2. Stomach Flashcards
What are the three phases of Digestion?
Cephalic
Gastric
Intestinal!
Desribe the Cephalic Phase
This phase occurs before food enters the stomach and involves the preparation of the body for eating and digestion. Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) and G cell (secretes gastrin) activity via D cell secretion of somatostatin.
Describe the Gastric Phase
this phase takes 3-4 hours. It is stimulated by the distension of the stomach, presence of food in stomach and decrease in pH. Distension activates long and myenteric reflexes. This activates the release of acetylcholine, which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach. Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete gastric acid. Gastric acid is about 0.5% hydrochloric acid (HCl) which lowers the pH to the desired pH of 1-3. Acid release is also triggered by acetylcholine and histamine.
Intestinal Phase
Intestinal Phase- this phase has 2 parts, the excitatory and inhibitory. Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.
CEPHALIC PHASE*
Describe the preparation of GIT for food!
Preparing GIT for food:
• Triggers (smell, taste, thought of food)
• Parasympathetic outflow (CNX); Ach
Salivation
Increases stomach secretion: mucous, chief, parietal, G cells
Stimulates pancreatic secretion (Readiness)
• 30% of stomach secretions
• 25% of pancreatic secretions
How does Salivation occur?
Salivation
• Both conditioned and simple reflexes
• Sensory: cranial nerves
o Touch: Trigeminal (CNV)
o Taste: Facial (CNVII), glossopharyngeal (IX), vagus (X)
• Integrated by salivatory nuclei in the brainstem
• Visceral motor nerves (PSNS via CNs)
o Submandibular, sublingual: facial (VII)
o Parotid: glossopharyngeal (IX)
Describe Deglutition
Neural control only
Voluntary initiation, then pattern generator:
• Food in pharynx initiates swallowing reflex
• Pharyngeal constrictor muscles
• First third of the oesophagus
Continues via ENS
• Lower third of oesophagus
• Peristaltic waves
• Upper/lower oesophageal sphincters (UES/LES) coordinate.
Describe the swallowing Reflex
• Counscious or (more often) reflex pharynx stimulation
• Around 1000 times a day, including sleep
• Sensory: tactile receptors in entry to pharynx
• Activates swallowing center in medulla & lower pons
• Motor:
o Cranial nerves to pharynx and upper oesophagus
o Vagus to rest of oesophagus
o “Pauses” pontine respiratory center
Describe the Oropharyngeal Sequence
- Soft Palate Elevates, closes entry to nose
- Vocal cords close, larynx elevates
- Epiglottis closes
- Pause in breathing
- Briefly relax UES (reflex opening after food passes)
- Contract upper oesophagus (skeletal muscle)
- Initiation of peristaltic wave
Describe the Oesophageal Sequence
• Primary peristaltic wave moves at 3-4cm/s for 6-8s
• If bolus is not cleared, oesophageal distension triggers a secondary peristaltic wave (ENS reflex)
• Upper third is skeletal muscle (longitudinal/circular); lower third is smooth muscle, with a gradient in between
• Skeletal and smooth muscles innervated by vagal efferents
o Neuromuscular junctions on skeletal muscle
o Control ENS motor neurons in smooth muscle part.
How does vomitting occur?
• Usually short-lived / adaptive • Often preceded by nausea • Medullary trigger zone • SNS, PSNS activity o Sweating, pallor o Tachycardia, tachypnea o Salivation (protect teeth) o Increased gastric tone • Deep inspiration against closed glottis (decreased thoracic pressure, increase abdominal pressure) • Contract abs, relax LES/ UES • Close nasopharynx with soft palate! (danger of aspiration)
GASTRIC PHASE*
Digestion of food in the stomach
Stimulated by distension, increased pH, proteins, peptides
Vagus/ENS: stimulates parietal and chief cells (ACh); G cells (GRP); increases motility (mixing waves)
Hormonal: peptides and AAs in the antrum detected → G cells → Gastrin → parietal/chief cell secretion + increases motility
Paracrine: distension + vagus stimulate ECL cells → histamine → parietal acid production
60% of stomach secretions
5-10% of pancreatic (weak gastrin, vagal stimulus)
Describe the stomach secretions
- Into Lumen
Onto Mucosal surface
0Into Blood
Into lumen o Acid o Pepsinogen o Intrinsic Factor Onto mucosal surface o Mucous o Bicarbonate- resist breakdown Into blood o Gastrin
Describe the cells present in Gastric Pits
Parietal (oxyntic) cells
o Secrete HCl and intrinsic factor
o Composition changes with flow rate:
---> Na+, K+, Cl- (low) to H+, Cl- (high)
Chief (peptic) cells
o Secrete pepsinogen
Mucous cells
• Secrete protective mucous
Glycoprotein layer
Resists pepsin and acid
G (endocrine) cells
o Secrete gastrinWhat is Intrinsic Factor? What does it do?
- Secreted by parietal cells along with acid
- Forms a dimer complex (blue) with vitamin B12
- Resistant to pepsin digestion
- IF-B12 complex binds to a specific receptor (green) in the ileum, internalized
- B12 is needed for erythrocyte production: IF deficiency -> pernicious anemia.
