Muscle physiology

skeletal muscle

Voluntary striated muscle attached to bones for body movement

cardiac muscle

Involuntary striated muscle found in the heart with intercalated discs

smooth muscle

Involuntary non-striated muscle in walls of organs and vessels

sarcomere

Functional contractile unit of muscle between two Z-discs

myofibril

Long cylindrical organelle containing actin and myosin filaments

myosin

Thick filament that forms cross-bridges with actin to produce contraction

actin

Thin filament that myosin binds during contraction

A band

Region of overlapping actin and myosin filaments

I band

Region containing only actin filaments

H zone

Region containing only myosin filaments in relaxed muscle

M line

Center of sarcomere anchoring myosin filaments

Z disc

Boundary of sarcomere anchoring actin filaments

sliding filament theory

Muscle shortens as actin slides over myosin using ATP

cross-bridge cycle

Myosin binds actin

tropomyosin

Protein blocking actin binding sites at rest

troponin

Protein complex that binds Ca²⁺ to move tropomyosin and allow contraction

excitation-contraction coupling

Process linking muscle fiber stimulation to Ca²⁺ release and contraction

neuromuscular junction

Synapse between motor neuron and muscle fiber using acetylcholine

acetylcholine (ACh)

Neurotransmitter triggering depolarization of muscle membrane

t-tubule

Membrane extension transmitting action potentials into muscle fiber

sarcoplasmic reticulum

Organelle storing and releasing Ca²⁺ for contraction

motor unit

A single motor neuron and all muscle fibers it innervates

recruitment

Increasing the number of active motor units to increase strength

tetanus

Sustained muscle contraction without relaxation

isometric contraction

Muscle tension increases without length change

isotonic contraction

Muscle changes length while maintaining tension

muscle fatigue

Reduced ability to generate force due to ATP depletion or ion imbalance

slow oxidative fibers (Type I)

Small

fast glycolytic fibers (Type IIb)

Large