Smooth muscles are involuntary muscles without striations, found in the walls of internal (visceral) organs.
Important points
- Muscle fibres are spindle-shaped or fusiform.
- Smooth muscles are unbranched.
- Striations are absent, and hence they are called smooth muscles.
- Smooth muscles are involuntary.
- Muscle contractions are slow.
- These muscles do not develop fatigue.
- Location – present in visceral organs like the gut, blood vessels and urinary bladder.
Each smooth muscle fibre has a single, centrally placed nucleus (uninucleate). NCERT notes that the cells are held together by cell junctions and bundled together in a connective tissue sheath. Because they are found in the walls of internal organs, smooth muscles are also called visceral muscles.
Smooth, skeletal and cardiac muscles
Smooth muscles are one of the three types of muscles, along with skeletal (striated) muscles and cardiac muscles.
| Feature | Smooth | Skeletal | Cardiac |
|---|---|---|---|
| Shape of fibres | Spindle-shaped | Long, cylindrical | Cylindrical, branched |
| Striations | Absent | Present | Present |
| Nuclei | One, central | Many, peripheral | One or two, central |
| Control | Involuntary | Voluntary | Involuntary |
| Fatigue | Does not fatigue | Fatigues quickly | Does not fatigue |
| Location | Walls of gut, blood vessels, urinary bladder | Attached to bones | Heart wall |
Cardiac muscle cells are joined by intercalated discs, which let the cells contract as a single unit. Smooth and cardiac muscles are both involuntary, but only cardiac muscle is striated.