Which statement about smooth muscle is true?
A) Acetylcholine decreases the likelihood of contraction of smooth muscle cells of the gut.
B) Smooth muscle contraction is controlled by the same troponin-tropomyosin mechanism as skeletal muscle.
C) Norepinephrine increases the likelihood of contraction of smooth muscle cells of the gut.
D) Action potentials, hormones, and stretching can cause an influx of calcium into the sarcoplasm of smooth muscles.
E) Norepinephrine causes smooth muscle cells of the arteries serving the gut to relax.

Question

Quizplus · Accepted Answer

Smooth muscle cells do not have well-developed sarcoplasmic reticula (SR), so calcium ions needed for contraction are usually acquired from the extracellular fluid. Action potentials, hormones, and stretching can cause an influx of calcium into the sarcoplasm of smooth muscles, so choice D is the correct answer.

Note: A, C, and E are all incorrect statements about smooth muscle cells. Choice B is also incorrect - smooth muscle contraction is regulated by calcium ions binding to the protein calmodulin, not the troponin-tropomyosin mechanism found in skeletal muscle.

Which Statement About Smooth Muscle Is True