Question

Multi-unit smooth muscle is present at all of the following except?

• A. Blood vessels
• B. Iris
• C. Gut
• D. Ductus deferens

Hint:

Gut is single-unit (syncytial) smooth muscle; iris, vessels and vas deferens are multi-unit.

Answer 1

The Correct Option is C

Step 1: Smooth muscle is classified into two types - multi-unit and single-unit (unitary or visceral). In multi-unit smooth muscle each fibre is innervated separately and contracts independently, allowing fine, graded control.

Step 2: Multi-unit smooth muscle is found in the iris and ciliary muscle of the eye, the piloerector muscles, the walls of large blood vessels, and the ductus deferens (vas deferens). So options a, b and d are correct sites of multi-unit muscle.

Step 3: The gut wall contains single-unit (visceral) smooth muscle. Here the fibres are electrically coupled by gap junctions and act as a syncytium, contracting together to produce peristalsis. This is the exception.

Step 4: Therefore the gut does not contain multi-unit smooth muscle, and the correct answer is gut.