Muscle cells vocab

Muscle fiber

a single long muscle cell  

Fascicle

bundle of muscle fibers (cells)  

Myofibril

thread-like structures inside a muscle fiber

Sarcomere

Region between two Z discs where contraction occurs 

Myofilament

protein filaments inside sarcomeres (actin + myosin)  

Actin (thin filament)

protein filament myosin binds to; anchored at Z discs;has myosin-binding sites 

Myosin (thick filament)

motor protein that pulls actin during contraction 

Tropomyosin

regulatory protein that blocks myosin-binding sites on actin when relaxed

Troponin

regulatory protein that binds calcium and moves tropomyosin 

• binds Ca²⁺ → causes tropomyosin to move 

A band

region containing full length of myosin (includes overlap with actin)

I band

region with only actin (no myosin overlap)  

H zone

center of A band with only myosin (no actin overlap)

Sarcolemma

muscle cell membrane that conducts action potentials  

Sarcoplasmic reticulum

stores and releases calcium (Ca²⁺)  

• releases Ca²⁺ when stimulated

DHP receptor

• voltage sensor in T-tubules that detects action potential

• voltage sensor in T-tubules that activates ryanodine receptor

Ryanodine receptor (RyR)

calcium channel on SR that releases Ca²⁺ into cytoplasm 

Sliding filament theory

muscle contracts when actin slides past myosin, shortening the sarcomere 

• myosin heads pull actin inward using ATP, shortening the sarcomere without changing filament length 

 

Motor end plate

pecialized region of the muscle fiber’s sarcolemma where acetylcholine (ACh) is released to trigger a muscle action potential  

T-tubules (transverse tubules)

deep folds of the sarcolemma that rapidly carry the action potential into the interior of the muscle fiber 

Motor unit

one motor neuron and all the muscle fibers it controls; determines how much force is produced

Fatigue

reduced ability of a muscle to generate force due to factors like ATP depletion, ion imbalance, or metabolite buildup  

Twitch

a single, brief contraction-relaxation cycle in response to one stimulus  

Treppe (staircase effect)

gradual increase in contraction strength after repeated stimulation following rest, due to rising Ca²⁺ availability and enzyme efficiency  

Twitch summation (wave summation)

increased force when stimuli arrive before full relaxation, causing contractions to “add together”

Tetanus (tetanic contraction)

sustained, strong contraction with no relaxation due to very rapid repeated stimulation 

Isometric contraction →

muscle produces tension without changing length 

Isotonic contraction

muscle changes length while maintaining constant tension  

Concentric contraction

muscle shortens while generating force (lifting phase)

Eccentric contraction

muscle lengthens while still generating force (lowering phase or resisting load) 

 

Calmodulin

calcium-binding protein that activates smooth muscle contraction (replaces troponin in smooth muscle)  

Myosin light chain kinase (MLCK)

enzyme activated by Ca²⁺–calmodulin that phosphorylates myosin, allowing contraction to begin 

Multi-unit smooth muscle

smooth muscle cells that act independently with separate neural input; allows fine control (e.g., iris of eye)  

Single unit smooth muscle

smooth muscle cells connected by gap junctions that contract together as one unit (e.g., intestines, uterus)

Functional syncytium

tissue that behaves as a single coordinated unit due to electrical coupling through gap junctions