Neurotransmitters, Reflexes, and Hormones Oh My!

Cell Bio Lecture 2 2026

Neurocrine

Hormone like substance that comes from a neuron, interacts on post synaptic cell

Acetylcholine (ACh)

Actions:

Contraction of smooth muscle in GI Wall

Relaxes sphincters

Increases salivary secretion

Increases gastric secretion

Increases Pancreatic secretion

Norepinephrine (NE)

Actions:

Relaxation of smooth wall muscle in GI wall

Contraction of sphincter

Increases salivary secretion

Vasoactive Intestinal Peptide (VIP)

Actions:

Relaxation of smooth muscle in GI wall

Increases intestinal secretion

Increases pancreatic secretion

Gastrin-Releasing Peptide (GRP) or Bombesin

Actions:

Increases gastric secretion

Enkephalins (opiates)

Actions:

Contraction of smooth muscle

Decreases intestinal secretion

Neuropeptide Y

Actions:

Relaxation of smooth muscle

Decreases intestinal secretion

Substance P

Actions:

Contraction of smooth muscle

Increases salivary secretion

Nitric Oxide (NO)

Actions:

Relaxation of smooth muscle

What kind of neurons secrete ACh?

Cholinergic neurons

What kind of neurons secrete NE?

Adrenergic neurons

What kind of neurons secrete VIP?

Neurons of mucosa and smooth muscle

What kind of neurons secrete GRP?

Neurons of gastric mucosa

What kind of neurons secrete enkephalins?

Neurons of mucosa and smooth muscle

What kind of neurons secrete Neuropeptide Y?

Neurons of mucosa and smooth muscle

What is Substance P cosecreted with?

ACh

What kind of neurons secrete NO?

Neurons of the enteric nervous system

Purpose of the swallowing reflex

Propel food to esophagus

Purpose of Receptive Relaxation

Accommodate ingested meal in the stomach

Purpose of peristaltic reflex

Propulsion of a bolus through the intestine

Purpose of gastrocolic reflex

Prepare the colon for the next meal

Purpose of enterogastric reflex

Slow delivery of chyme to allow for digestion and absorption

Purpose of Colonoileal reflex (“Ileal Brake”)

Control delivery of contents to large intestine

Purpose of rectosphincteric reflex

Anal sampling, can lead to defecation

What do the afferent nerves detect in the swallowing reflex?

Ingested material in pharynx

What do the afferent nerves detect in receptive relaxation?

Distension of the stomach

What do the afferent nerves detect in the peristaltic reflex?

Distension of the intestine by the bolus

What do the afferent nerves detect in the gastrocolic reflex?

Distension of the stomach

What do the afferent nerves detect in the enterogastric reflex?

Acid and chyme in the duodenum

What do the afferent nerves detect in the colonoileal reflex (“Ileal Brake”)?

Distension of the proximal colon

What do the afferent nerves detect in the rectosphincteric reflex?

Increased pressure due to contents in the rectum

What do the efferent nerves cause in the swallowing reflex?

A swallow (from Lecture 1)

What do the efferent nerves cause in receptive relaxation?

Relaxation of the proximal stomach

What do the efferent nerves cause in the peristaltic reflex?

Contraction proximal to food bolus and relaxation distal to food bolus

What do the efferent nerves cause in the gastrocolic reflex?

Increased motility in the colon

What do the efferent nerves cause in the enterogastric reflex?

Reduced gastric emptying

What do the efferent nerves cause in the colonoileal reflex?

Contraction of the ileocecal

What do the efferent nerves cause in the rectosphincteric reflex?

Relaxation of the internal anal sphincter

Source of gastrin

G cells of stomach

Source of Cholecystokinin (CCK)

i cells of the duodenum and jejunum

Source of secretin

S cells of the duodenum

Source of glucose-dependent insulinotropic peptide, GIP (gastric inhibitory peptide)

K cells of the duodenum and jejunum

Stimulant of gastrin

Peptides

Amino acids

Stomach distension

Vagal stimulation (GRP)

Stimulant of CCK

Peptides

Amino Acids

Fatty Acids

Stimulant of secretin

H+ in duodenum

Fatty acids in the duodenum

Stimulant of GIP

Fatty acids

Amino acids

Oral glucose

What is the function of gastrin?

Increases gastric H+ secretion (directly and indirectly through histamine), Stimulate gastric mucosa growth, Increases gastric motility

What is the function of CCK?

Increases pancreatic enzyme secretion, Increases pancreatic HCO3- secretion, Stimulates gallbladder contraction, Relaxes the sphincter of Oddi, Stimulates exocrine pancreas and gallbladder growth, Inhibits gastric emptying

What is the function of secretin?

Increases pancreatic and biliary HCO3- secretion, Decreases gastric H+ secretion, Inhibits trophic effects of gastrin

What is the function of GIP?

Increases pancreatic insulin secretion, Decreases gastric H+ secretion, Regulates lipid metabolism by promoting lipid storage by increasing adipose tissue blood flow and triglyceride uptake

Source of serotonin (5-hydroxy-tryptamine[5-HT])

Enterochromaffin cells and enteric neurons

Source of histamine

Enterochromaffin-like cells of the stomach (ECL)

Source of somatostatin

D cells of the stomach, duodenum and jejunum

Stimulus of 5-HT

Luminal contents and luminal irritants

Stimulus of histamine

Gastrin

Stimulus of somatostatin

Acid and gastrin

Function of 5-HT

Increased GI motility, decreased water absorption, stimulates ENS interneurons and stimulates vagal afferents

Function of histamine

Primary stimulant of H+ secretion

Function of somatostatin

Inhibits gastrin, H+ and histamine

Source of Motilin (candidate hormone)

M cells of the duodenum and jejunum

Source of Glucagon-like peptide (GLP)

L cells of the ileum and colon

Source of Leptin

Adipocytes

Source of Peptide YY

L cells of the ileum and colon (primary)

Source of Ghrelin

P/D1 cells in the fundus of the stomach (some pancreas)

What is the stimulus of motilin?

Fatty foods, acidification of the duodenum, increases during fasting (cyclic)

What is the stimulus of GLP?

Fatty acids, amino acids and oral glucose

What is the stimulus of leptin?

Proportionate to amount of adipose tissue present

What is the stimulus of peptide YY?

Increased caloric intake (nutrient dependent)

What is the stimulus of ghrelin?

Decreased caloric intake (fasting) stimulates secretion, inhibited by stretch of the stomach and macronutrients

What is the function of motilin?

Stimulates gastric and intestinal motility, stimulates the migrating motor complex (MMC) and stimulates pepsin

What is the function of GLPs?

Product of proglucagon cleavage; stimulates pancreatic insulin secretion, inhibits glucagon secretion, increases pancreatic beta cell sensitivity to glucose, decreases gastric emptying and inhibits appetite, energy balance

What is the function of leptin?

Decreases appetite (rapidly becomes desensitized in obesity)

What is the function of peptide YY?

Mediates satiety and influences energy homeostasis, increases postprandially to reduce appetite, inhibits motility, increases electrolyte absorption in colon

What is the function of ghrelin?

Feeding behavior (appetite stimulating), energy homeostasis (increased fat storage), and carbohydrate metabolism, inhibition of insulin secretion, increased gastric acid secretion and intestinal motility