Glossary — Key Terms for HP-05 — Notes

Term	Definition	NEET Relevance
Sarcomere	Smallest functional contractile unit of skeletal muscle; bounded by two Z-lines	Foundation of all contraction questions
A band	Anisotropic (dark) band; contains full myosin length + overlapping actin; CONSTANT during contraction	Most tested fact in this chapter
I band	Isotropic (light) band; contains only actin; bisected by Z-line; DECREASES during contraction	Paired with A band in every sarcomere question
H zone	Central myosin-only zone within A band; DECREASES during contraction; may disappear completely	Third element of band-change trio
M-line	Midpoint of H zone; anchors myosin filaments	Less common but important structural landmark
Z-line	Boundary between sarcomeres; anchors actin filaments; bisects I band	Defines sarcomere boundaries
Sarcoplasmic reticulum (SR)	Specialized ER in muscle; stores and releases Ca2+ for contraction	Ca2+ source — key to contraction initiation
T-tubules	Transverse tubules; invaginations of sarcolemma; conduct action potential into muscle interior	Bridge between electrical signal and Ca2+ release
Sarcolemma	Plasma membrane of muscle cell	Site of ACh receptor; receives action potential
Troponin-C	Ca2+-binding subunit of troponin complex; triggers tropomyosin shift	Direct Ca2+ sensor in contraction regulation
Tropomyosin	Coiled-coil protein; blocks myosin-binding sites on actin at rest; shifts when Ca2+ binds troponin	The "gate" for actin-myosin interaction
Cross-bridge	Temporary attachment between myosin head and actin binding site	The mechanical coupling unit of contraction
Power stroke	Rotation of myosin head after cross-bridge formation; pulls actin toward M-line	The actual movement step in contraction
Intercalated disc	Junction between adjacent cardiac cells; contains gap junctions + desmosomes	Unique to cardiac muscle; enables functional syncytium
Myasthenia gravis	Autoimmune destruction of ACh receptors at NMJ; progressive skeletal muscle weakness	Disease linking immunology to NMJ physiology
Tetany	Sustained involuntary muscle contraction due to hypocalcemia	Distinct from tetanus (bacterial); calcium-ion related
Osteoporosis	Decreased bone mineral density; fragile bones; post-menopausal estrogen deficiency	Endocrine-musculoskeletal connection
Gout	Uric acid crystal deposition in joints; hyperuricemia; big toe most common site	Metabolic cause of joint pain
Axial skeleton	80 bones forming the central axis (skull, spine, ribs, sternum, hyoid)	Must know 80 bones and their distribution
Appendicular skeleton	126 bones of limbs and girdles	126 = 206 - 80; includes pectoral and pelvic girdles
Synarthrosis	Fibrous joint; immovable; skull sutures	Classification terminology
Amphiarthrosis	Cartilaginous joint; slightly movable; pubic symphysis, intervertebral discs	Classification terminology
Diarthrosis	Synovial joint; freely movable; includes hinge, pivot, ball-and-socket, gliding, saddle, ellipsoid	The freely movable joint class