Muscular Crosswork

1. Contractile protein forming thick filaments with ATP and actin binding sites
3. Boundary of a sarcomere where thin filaments are anchored
5. Dark region of sarcomere containing thick filaments
6. Enlarged sacs of the sarcoplasmic reticulum that store calcium ions
9. Central region with only thick filaments that disappears during contraction
11. Electrical signal that travels along the sarcolemma to trigger contraction
14. Cell membrane of a muscle fiber that regulates transport and maintains electrical charge differences
17. One motor neuron and all the muscle fibers it controls
19. Neurotransmitter released to stimulate muscle contraction
21. Specialized smooth ER that stores and releases calcium ions for contraction
22. Protein that blocks myosin binding sites on actin at rest

1. Cylindrical organelle made of filaments that enables muscle contraction
2. Invaginations of the sarcolemma that carry signals into the muscle fiber
4. Active transport mechanism that pumps 3 Na+ out and 2 K+ into the cell using ATP
6. Filaments made of actin, tropomyosin, and troponin involved in contraction
7. Cytoplasm of a muscle cell containing organelles and nutrients
8. Difference in electrical charge across the sarcolemma at rest (-75 mV)
10. Synapse where a motor neuron communicates with a muscle fiber
12. Regulatory protein that binds calcium and moves tropomyosin
13. Functional unit of muscle contraction between two Z discs
15. Protein that makes up thin filaments and contains myosin binding sites
16. Light region containing only thin filaments
18. Filaments made of myosin proteins with heads that bind actin and ATP
20. Protein structure that holds thick filaments together in the center