Three movement types: ciliary (respiratory epithelium, fallopian tube), flagellar (spermatozoa), muscular (locomotion and organ function).

Skeletal muscle = striated + voluntary + multinucleated. Only voluntary muscle type.

Smooth muscle = non-striated + involuntary + uninucleate (fusiform/spindle-shaped). Found in visceral organ walls.

Cardiac muscle = striated + INVOLUNTARY + uni/binucleate. The only striated involuntary muscle — the most tested NEET fact in this chapter.

Intercalated discs are found ONLY in cardiac muscle. They contain gap junctions (electrical coupling) and desmosomes (mechanical coupling).

Cardiac muscle NEVER fatigues. Skeletal fatigues readily. Smooth is resistant to fatigue.

Sarcomere = Z-line to Z-line = functional contractile unit of skeletal muscle.

A band = dark = anisotropic = contains FULL myosin length + overlapping actin. ALWAYS CONSTANT during contraction.

I band = light = isotropic = contains ONLY actin. DECREASES during contraction.

H zone = central myosin-only zone within A band. DECREASES during contraction; may completely disappear at maximum contraction.

M-line bisects the H zone and anchors myosin. Z-line bisects the I band and anchors actin.

During contraction, NEITHER actin nor myosin filaments change their own length. Sarcomere shortens because actin SLIDES over stationary myosin.

Cross-bridge cycle: Ca2+ from SR → troponin-C activated → tropomyosin shifts → cross-bridge → power stroke → ATP binds (detach) → ATP hydrolyzed (re-energize) → repeat.

ATP has TWO roles: hydrolysis re-energizes myosin head; binding causes detachment from actin.

Without ATP, myosin cannot detach from actin = RIGOR MORTIS.

Ca2+ re-uptake into SR by SERCA (Ca2+-ATPase) is required for RELAXATION — an active, ATP-requiring process.

Total adult human bones = 206 = axial 80 + appendicular 126.

Axial 80: skull (22) + vertebrae (26) + ribs (24) + sternum (1) + hyoid (1).

Vertebral formula: C7 + T12 + L5 + S1 (fused) + Cx1 (fused) = 26.

Rib classification: 7 true pairs (attach directly to sternum) + 3 false pairs + 2 floating pairs = 12 pairs = 24 ribs.

Hyoid = only bone in the body NOT articulating with any other bone. Part of axial skeleton.

Fibrous joints = immovable (skull sutures). Cartilaginous = slightly movable (pubic symphysis, intervertebral discs). Synovial = freely movable (6 subtypes).

Six synovial joints: hinge (knee, elbow), pivot (atlas-axis = rotation only), ball-and-socket (shoulder, hip = all planes), gliding (intercarpal), saddle (thumb CMC), ellipsoid (wrist).

Myasthenia gravis = autoimmune destruction of ACh receptors at NMJ → progressive SKELETAL muscle weakness.

Muscular dystrophy = X-linked genetic dystrophin deficiency → progressive skeletal muscle degeneration.

Tetany = HYPOCALCEMIA → hyperexcitable neurons → sustained involuntary contractions.

Osteoarthritis = degenerative cartilage wear-and-tear (NOT autoimmune). Weight-bearing joints in elderly.

Rheumatoid arthritis = AUTOIMMUNE attack on synovial membrane. Symmetric joints. Morning stiffness > 1 hour.

Osteoporosis = decreased bone mineral density. Post-menopausal estrogen deficiency → increased osteoclast activity.

Gout = uric acid crystal deposition in joints. Big toe most common site. Hyperuricemia from purine metabolism.