Final answer:
Smooth muscle excitation-contraction coupling involves single-unit smooth muscle contracting as a whole unit through gap junctions, beneficial in digestive tract movements, and multiunit smooth muscle cells responding individually to autonomic nervous system or hormonal stimuli, seen in blood vessels and respiratory airways.
Step-by-step explanation:
In smooth muscle excitation-contraction coupling, single-unit and multiunit types demonstrate different physiological mechanisms for initiating contractions. Single-unit smooth muscle, found in the walls of the viscera, is characterized by fibers joined through gap junctions, allowing for synchronized contractions to occur across cells. This coordination causes the muscle to contract as a whole unit, which is beneficial for the rhythmic passage of food in the digestive tract, for instance. These muscles have a remarkable stress-relaxation response, which means they can stretch and relax to adapt to changes in organ volume without premature emptying of contents.
In contrast, multiunit smooth muscle cells lack gap junctions and respond to stimuli individually rather than as a unit. The stimuli originate from the autonomic nervous system or are hormone-induced, and contraction is confined to the cell that was initially stimulated. This type of smooth muscle is present in structures such as large blood vessels, respiratory airways, and the iris of the eye.