| A | B |
|---|
| Sarcolemma | The plasma membrane of a muscle fiber |
| Sarcoplasm | The cytoplasm of a muscle fiber |
| Glycogen | An energy-storage polysaccharide abundant in muscle |
| Myoglobin | An oxygen-storing red pigment of muscle |
| T tubule | A tunnel-like extension of the sarcolemma extending from one side of the muscle fiber to the other; conveys electrical signals from the cell surface to its interior |
| Sarcoplasmic reticulum | The smooth ER of a muscle fiber; a Ca2+ reservoir |
| Terminal cisternae | The dilated ends of sarcoplasmic reticulum adjacent to a T tubule |
| Myofibril | A bundle of protein microfilaments (myofilaments) |
| Myofilament | A threadlike complex of several hundred contractile protein molecules |
| Thick filament | A myofilament about 11 nm in diameter composed of bundled myosin molecules |
| Elastic filament | A myofilament about 1 nm in diameter composed of a giant protein |
| Thin filament | A myofilament about 5 to 6 nm in diameter composed of actin |
| Myosin | A protein with a long shaftlike tail and a globular head; constitutes the thick myofilament |
| F actin | A fibrous protein made of a long chain of G actin molecules twisted into a helix; main protein of the thin myofilament |
| G actin | A globular subunit of F actin with an active site for binding a myosin head |
| Regulatory proteins | Troponin and tropomyosin |
| Tropomyosin | A regulatory protein that lies in the groove of F actin and |
| Troponin | A regulatory protein associated with tropomyosin that acts as a calcium receptor |
| Titin | A springy protein that forms the elastic filaments |
| Dystrophin | A large protein that links thin filaments to transmembrane proteins; transfers the force of sarcomere contraction to the extracellular proteins of the muscle |
| Striations | Alternating light and dark transverse bands across a myofibril |
| A band | Dark band formed by parallel thick filaments that partly overlap the thin filaments |
| H band | A lighter region in the middle of an A band that contains thick filaments only; thin filaments do not reach this far into the A band in relaxed muscle |
| I band | A light band composed of thin filaments only |
| Z disc | A protein disc to which thin filaments and elastic filaments are anchored at each end of a sarcomere; appears as a narrow dark line in the middle of the I band |
| Sarcomere | The distance from one Z disc to the next; the contractile unit of a muscle fiber |
| Neuromuscular junction | A functional connection between the distal end of a nerve fiber and the middle of a muscle fiber |
| Synaptic knob | The dilated tip of a nerve fiber; contains synaptic vesicles |
| Synaptic cleft | A gap of about 60 to 100 nm between the synaptic knob and sarcolemma |
| Synaptic vesicle | A secretory vesicle in the synaptic knob; contains acetylcholine |
| Junctional folds | Invaginations of the sarcolemma where ACh receptors are especially concentrated |
| Acetylcholine (ACh) | The neurotransmitter released by a somatic motor fiber that stimulates a skeletal muscle fiber (also used elsewhere in the nervous system) |
| ACh receptor | A transmembrane protein in the sarcolemma of the neuromuscular junction that binds to ACh |
| Acetylcholinesterase (AChE) | An enzyme in the sarcolemma and basal lamina of the muscle fiber in the synaptic region; responsible for degrading ACh and stopping the stimulation of the muscle fiber |
