Q.16 Which one of the following is not present in the smooth muscles?
1. Z-disc
2. H-band
3. Thick myofilaments
4. Thin myofilaments
Smooth muscles lack the organized sarcomere structure found in striated muscles, so they do not have Z-discs, H-bands, or other striated features.
The correct answer is 1. Z-disc.
Correct Answer
1. Z-disc
Z-discs (or Z-lines) anchor thin actin filaments at sarcomere ends in skeletal/cardiac muscle, defining striations under microscopy. Smooth muscle has dense bodies instead, scattering actin attachments without sarcomeres or Z-discs.
Option Explanations
| Option | Description | Why Incorrect/Correct |
|---|---|---|
| 1. Z-disc | Boundaries of sarcomeres anchoring actin in striated muscle. | Correct: Absent in smooth muscle; replaced by cytoplasmic dense bodies and membrane plaques. |
| 2. H-band | Central myosin region in sarcomere A-band, shortens in contraction. | Incorrect: Smooth lacks sarcomeres/H-bands; myosin disorganized between actin. |
| 3. Thick myofilaments | Myosin filaments sliding past actin for contraction. | Incorrect: Present in smooth muscle, though side-polar (not bipolar like striated). |
| 4. Thin myofilaments | Actin filaments attached to dense structures. | Incorrect: Abundant in smooth muscle cytoplasm, forming contractile mesh. |
Smooth Muscle Structure
Spindle-shaped cells with actin/myosin myofilaments, intermediate filaments (desmin/vimentin), and caveolae (T-tubule analogs) enable slow, sustained contractions in organs like gut/vessels. No striations due to oblique myofilament lattice. Ties to your muscle physiology in molecular biology studies.
