Skeletal Muscle Fibers, Contraction Mechanics, and Metabolism

Flashcards covering sarcomere structure, length–tension relationship, ATP sources, twitch mechanics, summation and tetanus, motor unit recruitment, and key distinctions between slow- and fast-twitch skeletal muscle fibers.

What is a sarcomere?

The region of a myofibril between two Z discs—the functional contractile unit of skeletal muscle.

Which proteins make up the thick and thin filaments of a sarcomere?

Thick filaments are composed of myosin; thin filaments are composed of actin (plus regulatory proteins).

What happens to the H zone during muscle contraction?

It narrows (or disappears) as thin filaments slide toward the M line.

What is meant by the 'optimal length-tension relationship'?

A sarcomere length where the maximum number of cross-bridges can form, producing the greatest force.

Why does excessive sarcomere stretch reduce force production?

Few myosin heads overlap with thin filaments, so few cross-bridges can form.

Why does excessive sarcomere shortening reduce force production?

Thin filaments already overlap; further sliding is limited, so little additional tension can develop.

List the four immediate sources of ATP for skeletal muscle contraction in order of use.

1) Residual (stored) ATP, 2) Substrate-level phosphorylation via creatine phosphate, 3) Glycolysis, 4) Oxidative phosphorylation.

Which ATP-generating pathway produces the most ATP but requires the most time and oxygen?

Oxidative phosphorylation in the mitochondria.

During intense, anaerobic activity, pyruvate from glycolysis is converted into what product?

Lactic acid (via lactic acid fermentation).

Define a muscle 'twitch.'

One complete contraction-relaxation cycle of a muscle fiber.

What is the latent period of a twitch?

The short delay between the action potential and the onset of contraction, when excitation-contraction coupling occurs.

Explain 'summation' in skeletal muscle.

Rapid, repeated action potentials keep Ca²⁺ levels elevated, producing progressively stronger contractions.

What is tetanus (in muscle physiology)?

A sustained, maximal contraction produced by high-frequency stimulation with no relaxation between twitches.

Differentiate between incomplete (unfused) and complete (fused) tetanus.

Incomplete tetanus allows slight relaxation between stimuli; complete tetanus shows no relaxation—force plateaus at maximum.

Define a motor unit.

One somatic motor neuron and all of the skeletal muscle fibers it innervates.

What is 'recruitment' in muscle contraction?

Activation of additional motor units to increase overall muscle force.

Name three physiological ways to increase muscle tension.

1) Optimal length-tension relationship, 2) Summation leading to tetanus, 3) Recruitment of more motor units.

Which two ATPases are especially important for twitch speed differences between fiber types?

Myosin ATPase (cross-bridge cycling) and Ca²⁺-ATPase (pumping Ca²⁺ back into SR).

Give two alternate names for slow-twitch and fast-twitch fibers.

Slow-twitch = Type I fibers; Fast-twitch = Type II fibers.

What primary metabolic pathway do slow-twitch fibers rely on?

Oxidative phosphorylation (aerobic metabolism).

Why are slow-twitch fibers darker in color?

High levels of myoglobin, mitochondria, and capillaries give them a dark (red) appearance.

Which fiber type has a larger diameter and stronger, quicker contractions but fatigues rapidly?

Fast-twitch (Type II) fibers.

Typical twitch duration: compare fast-twitch vs. slow-twitch fibers.

Fast-twitch: as short as ~7 ms; Slow-twitch: up to ~70 ms.

How does creatine phosphate help regenerate ATP?

It donates a phosphate group to ADP in substrate-level phosphorylation, forming ATP quickly.