Final answer:
Single-unit smooth muscles contain gap junctions allowing them to contract as a unit, commonly found in visceral organs, while multiunit smooth muscles lack gap junctions, require individual nerve stimulation, and are found in large blood vessels, airways, and eyes.
Step-by-step explanation:
Single-unit smooth muscles have numerous gap junctions and contract as one group while multiunit smooth muscles have few if any gap junctions and require direct nerve stimulation.
Single-unit smooth muscle, found in the walls of visceral organs except the heart, contains gap junctions to synchronize membrane depolarization and contractions. This type of muscle contracts as a whole, allowing for coordinated movements such as food moving through the digestive tract. It can maintain a muscle tone even when the organ empties, preventing "flabbiness" and is able to undergo a stress-relaxation response to adjust to the changing volume of the organ it surrounds.
In contrast, multiunit smooth muscle cells rarely possess gap junctions, and thus they do not contract as a unit. These cells are not electrically coupled, and each cell's contraction is confined to that individual cell originally stimulated. Stimuli for these muscles primarily come from the autonomic nervous system, or by hormones, but not from stretching. Multiunit smooth muscles are found in places such as around large blood vessels, in the respiratory airways, and in the eyes.