Final answer:
Smooth muscle is found in the walls of hollow organs and passageways, including the digestive, respiratory, and reproductive tracts. It's an involuntary, nonstriated muscle with a single nucleus per cell that plays a key role in the expansion and contraction of these organs.
Step-by-step explanation:
Smooth muscle tissue is the type of muscle found in the walls of hollow organs like the urinary bladder, stomach, intestines, and in the respiratory tract. Unlike skeletal muscles, smooth muscle has no striations and is not under voluntary control, which is why it is also known as involuntary muscle. Each cell contains a single nucleus and is tapered at both ends. In addition to being found in various organ systems, smooth muscle is also present in the eye, participating in functions like changing the iris size or altering the lens shape. Further, smooth muscle is categorized as either single-unit or multi-unit muscle, with the former being more common and found in the walls of visceral organs. Single-unit smooth muscle fibers are connected by gap junctions, allowing them to contract as one unit, which is essential for the regular expansion and contraction of organs as they fill and empty.