the binding sites on myosin and actin?
A:

  • Myosin head: Contains actin-binding and ATP-binding sites.

  • Actin: Contains myosin-binding sites.

Motor Unit & Neuromuscular Junction (NMJ)

Q: Define a motor unit.
A: A motor neuron and all the muscle fibers it innervates.

Q: Outline the steps of motor unit activation at the NMJ.
A:

  1. Action potential reaches the axon terminal.

  2. Acetylcholine (ACh) is released into the synaptic cleft.

  3. ACh binds to nicotinic receptors on the muscle fiber.

  4. End plate potential (EPP) is generated.

  5. Action potential spreads along the sarcolemma.

  6. Acetylcholinesterase degrades ACh, stopping the signal.

Excitation-Contraction Coupling

Q: Differentiate excitation and contraction.
A:

  • Excitation: Electrical signal (action potential) in the muscle fiber.

  • Contraction: Mechanical response (muscle shortening).

Q: Define the latent period in muscle contraction.
A: The short delay between excitation and contraction, where calcium is released.

Q: What is the role of calcium in muscle contraction?
A:

  • Stored in the sarcoplasmic reticulum.

  • Released into the cytoplasm when stimulated.

  • Binds to troponin, shifting tropomyosin to expose actin-binding sites.

  • Allows myosin to form cross-bridges with actin.

Sliding Filament Mechanism

Q: Describe the steps of the sliding filament mechanism.
A:

  1. Myosin head binds to actin (cross-bridge formation).

  2. Power stroke—myosin pulls actin, shortening the sarcomere.

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

  4. ATP hydrolysis resets the myosin head for another cycle.

Q: When is ATP required in muscle contraction?
A:

  • Cross-bridge detachment

  • Resetting myosin heads

  • Calcium reuptake into the sarcoplasmic reticulum

Muscle Tension & Contraction Types

Q: Define tension and load.
A:

  • Tension: Force generated by muscle contraction.

  • Load: Resistance the muscle works against.

Q: Differentiate between isotonic and isometric contractions.
A:

  • Isotonic: Muscle shortens, load moves.

  • Isometric: Muscle does not shorten, load does not move.

Twitch & Frequency-Tension Relationship

Q: Define a muscle twitch.
A: A single contraction in response to one action potential.

Q: How do isotonic and isometric twitches differ in latent period and contraction time?
A:

  • Isotonic: Longer latent period, shorter contraction time.

  • Isometric: Shorter latent period, longer contraction time.

Q: Define summation in muscle contractions.
A: Increased force generation by rapid successive stimuli.

Q: What is the difference between unfused and fused tetanus?
A:

  • Unfused tetanus: Partial relaxation between contractions.

  • Fused tetanus: Continuous contraction with no relaxation.

Types of Muscle Fibers

Q: Compare the structural and functional differences between the three muscle fiber types.
A:

  1. Slow oxidative (Type I): High endurance, many mitochondria, uses oxygen.

  2. Fast oxidative (Type IIa): Moderate endurance, intermediate mitochondria, uses oxygen & glycolysis.

  3. Fast glycolytic (Type IIb): Low endurance, few mitochondria, uses glycolysis.

Q: Define recruitment and explain its effect on muscle contraction.
A:

  • Recruitment: Activation of more motor units to increase force.

  • More motor units increase contraction strength.

ATP Generation in Skeletal Muscle

Q: What is the first source of ATP in muscle contraction, and how long does it last?
A: Stored ATP, lasts a few seconds.

Q: How does creatine phosphate contribute to ATP generation, and for how long?
A: Transfers phosphate to ADP to regenerate ATP, lasts ~10-15 seconds.

Q: How long does stored glycogen provide ATP?
A: 30 seconds to 2 minutes, depending on fiber type.

Q: How long does oxidative phosphorylation provide ATP?
A: Hours, if oxygen is available.

Muscle Fatigue

Q: What conditions lead to muscle fatigue?
A:

  • Depletion of ATP, glycogen, or creatine phosphate.

  • Accumulation of metabolic byproducts (e.g., lactic acid).

  • Impaired calcium handling in the sarcoplasmic reticulum.

Q: What happens to the muscle during fatigue?
A: Reduced ability to generate force, slower contractions, longer relaxation times.