Errors in Sarcomere and Contraction Questions

Error 1 — A band changes during contraction Students write "all bands decrease" in contraction questions. The A band NEVER changes — it spans the entire length of myosin filaments, which are stationary. Correct answer: only I band and H zone decrease. Drilling point: A = Always constant.

Error 2 — Actin filaments shorten during contraction The sliding filament theory states that neither actin nor myosin filaments change their own length. The sarcomere shortens because actin slides over myosin. No filament shortening occurs. This is tested in "what changes vs what stays the same" questions.

Error 3 — Confusing troponin and tropomyosin roles Troponin-C binds Ca2+ (it is the calcium sensor). Tropomyosin physically blocks the myosin-binding sites. Ca2+ binds troponin, which then moves tropomyosin. Students often write "Ca2+ binds tropomyosin" — this is wrong.

Error 4 — ATP hydrolysis causes detachment ATP binding causes myosin detachment from actin (allowing cycle reset). ATP hydrolysis re-energizes (re-cocks) the myosin head. These two ATP roles are frequently swapped by students. Memory: Bind to Break free; Hydrolyse to Heave (do the power stroke).

Errors in Muscle Classification

Error 5 — Cardiac muscle is voluntary Cardiac is striated but involuntary. Students associate striations exclusively with skeletal (voluntary) muscle. Both skeletal and cardiac are striated; only skeletal is voluntary.

Error 6 — Smooth muscle has T-tubules Smooth muscle lacks T-tubules. It has caveolae for calcium handling. Only skeletal and cardiac muscle have T-tubules (best developed in skeletal; moderately developed in cardiac).

Errors in Skeleton and Joints

Error 7 — Wrist = ball-and-socket joint Wrist = ellipsoid (condyloid) joint — allows biaxial movement without rotation. Ball-and-socket (shoulder, hip) allows rotation. Students confuse them because both allow multiplanar movement, but rotation distinguishes ball-and-socket.

Error 8 — Hyoid is appendicular Hyoid is axial (counted in the 80). The hyoid is in the neck and is part of the central axis skeleton.

Error 9 — 33 vertebrae in adult 33 is the number of vertebral elements before sacral and coccygeal fusion. Adults have 26 vertebral bones (sacrum = 1 fused bone; coccyx = 1 fused bone).

Errors in Disorders

Error 10 — Myasthenia gravis is genetic Myasthenia gravis is autoimmune (ACh receptor antibodies). Muscular dystrophy is genetic (dystrophin). Students frequently swap these two, especially under exam pressure.