Question

Smooth muscle fibres are
A. Spindle-shaped, unbranched, non-striated, multinucleate and involuntary
B. Spindle-shaped, unbranched, unstriated, uninucleate and involuntary
opi. Cylindrical, striated, unbranched, multinucleate and voluntary
D. Cylindrical, non-striated, unbranched, multinucleate and involuntary

Answer 1

Hint: A non-striated muscle that is involuntary is a smooth muscle. It is split into two subtypes: the single unit and the smooth muscle multi-unit. Within single-unit cells, the whole package or sheet contracts as a syncytium.

Complete answer:

Smooth muscle fibers and the tracts of the respiratory, urinary, and reproductive systems are found in the walls of hollow organs such as the liver, intestines, urinary bladder, and uterus, and in the walls of archways, such as the arteries and veins of the circulatory system.
With the binding of calcium directly to myosin and then rapidly cycling cross-bridges, producing force, contraction is triggered in the invertebrate smooth muscle. There is a low calcium and low energy utilization capture process, comparable to the vertebrate smooth muscle mechanism.
An involuntary, non-striated muscle is a smooth muscle. They are spindle-shaped, have a single nucleus with a central position, and striations are absent.

Hence, the correct answer is option (B).

Note: Much of the single-unit variety is smooth muscle, that is, either the whole muscle contracts or the whole muscle relaxes. The trachea, the large elastic vessels, and the iris of the eye have a multi-unit smooth muscle. However, smooth muscle is most common in a single unit and lines blood vessels, the urinary tract, and the digestive tract. The ciliary muscle, a kind of smooth muscle in the pupils, dilates and contracts the iris and changes the shape of the lens. In the skin, in reaction to cold weather or anxiety, smooth muscle cells cause hair to stand erect.