A&P: Whole Muscle Function

What is a muscle twitch?

A period of contraction and relaxation in a muscle fiber in response to a single stimulus.

Does a single muscle twitch produce maximum force?

No.

What are the three phases of a muscle twitch?

Latent period, contraction period, relaxation period.

What happens during the latent period?

Delay after stimulus while Ca²⁺ is released and binds to troponin.

What happens during the contraction phase?

Muscle tension increases.

What happens during the relaxation phase?

Ca²⁺ is pumped back into the SR and the muscle relaxes.

Why do different muscles have different twitch lengths?

They have different speeds of contraction and relaxation.

What is an isometric contraction?

Muscle produces force but does not change length.

What is an isotonic contraction?

Muscle changes length while producing force.

What is a concentric contraction?

Muscle shortens while generating force.

What is an eccentric contraction?

Muscle lengthens while generating force.

Example of concentric contraction?

Lifting a weight.

Example of eccentric contraction?

Lowering a weight.

Example of isometric contraction?

Holding a weight still.

What is a motor unit?

A motor neuron and all muscle fibers it innervates.

What determines how much force a whole muscle produces?

The number of motor units activated.

What is motor unit recruitment?

Activating more motor units to increase force.

What happens when more motor units are recruited?

More force is produced.

What four factors affect the strength of a whole muscle contraction?

Motor unit recruitment, stimulus frequency, fiber diameter, and length-tension relationship.

What is threshold stimulus?

The minimum stimulus needed to cause contraction.

What is maximal stimulus?

The stimulus that recruits all motor units.

What is summation?

A stronger contraction when stimuli occur before full relaxation.

What is wave summation?

Multiple twitches add together due to repeated stimulation.

What is incomplete tetanus?

Sustained but wavering contraction.

What is complete tetanus?

Smooth sustained contraction with no relaxation.

Why would tetanus be dangerous in breathing muscles?

The diaphragm must relax to allow breathing.

How does fiber diameter affect force?

Larger fibers produce more force.

Why do larger fibers produce more force?

They contain more myofibrils and sarcomeres.

What determines force in the length-tension relationship?

Overlap between actin and myosin filaments.

When does a muscle produce maximum force?

At optimal resting length.

Why does overstretching reduce force?

Too little actin-myosin overlap.

What are the three skeletal muscle fiber types?

Type I, Type IIa, Type IIb.

What are Type I fibers also called?

Slow oxidative fibers.

What energy system do Type I fibers use?

Oxidative phosphorylation.

What are key traits of Type I fibers?

Many mitochondria, many capillaries, small diameter, fatigue resistant.

What are Type IIa fibers also called?

Fast oxidative-glycolytic fibers.

What energy systems do Type IIa fibers use?

Oxidative phosphorylation and glycolysis.

What is their fatigue resistance?

Intermediate.

What are Type IIb fibers also called?

Fast glycolytic fibers.

What energy system do Type IIb fibers use?

Glycolysis.

What are key traits of Type IIb fibers?

Largest diameter, few mitochondria, fatigue quickly.

What are the three ATP sources for muscle?

Creatine phosphate, glycolysis, aerobic respiration.

How does creatine phosphate produce ATP?

Directly phosphorylates ADP.

Is creatine phosphate short-term or long-term energy?

Short-term.

Where does glycolysis occur?

Cytosol.

Net ATP from glycolysis?

2 ATP.

What happens if oxygen is absent?

Lactic acid forms.

Where does aerobic respiration occur?

Mitochondria.

ATP produced from aerobic respiration?

About 38 ATP per glucose.

What are the three roles of ATP in skeletal muscle?

Break cross-bridges, pump Ca²⁺ into SR, power Na⁺/K⁺ pumps.

What determines speed of contraction?

Cross-bridge cycling and myosin ATPase.

What determines speed of relaxation?

Speed of Ca²⁺ reuptake into the SR.

Which muscle fibers are recruited first to last?

Type I → Type IIa → Type IIb.

What is muscle fatigue?

Decreased force after prior contraction.

Which fibers fatigue slowest?

Type I.

Which fibers fatigue fastest?

Type IIb.

What are the two main types of fatigue?

Central fatigue and peripheral fatigue.

What is hypertrophy?

Increase in muscle size.

What is atrophy?

Decrease in muscle size.

What cells repair damaged muscle fibers?

Satellite (progenitor) cells.

What is a muscle strain?

Tearing or damaging muscle fibers.

What is muscle soreness (DOMS)?

Delayed onset muscle soreness after exercise; the exact mechanism is not fully known.

What likely causes muscle cramps?

Ionic imbalances and increased stimulation of the muscle.

What type of contraction commonly causes delayed onset muscle soreness (DOMS)?

Eccentric contractions.

What is one indication of muscle damage after exercise?

Muscle proteins appear in the blood plasma.

Which muscle proteins increase in the blood after muscle damage?

CK (creatine kinase), LDH (lactate dehydrogenase), and Mb (myoglobin).

What is the 'training effect' after eccentric exercise?

Muscles adapt so they become more resistant to future damage.

What structural change can occur in muscle after repeated eccentric exercise?

Increase in the number of sarcomeres.

What happens first after muscle fiber damage (myotrauma)?

Satellite/progenitor cells are activated and proliferate.

What does chemotaxis do during muscle repair?

Progenitor cells move toward the injured muscle fiber.

How do satellite cells contribute to hypertrophy?

They fuse with existing damaged muscle fibers.

How can satellite cells form new muscle fibers?

They fuse together to create new myofibers (hyperplasia).

What feature is seen in regenerated muscle fibers?

Central nuclei in the muscle fiber.