Stomach Nerve supply Sympathe0c and parasympathe0c fibers of the autonomic nervous system Blood supply Celiac trunk, and corresponding veins (part of the hepa0c portal system) Stomach Figure 23.14a Chapter 23: Digestive System 1
Microscopic Anatomy of the Stomach Muscularis externa Has an addi0onal oblique layer that: Allows the stomach to churn, mix, and pummel food physically Breaks down food into smaller fragments Epithelial lining is composed of: Goblet cells that produce a coat of alkaline mucus The mucous surface layer traps a bicarbonate rich fluid beneath it Gastric pits contain gastric glands that secrete gastric juice, mucus, and gastrin Microscopic Anatomy of the Stomach Figure 23.15 Chapter 23: Digestive System 2
Mucosa Histology of the Stomach Simple columnar epithelium Transi0on from stra0fied squamous at gastricesophageal junc0on Mostly mucous cells To keep from diges0ng itself, the stomach has a mucosal barrier with: A thick coat of bicarbonate rich mucus on the stomach wall Epithelial cells that are joined by 0ght junc0ons Gastric glands that have cells impermeable to HCl Damaged epithelial cells are quickly replaced 3 6 days Muscularis Externa Histology of the Stomach Three layers instead of two Mucosa Addi0onal internal oblique layer Middle circular layer Submucosa Outer longitudinal layer Muscularis externa Serosa Chapter 23: Digestive System 3
Gastric pits and Glands Gastric pits Apical surface to lamina propria Mucous neck cells Secrete acid mucus Parietal cells Secrete HCl and intrinsic factor ph 1.5 3.5 Gastric Glands Chief cells Produce pepsinogen Ac0vated to pepsin by: HCl in the stomach Pepsin itself via a posi0ve feedback mechanism Enteroendocrine cells Secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatosta0n into the lamina propria Chapter 23: Digestive System 4
Diges9on in the Stomach The stomach: Holds ingested food Degrades this food both physically and chemically Enzyma0cally digests proteins with pepsin Delivers chyme to the small intes0ne Secretes intrinsic factor Required for absorp0on of vitamin B 12 Regula9on of Gastric Secre9on Neural and hormonal mechanisms regulate the release of gastric juice Parasympathe0c nervous control Vagus nerve (X) S0mulates dras0c increase in all gastric gland secre0ons Hormonal Control Gastrin Same result as parasympathe0c s0mulus Chapter 23: Digestive System 5
Regula9on of Gastric Secre9on S0mulatory and inhibitory events occur in three phases Cephalic, Gastric, Intes0nal Cephalic (reflex) phase Prior to food entry, ini0ated by sight, smell, taste of food Reflex arc sensory receptor cerebral cortex/hypothalamus Vagal nuclei in medulla stomach glands via vagal motor fibers Inhibited by fear or sa0a0on Regula9on of Gastric Secre9on Gastric phase: once food enters the stomach Hormonal and neural control Vagovagal reflex Stomach distension Ac0va0on of stretch receptors (neural ac0va0on) Impulse sent to medulla and back to myenteric plexus triggering acetylcholine release, causing gastrin release Ac0va0on of chemoreceptors by protein, caffeine, and rising ph, alcohol Inhibitory events include: A ph lower than 2 S0mulate release of gastrin and increased secre0on and mo0lity Emo0onal upset that overrides the parasympathe0c division Chapter 23: Digestive System 6
Intes9nal Phase Two Phases excitatory and inhibitory Excitatory phase Low ph; par0ally digested protein enters the duodenum S0mulates duodenal mucosa to secrete enteric (intes0nal) gastrin Causes gastric gland ac0vity and maintains mo0lity Inhibitory phase Intes9nal Phase Distension of duodenum, presence of fa`y acids, low ph, or hypertonic chyme, and/or irritants in the duodenum Triggers enterogastric reflex Inhibi0on of local (gastric) reflexes and vagal (parasympathe0c) nuclei in medulla Sympathe0cally closes the pyloric sphincter Increased vasoconstric0on Releases enterogastrones that inhibit gastric secre0on Secre0n: lowers gastric secre0ons Cholecystokinin (CCK): decreases gastric mo0lity Gastric inhibitory pep0de (GIP): lowers gastric secre0ons and mo0lity Chapter 23: Digestive System 7
Release of Gastric Juice Figure 23.16 Regula9on and Mechanism of HCl Secre9on HCl secre0on S0mulated by ACh, histamine, and gastrin Through second messenger systems Release of hydrochloric acid: Is low if only one ligand binds to parietal cells Is high if all three ligands bind to parietal cells An0histamines block receptors and decrease HCl release Chapter 23: Digestive System 8
Regula9on and Mechanism of HCl Secre9on Figure 23.17 Response of the Stomach to Filling Stomach pressure Remains constant un0l about 1L of food is ingested Rela0ve unchanging pressure results from reflex mediated relaxa0on and plas0city Reflex mediated events include: Recep0ve relaxa0on as food travels in the esophagus, stomach muscles relax Adap0ve relaxa0on the stomach dilates in response to gastric filling Plas0city intrinsic ability of smooth muscle to exhibit the stress relaxa0on response Chapter 23: Digestive System 9
Gastric Contrac9le Ac9vity Peristal0c waves move toward the pylorus at the rate of 3 per minute This basic electrical rhythm (BER) is ini0ated by pacemaker cells (cells of Cajal) Most vigorous peristalsis and mixing occurs near the pylorus Chyme is either: Delivered in small amounts to the duodenum or Forced backward into the stomach for further mixing Chapter 23: Digestive System 10