Untitled Flashcard Set
Skeletal muscle rule #1 – Muscles have at least two attachments and cross at least one joint.
Skeletal muscle rule #2 – Muscles always pull (never push) and get shorter to make movement.
Skeletal muscle rule #3 – Muscle fiber striations point to their attachments and show direction of pull.
Muscle fiber – A single muscle cell (like one strand of spaghetti).
Fascicle – A bundle of muscle fibers wrapped in perimysium.
Perimysium – Connective tissue sheath around a fascicle.
Muscle belly – A bundle of fascicles wrapped in epimysium.
Epimysium – Connective tissue sheath around the whole muscle belly.
Tendon – Tough extensions of perimysium/epimysium that attach muscle to bone; interwoven in the muscle and very strong.
Origin – The fixed attachment point that stays stationary during movement.
Insertion – The movable attachment that moves toward the origin.
Sarcomere – The functional unit of muscle contraction.
H-zone – The center region that narrows during contraction, shortening the sarcomere.
A band (myosin region) – The part that does not change length during contraction.
Myosin heads – “Grabber” parts of thick filaments that form cross-bridges and pull actin.
Actin filament – Thin filament that myosin pulls to shorten the sarcomere.
G-actin – A single “bead” of actin (globular actin).
F-actin – A strand made by linking many G-actins (fibrous actin).
Actin double helix – Two F-actin strands twisted together to make the thin filament.
Tropomyosin – A supportive “scaffold” on actin that also blocks myosin-binding sites.
Troponin complex – Proteins that, when bound by Ca²⁺, move tropomyosin to uncover binding sites.
Calcium (Ca²⁺) – The trigger that binds troponin, shifts tropomyosin, and exposes actin sites.
Cross-bridge – The connection formed when a myosin head binds to exposed actin; basis of pulling.
Sarcoplasmic reticulum (SR) – A calcium reservoir wrapped around the myofibrils.
Ca²⁺ release – Voltage change causes the SR to release calcium onto troponin to start contraction.
Ca²⁺ reuptake – SR pulls calcium back in, reversing steps and relaxing the muscle.
Resting membrane potential (polarization) – Outside of the fiber is more positive; inside is relatively negative.
Na⁺/K⁺ pump (NaK pump) – Membrane protein that sends 3 Na⁺ out and brings 2 K⁺ in to keep the resting gradients.
Sodium (Na⁺) at rest – Kept high outside the cell by Na⁺/K⁺ pumps.
Potassium (K⁺) at rest – Kept higher inside the cell by Na⁺/K⁺ pumps.
Sarcolemma – The muscle fiber’s cell membrane.
T-tubules – “Tunnels” in the sarcolemma that carry the electrical signal deep to the SR.
Motor neuron – The nerve cell that stimulates the muscle fiber.
Acetylcholine (ACh) – Main neurotransmitter released by the motor neuron at the neuromuscular junction.
ACh receptor/ligand-gated Na⁺ channel – When ACh binds, Na⁺ channels open and Na⁺ rushes in.
Depolarization – Inside becomes more positive than outside; the electrical switch that starts contraction.
Action potential – The wave of depolarization that travels down T-tubules and tells the SR to release Ca²⁺.
Acetylcholinesterase (AChE) – Enzyme that breaks down ACh so the signal stops.
Repolarization – Pumps and K⁺ channels restore normal Na⁺/K⁺ balance; Ca²⁺ returns to SR; fiber relaxes.
Sliding/shortening result – With binding sites open, myosin pulls actin so sarcomeres shorten and the muscle contracts.
Tendon strength note – Tendon fibers are interwoven with muscle sheaths, making the attachment very strong.
Direction of pull note – The orientation of striations indicates where a muscle will pull when it shortens.
Contraction control summary – Nerve ACh → Na⁺ influx & depolarization → T-tubule signal → SR Ca²⁺ release → troponin/tropomyosin shift → cross-bridges pull → Ca²⁺ reuptake & repolarization to relax.