How many neurons does the ENS contain, and what is its defining functional property?
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Answer
~500 million neurons (more than the spinal cord)
Defining property: functions completely independently of the CNS — can coordinate peristalsis, secretion, and absorption without brain/spinal cord input ("second brain")
LO 4 — ENS Neurotransmitters
What do excitatory ENS motor neurons release? What do inhibitory ENS motor neurons release?
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Answer
Excitatory: Acetylcholine (ACh) + Substance P → smooth muscle CONTRACTION
Prepares the stomach to receive and begin digesting food
FIB 01 — LO 1
The myenteric plexus is located between the circular and longitudinal muscle layers and controls GI ___________. Its alternate name is ___________'s plexus.
Hint: "My Sub" mnemonic — Myenteric = ?
FIB 02 — LO 1
Hirschsprung disease results from failure of ___________ cells to migrate into the distal colon during embryogenesis, leaving the segment ___________.
Hint: The affected segment tonically contracts because it has no ___.
FIB 03 — LO 1
Only two regions of the GI tract use skeletal muscle: the mouth/pharynx/upper esophagus, and the ___________ sphincter.
Hint: Allows voluntary control of defecation.
FIB 04 — LO 2
DEEP MASS: the "E" for Endocrine function refers to the fact that the GI tract is the ___________ endocrine organ in the body. GI hormones are secreted into the ___________, not the lumen.
FIB 05 — LO 3
Approximately ___________ liters of fluid enter the small intestine daily. Of this, approximately ___________ L is absorbed by the small intestine, and only ~100–200 mL is lost in feces.
FIB 06 — LO 3
Vomiting causes loss of ___________, leading to metabolic ___________. Diarrhea causes loss of ___________, leading to metabolic ___________.
Hint: Both also cause hypokalemia.
FIB 07 — LO 4
The ENS contains approximately ___________ million neurons and is sometimes called the "___________ brain." It can function independently of the ___________.
FIB 08 — LO 5
In the peristaltic reflex, ascending excitation releases ___________ and Substance P causing contraction ABOVE the bolus, while descending inhibition releases ___________ and NO causing relaxation BELOW.
FIB 09 — LO 6
The celiac trunk supplies the ___________; the SMA supplies the ___________; the IMA supplies the ___________.
FIB 10 — LO 7
The ___________ phase of digestion is triggered by the sight, smell, or thought of food. It is mediated by the ___________ nerve, which pre-stimulates gastric acid secretion before food arrives.
Instructions: Drag and drop the items into the correct order, then click "Check Order." Use "Show Answer" if you get stuck.
Drill 1 — GI Wall Layers: Innermost → Outermost
Arrange these four layers from the innermost (luminal side) to the outermost.
Instructions: Click an item on the left, then click its match on the right. Correct pairs turn green; wrong pairs flash red.
Set 1 — Gastric Cell Type → Secretory Product
Match each cell type to its primary secretory product.
Cell Type
Parietal cell
Chief cell
G cell
Paneth cell
Primary Product
Gastrin
HCl + Intrinsic factor
Defensins + Lysozyme
Pepsinogen
Score: 0/4 correct
Set 2 — ENS Neurotransmitter → Primary GI Effect
Match each neurotransmitter to its primary GI effect.
Neurotransmitter
Acetylcholine (ACh)
VIP
Nitric Oxide (NO)
Serotonin (5-HT)
Primary GI Effect
↑Secretion + smooth muscle relaxation
Initiates peristaltic + secretory reflexes via ENS
↑Motility + ↑Secretion (excitatory)
Sphincter and smooth muscle relaxation (LES)
Score: 0/4 correct
Set 3 — Artery → GI Territory
Match each artery to the GI structures it primarily supplies.
