Gastrointestinal Motility and the Enteric Nervous System Quiz

See Four Layers of the Gut Wall. Option D is correct. The hollow gut is a layered tube, and from the lumen (the hollow space inside the tube) outwards the four classical layers are the mucosa, the submucosa, the muscularis externa and the serosa or adventitia. The mucosa is the lining that meets the food: it contains epithelium (the surface cells that absorb and secrete), the lamina propria (loose connective tissue with vessels and immune cells) and a thin sheet of smooth muscle, the muscularis mucosae, which ripples the lining into folds. The submucosa is loose connective tissue carrying blood vessels, lymphatics and a nerve network called the submucosal plexus. The muscularis externa is usually an inner circular and an outer longitudinal layer of smooth muscle, and this is the layer that mixes and moves the contents. The outer coat is serosa, a covering of peritoneum (the smooth membrane lining the abdominal cavity), where the organ lies inside the peritoneal cavity, or adventitia, a plain connective-tissue covering, where it does not. A memory aid: read the names in the order the food meets them. The lining touches the food, the layer under the lining is the sub-mucosa, then comes the muscle that squeezes, then the outer coat.

• Four layers: From the lumen outwards: mucosa, submucosa, muscularis externa, then serosa or adventitia.
• Muscularis externa: An inner circular and an outer longitudinal layer of smooth muscle that mixes and moves the contents.
• Outer coat: Serosa where the organ lies in the peritoneal cavity, adventitia where it does not.