Organization and regulation of the digestive system

The visceral layer covers the organ itself; the parietal layer lines the wall of the cavity. The thin space between them holds serous fluid.

• The parietal layer:
• Correct: The visceral layer:
• The fibrous layer:

Smooth muscle forms the walls of hollow organs. It is involuntary and is controlled by the autonomic nervous system, hormones and local signals.

• Skeletal muscle:
• Cardiac muscle:
• Correct: Smooth muscle:

The rest-and-digest pattern increases both gut movement and gut secretion; sympathetic activity reduces both.

• Correct: Both increase:
• Both decrease:
• Movement increases but secretion decreases:

Mechanical digestion makes pieces smaller and mixes them without breaking chemical bonds: chewing, churning and segmentation. Enzymes splitting molecules is chemical digestion, and glucose crossing the lining is absorption.

• Correct: Your molars grinding a peanut: Correct. Grinding breaks the peanut into smaller pieces without changing its molecules.
• An enzyme in saliva splitting starch into sugars: Splitting starch breaks chemical bonds by hydrolysis, so this is chemical digestion.
• Correct: The stomach wall churning its contents: Correct. Churning breaks up and mixes food physically.
• Correct: Rings of intestinal muscle chopping and remixing the contents: Correct. This is segmentation, a mixing movement of the wall.
• Glucose crossing the intestinal lining into the blood: Moving a nutrient across the lining into the blood is absorption, not digestion.
• Protein being split into amino acids: Breaking peptide bonds is chemical digestion, done by enzymes.

The mucosa comes first: the epithelium, then the lamina propria beneath it (with the thin muscularis mucosae at its base). Next is the submucosa, then the muscularis externa with its inner circular and outer longitudinal layers, and finally the serosa.

• Correct order: 1. Simple columnar epithelium 2. Lamina propria 3. Submucosa 4. Circular layer of smooth muscle 5. Longitudinal layer of smooth muscle 6. Serosa (visceral peritoneum)

Ahead of the food, inhibitory enteric neurons release nitric oxide, which relaxes the circular muscle and widens the tube. If the muscle ahead contracted too, the food would be squeezed from both sides and could not move.

• Food stretches a short length of the wall: This step is right. Distension of the wall is the stimulus that starts the reflex.
• Enteric sensory neurons detect the stretch: This step is right. Stretch-sensitive enteric sensory neurons start the short reflex.
• Circular muscle behind the food contracts: This step is right. Excitatory motor neurons release acetylcholine behind the food.
• Correct: Circular muscle ahead of the food also contracts, gripping it: This is the error. The muscle ahead relaxes; the narrowing behind and widening ahead together push the food forward.
• The food is pushed forward and stretches the next length of wall: This step is right. The new stretch repeats the reflex, so the wave travels along the tube.

Peristalsis needs relaxation ahead of the contents, and that relaxation comes from enteric inhibitory neurons releasing nitric oxide. Without them the smooth muscle stays contracted, the segment stays narrow, and no wave can pass through it. The normal intestine above it fills and swells. This condition is called Hirschsprung disease.

• Its muscle has no nerve input, so it is paralyzed and floppy: The segment is narrow and contracted, not floppy. Smooth muscle has its own tone; what the missing neurons remove is the signal that relaxes it.
• Its mucosa cannot absorb water, so stool becomes too large to pass: The problem is movement, not absorption. The block is in the muscle wall, and stool never reaches the segment's lining in quantity.
• Its sympathetic fibers are missing, so its sphincters stay open: Sympathetic fibers tighten sphincters rather than open them, and the biopsy showed missing enteric neurons, not missing sympathetic fibers.
• Correct: Its circular muscle cannot relax, so no peristaltic wave can pass: Correct. Without inhibitory enteric neurons, the muscle cannot relax, so the segment acts as a fixed narrowing.

Acid releases secretin and fat releases CCK. Secretin brings in bicarbonate to neutralize the acid; CCK brings in bile and pancreatic enzymes to digest the fat. Both hold back the stomach, so the intestine receives new material only as fast as it can process it.

• Secretin in the blood: up. Acid arriving in the first part of the small intestine releases secretin.
• Cholecystokinin (CCK) in the blood: up. Fats and protein fragments in the first part of the small intestine release CCK.
• Bicarbonate-rich fluid from the pancreas: up. Secretin acts on the pancreas and the ducts that carry bile to release a watery bicarbonate-rich fluid.
• Contraction of the gallbladder: up. CCK contracts the gallbladder's smooth muscle, pushing bile into the intestine.
• Rate of stomach emptying: down. Both secretin and CCK slow stomach emptying, so less arrives until the intestine has handled what it has.

Acid in the first part of the small intestine releases secretin, which makes the pancreas and the ducts that carry bile secrete bicarbonate. Once the acid is neutralized, secretin release falls: a negative feedback loop.

• Gastrin is released into the blood: Gastrin comes from the stomach and raises stomach acid secretion, which would add to the acid rather than neutralize it.
• Motilin is released into the blood: Motilin is released between meals and triggers housekeeping contractions; it does not control bicarbonate secretion.
• Correct: Secretin is released into the blood: Correct. Secretin is the hormone released by acid in the intestine, and its main target is the bicarbonate secretion of the pancreas.
• The myenteric plexus stops all contractions: The myenteric plexus controls muscle, not the pancreas, and stopping all contractions would not neutralize acid.

The pellet stretches the wall and triggers the peristaltic short reflex through the myenteric plexus, which is entirely within the wall. The intestine propels the pellet with no connection to the central nervous system.

• It stays put until vagal signals are supplied to the muscle: The vagus nerve adjusts gut activity but does not start it. The enteric nervous system can run peristalsis with no outside input.
• Correct: Peristalsis run by the wall's own neurons pushes it along: Correct. The ENS contains sensory neurons, interneurons and motor neurons, so the whole reflex can run inside the wall.
• It moves only after epinephrine is added to the solution: Sympathetic signals and epinephrine inhibit gut motility; they do not start peristalsis.
• It is squeezed by the whole length contracting at once: Gap junctions spread contraction locally, but enteric neurons shape it into a wave with relaxation ahead. The whole length does not contract at once.