Final answer:
Smooth muscle cells are involuntary, found within various organs, and classified as either single-unit or multiunit. Single-unit cells have gap junctions that allow for synchronized contractions, whereas multiunit cells respond independently and lack these junctions. Stimuli for multiunit smooth muscles come from the autonomic nervous system or hormones but not from stretching.
Step-by-step explanation:
Smooth muscle is composed of involuntary muscles found within the walls of various organs and structures such as the esophagus, stomach, intestines, and blood vessels. Unlike neurogenic muscles, which are stimulated by the nervous system, smooth muscles can be classified as either single-unit or multiunit, with distinct differences between the two.
Differences between Single-Unit and Multiunit Smooth Muscle
- Single-unit smooth muscle cells are connected by gap junctions, which allow them to synchronize their contractions and behave as a single unit. This type is commonly found in the walls of viscera.
- Multiunit smooth muscle cells do not have gap junctions, therefore they do not synchronize their contractions. Instead, each cell responds independently to stimulation. This type is found around large blood vessels, in the respiratory airways, and in the eyes.
- Stimuli for multiunit smooth muscles come from the autonomic nervous system or by hormones, but not from stretching, unlike single-unit smooth muscles which can be stimulated spontaneously or by stretching.
In addition, smooth muscle cells are capable of hyperplasia, meaning they can divide to produce new cells. This is particularly notable in organs like the uterus. Smooth muscles can also sustain contractions over a wider range of resting lengths than skeletal and cardiac muscle, partly due to the presence of latch-bridges in their fibers.