Question

What is the structure in the smooth muscle corresponding to Z line of skeletal muscle?

• A. Dense bodies
• B. Caveolae
• C. M line
• D. Bachmans bundle

Hint:

Remember that "Dense bodies" are the anchors for actin filaments in smooth muscle cells. Because they are scattered throughout the cell rather than aligned in rows, smooth muscle contracts in a twisting, corkscrew fashion rather than a straight line.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Skeletal and smooth muscle cells have different internal structures for contraction, but they share similar functional components to anchor thin filaments.

Step 2: Detailed Explanation:
In skeletal and cardiac muscle, actin (thin) filaments are anchored to the Z lines (or Z discs), which mark the boundaries of individual sarcomeres.
Smooth muscle cells do not have sarcomeres or Z lines, which is why they do not appear striated under a microscope.
Instead, smooth muscle cells contain structures called dense bodies distributed throughout the cytoplasm and attached to the cell membrane.
These dense bodies contain the protein \(\alpha\)-actinin, which is the same anchoring protein found in Z lines.
The actin filaments in smooth muscle attach directly to these dense bodies.
During contraction, tension is transmitted through these dense bodies to the cell membrane, shortening the cell.
Therefore, dense bodies are the smooth muscle equivalent of skeletal muscle Z lines.
Caveolae are small membrane indentations involved in calcium entry, and Bachmann's bundle is a conduction pathway in the heart.

Step 3: Final Answer:
The smooth muscle structure corresponding to the Z line is Dense bodies.