muscular system

Hierarchical organization of skeletal muscle

Whole muscle → Fascicle → Muscle fiber → Myofibril → Sarcomere → Myofilaments (actin & myosin)

What connective tissue surrounds the entire muscle?

Epimysium-Dense connective tissue surrounding the entire muscle

What connective tissue surrounds a fascicle?

Perimysium-Connective tissue surrounding a fascicle (bundle of muscle fibers)

What connective tissue surrounds each individual muscle fiber?

Endomysium-Thin connective tissue surrounding each individual muscle fiber

Myofibril

Rod-like contractile structure inside a muscle fiber made of sarcomeres

Sarcomere

Smallest functional unit of a muscle fiber

Thin myofilament

Actin

Thick myofilament

Myosin

Cause of muscle striations

Alternating dark A bands (myosin) and light I bands (actin)

What happens when an action potential reaches the axon terminal at the Neuromuscular Junction (NMJ)

Voltage-gated calcium (Ca²⁺) channels open and calcium enters the axon terminal

First event at NMJ

Action potential arrives at axon terminal

Ion that enters axon terminal at NMJ

Calcium (Ca²⁺)

Neurotransmitter released at NMJ

Acetylcholine (ACh)

Where ACh binds

Motor end plate receptors on the sarcolemma

Effect of ACh binding

Sodium channels open causing depolarization

Resulting event on sarcolemma

Muscle fiber action potential

What stops continuous muscle contraction

Breakdown of acetylcholine

Enzyme that breaks down acetylcholine

Acetylcholinesterase (AChE)

Role of calcium in contraction

Binds to troponin to initiate contraction

Troponin

Regulatory protein that binds calcium and changes shape

Tropomyosin

Protein that blocks myosin-binding sites on actin at rest

How binding sites are exposed

Calcium binds troponin, shifting tropomyosin

Role of ATP during power stroke

Energizes myosin head via ATP hydrolysis

Role of ATP in relaxation

ATP binding causes myosin to detach from actin

Motor unit

Motor neuron and all muscle fibers it innervates

All-or-none principle (motor unit)

All fibers in a motor unit contract fully or not at all

Fine motor movement control

Recruitment of small motor units

Powerful movement control

Recruitment of large motor units

Motor unit recruitment

Activation of additional motor units to increase force

Creatine Phosphate (CP) system use

Very short, high-intensity activity (0–10 seconds)

ATP production of CP system

Very fast but limited ATP

Anaerobic respiration (glycolysis) use

Moderate-intensity activity lasting ~30–120 seconds

ATP per glucose in glycolysis

2 ATP

Byproduct of anaerobic respiration

Lactic acid

Aerobic respiration use

Long-duration, low-intensity activity

ATP per glucose in aerobic respiration

~30–32 ATP

Is lactic acid produced in aerobic respiration

No

Muscle fatigue

Decreased ability to contract despite continued stimulation

Causes of muscle fatigue

ATP depletion, lactic acid buildup, ion imbalance, oxygen debt

Muscular hypertrophy

Increase in muscle fiber size due to increased myofibrils

Cause of hypertrophy

Repeated overload and resistance training

Muscle atrophy

Decrease in muscle size due to disuse

Use it or lose it principle

Muscles grow with use and shrink with inactivity

Excess Postexercise Oxygen Consumption (EPOC)

Increased oxygen intake after exercise to restore homeostasis

Another name for EPOC

Oxygen debt

EPOC recovery process 1

Replenish ATP and creatine phosphate

EPOC recovery process 2

Restore oxygen in blood and myoglobin

EPOC recovery process 3

Remove lactic acid and restore normal metabolism