Artery
Celiac trunk
Superior Mesenteric Artery (SMA)
Inferior Mesenteric Artery (IMA)
Portal vein
Territory
Distal transverse colon → upper rectum (hindgut)
Stomach, liver, spleen, pancreas (foregut)
Drains intestinal blood to liver (first-pass metabolism)
Small intestine + proximal 2/3 of transverse colon (midgut)
Score: 0/4 correct
Test Trap Mode: Each statement is either TRUE or FALSE as written. Many are designed to catch common exam errors. Click T or F — then read the explanation carefully.
Trap 01 — LO 1
"Auerbach's plexus controls secretion, while Meissner's plexus controls motility."
FALSE. This is reversed! Auerbach's (myenteric) plexus = MOTILITY (between circular & longitudinal muscle). Meissner's (submucosal) plexus = SECRETION. Mnemonic: "My Sub" — Myenteric = Motility, Submucosal = Secretion.
Trap 02 — LO 1
"In Hirschsprung disease, the DILATED segment of colon is the aganglionic segment that cannot relax."
FALSE. Classic trap! The AGANGLIONIC segment is NARROW and tonically contracted. The DILATED segment is PROXIMAL to the obstruction — it has NORMAL ganglion cells, it's just backed up with stool because nothing can pass through below it.
Trap 03 — LO 2
"Gastrin, CCK, and secretin are secreted directly into the GI lumen where they act on food to promote digestion."
FALSE. GI hormones are ALWAYS secreted into the BLOOD (portal circulation), not the lumen. They travel systemically to reach target organs (e.g., gastrin travels from G cells in the antrum through blood to stimulate parietal cells). Only digestive enzymes go into the lumen.
Trap 04 — LO 3
"Vomiting leads to metabolic acidosis because stomach acid is lost."
FALSE. Vomiting causes METABOLIC ALKALOSIS. When HCl is lost from the stomach, the body loses H⁺ → blood becomes relatively more basic. The correct pairing: vomiting → metabolic alkalosis + hypokalemia; diarrhea → metabolic acidosis + hypokalemia (lose HCO₃⁻).
Trap 05 — LO 4
"A patient with complete vagotomy (all vagal fibers cut) will have no peristalsis because the ENS requires vagal input to function."
FALSE. The ENS can function INDEPENDENTLY of extrinsic innervation (vagus, spinal cord). Transplanted, extrinsically denervated intestinal segments still peristalse. The vagus MODULATES and amplifies ENS activity but is NOT required for basic GI function.
Trap 06 — LO 5
"Nitric oxide (NO) from ENS neurons causes contraction of the LES to maintain the barrier against acid reflux."
FALSE. NO causes RELAXATION, not contraction. In fact, NO is responsible for relaxing the LES to allow swallowed food to pass into the stomach. When inhibitory NO neurons at the LES are destroyed (as in achalasia), the LES FAILS TO RELAX → food cannot enter the stomach → progressive esophageal dilation.
Trap 07 — LO 6
"B12 is absorbed in the jejunum, where the majority of nutrient absorption occurs."
FALSE. B12 (and bile salts) are specifically absorbed in the TERMINAL ILEUM — not the jejunum. The jejunum is the major site of general nutrient absorption, but B12 requires intrinsic factor (from parietal cells) and is exclusively absorbed in the ileum. This is why ileal disease/resection causes B12 deficiency.
Trap 08 — LO 7
"Sympathetic stimulation of the GI tract increases peristalsis and secretion to facilitate digestion during periods of stress."
FALSE. Sympathetic stimulation ("fight or flight") DECREASES GI motility and secretion, INCREASES sphincter tone, and causes vasoconstriction in the gut. It essentially shunts blood and energy away from digestion during stress. It is PARASYMPATHETIC ("rest and digest") that promotes GI function.
GI Wall Layers & Plexuses
Layer (Inside→Out)
Key Contents
Plexus
Mucosa
Epithelium, lamina propria, muscularis mucosae
—
Submucosa
Blood vessels, lymphatics, dense CT
Meissner's (submucosal) → SECRETION
Muscularis externa
Inner circular + outer longitudinal smooth muscle
Auerbach's (myenteric) → MOTILITY (between the two muscle layers)