Human Phys Chapter 9

What are the levels of skeletal muscle microanatomy?

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.

What is the function of tropomyosin?

Blocks myosin-binding sites on actin at rest.

What is the function of troponin?

Binds calcium to move tropomyosin, allowing contraction.

What are the binding sites on myosin?

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

What are the binding sites on actin?

Contains myosin-binding sites.

What is a motor unit?

A motor neuron and all the muscle fibers it innervates.

What are the first steps of motor unit activation at the NMJ?

Action potential reaches the axon terminal; Acetylcholine (ACh) is released into the synaptic cleft.

What activates muscle fibers at the NMJ?

ACh binds to nicotinic receptors on the muscle fiber, generating an end plate potential.

What is excitation in muscle physiology?

Electrical signal (action potential) in the muscle fiber.

What is contraction in muscle physiology?

Mechanical response (muscle shortening).

What is the latent period in muscle contraction?

The short delay between excitation and contraction, where calcium is released.

What role does calcium play in muscle contraction?

Calcium, when released into the cytoplasm, binds to troponin, causing tropomyosin to shift and expose actin-binding sites.

What is the first step in the sliding filament mechanism?

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

What occurs during the power stroke in muscle contraction?

Myosin pulls actin, shortening the sarcomere.

When is ATP required in muscle contraction?

Cross-bridge detachment, resetting myosin heads, and calcium reuptake into the sarcoplasmic reticulum.

What is tension in the context of muscle dynamics?

Force generated by muscle contraction.

What is load in the context of muscle dynamics?

Resistance the muscle works against.

What distinguishes isotonic contraction from isometric contraction?

Isotonic: Muscle shortens, load moves; Isometric: Muscle does not shorten, load does not move.

What is a muscle twitch?

A single contraction in response to one action potential.

How does isotonic twitch differ from isometric twitch in terms of contraction time?

Isotonic: Shorter contraction time; Isometric: Longer contraction time.

What is summation in muscle contractions?

Increased force generation by rapid successive stimuli.

What is the difference between unfused and fused tetanus?

Unfused tetanus: Partial relaxation between contractions; Fused tetanus: Continuous contraction with no relaxation.

What are the three muscle fiber types?

Slow oxidative (Type I): High endurance; Fast oxidative (Type IIa): Moderate endurance; Fast glycolytic (Type IIb): Low endurance.

What is recruitment in muscle contraction?

Activation of more motor units to increase force.

What is the first source of ATP in muscle contraction?

Stored ATP, lasts a few seconds.

How does creatine phosphate contribute to ATP generation?

Transfers phosphate to ADP to regenerate ATP, lasts ~10-15 seconds.

How long does stored glycogen provide ATP?

30 seconds to 2 minutes, depending on fiber type.

How long does oxidative phosphorylation provide ATP?

Hours, if oxygen is available.

What leads to muscle fatigue?

Depletion of ATP, glycogen, or creatine phosphate; Accumulation of metabolic byproducts; Impaired calcium handling.

What happens to the muscle during fatigue?

Reduced ability to generate force, slower contractions, longer relaxation times.