Question

which of the following is not characteristic of smooth muscle?

• the thin filaments of smooth muscle fibers are attached to dense bodies.
• neurons that innervate smooth muscles are under involuntary control.
• smooth muscle cells are uninucleate.
• the striations are due to the orderly arrangement of actin and myosin.
• smooth muscles do not contain sarcomeres.

Explanation:

🆕 New Concept Discovered: Muscle Tissue Classification
Comparing skeletal, cardiac, and smooth muscle structures.

Step 1: Analyze the structural features of smooth muscle

Smooth muscle is named "smooth" precisely because it lacks the visible stripes, or striations, seen in skeletal and cardiac muscle under a microscope.

• Sarcomeres and Striations: Striations in other muscle types are caused by highly organized, repeating functional units called sarcomeres (made of actin and myosin filaments). Because smooth muscle lacks sarcomeres, it does not have striations. Instead, its actin and myosin filaments are scattered throughout the cytoplasm in a loose, diagonal web.
• Dense Bodies: In smooth muscle, thin (actin) filaments anchor to structures called dense bodies, which function similarly to the Z-discs of striated muscles.

Step 2: Analyze the cellular and control features of smooth muscle

• Nucleation: Smooth muscle cells are spindle-shaped and contain a single, centrally located nucleus (uninucleate).
• Innervation: Smooth muscle is controlled involuntarily by the autonomic nervous system, meaning we cannot consciously control its contraction (unlike voluntary skeletal muscle).

Step 3: Evaluate the given options

Let's check each statement to find the one that is not true:

1. "The thin filaments of smooth muscle fibers are attached to dense bodies." — True. Dense bodies serve as the anchoring sites for actin in smooth muscle.
2. "Neurons that innervate smooth muscles are under involuntary control." — True. Smooth muscle contraction is regulated involuntarily by the autonomic nervous system.
3. "Smooth muscle cells are uninucleate." — True. They contain only one nucleus per cell.
4. "The striations are due to the orderly arrangement of actin and myosin." — False. Smooth muscle does not have striations because its actin and myosin are not arranged in highly organized, repeating sarcomeres.
5. "Smooth muscles do not contain sarcomeres." — True. The absence of sarcomeres is why they lack striations.

Answer:

The striations are due to the orderly arrangement of actin and myosin.