Which statement correctly describes the muscularis externa?

Which statement correctly describes the muscularis externa?

• A. It is the innermost lining.
• B. It is a bilayer of smooth muscle with circular and longitudinal fibers.
• C. It secretes enzymes into the lumen.
• D. It contains nerve plexuses only.

Answer: (B)

Muscularis externa is the muscular layer of the GI tract wall that drives movement. It consists of two smooth muscle layers: an inner circular layer and an outer longitudinal layer. This bilayer arrangement lets the tube constrict around its lumen and shorten along its length, producing peristalsis and segmentation to move and mix contents. Between these layers lies the myenteric (Auerbach) plexus, a nerve network that coordinates their contractions. So the statement is best because it accurately describes both the two-layer smooth muscle structure and its role in GI motility. The innermost lining is the mucosa, not the muscularis externa. Enzyme secretion into the lumen is a function of mucosal glands and the pancreas, not the muscularis externa. And while nerve plexuses are present within the muscularis externa, it is not comprised of nerve tissue alone; the muscle layers are the primary component coordinating motion.

Muscularis externa is the muscular layer of the GI tract wall that drives movement. It consists of two smooth muscle layers: an inner circular layer and an outer longitudinal layer. This bilayer arrangement lets the tube constrict around its lumen and shorten along its length, producing peristalsis and segmentation to move and mix contents. Between these layers lies the myenteric (Auerbach) plexus, a nerve network that coordinates their contractions.

So the statement is best because it accurately describes both the two-layer smooth muscle structure and its role in GI motility. The innermost lining is the mucosa, not the muscularis externa. Enzyme secretion into the lumen is a function of mucosal glands and the pancreas, not the muscularis externa. And while nerve plexuses are present within the muscularis externa, it is not comprised of nerve tissue alone; the muscle layers are the primary component coordinating motion.