D’YOUVILLE COLLEGE
PMD 604 - ANATOMY, PHYSIOLOGY, PATHOLOGY II
Lecture 18: Digestive System
G & H chapters 62 - 64
1.
Organization: two parts:
• alimentary canal (GI tract) (fig. 62 – 1 & ppts. 1 & 2)
- oral cavity – initial breakdown of food, swallowing
- pharynx – swallowing
- esophagus -- swallowing
- stomach – digestion, storage
- small intestine (duodenum, jejunum, ileum) – digestion, absorption
- large intestine (cecum , ascending colon, transverse colon, descending colon,
sigmoid colon, rectum & anus) – water recovery, defecation
- wall layers (fig. 62 – 2 & ppt. 3)
- mucosa – last stages of chemical digestion, absorption
- submucosa – blood supply, glands, nerve net (submucosal plexus)
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- p. 2 -
- muscularis – motility; inner circular & outer longitudinal smooth muscle,
nerve net (myenteric plexus) (ppt. 4)
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- p. 3 -
- serosa – outer limiting serous membrane, continuous with peritoneum
(visceral peritoneum)
• accessory glands
- major salivary glands (three pairs: sublingual, submandibular, parotid) –
secretion of saliva to aid oral digestion (ppt. 5)
- liver – metabolic regulation of absorbed nutrients, secretion of bile (ppt. 6)
- pancreas – secretion of buffering and enzyme – rich juices (ppt. 7)
2.
Blood Supply:
• splanchnic circulation (fig. 62 – 6 & ppt. 8): flow pattern is from arteries to
gut, pancreas, liver & spleen; portal vein conveys blood from gut, pancreas & spleen to
liver; liver drainage is via hepatic veins
- arterial supply (fig. 62 – 7, ppts. 9 to 12) – celiac trunk (from aorta)
supplies liver, stomach & spleen
- superior mesenteric artery (from aorta) supplies the small intestine,
pancreas & the right half of large intestine
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- p. 4 -
- inferior mesenteric artery (from aorta) supplies left half of large
intestine & rectum
- increased intestinal activity stimulates vasodilation of arterioles supplying
active area; several hormones released by the active GI tract also promote vasodilation
- villi are served by arteriole-capillary-venule system + lymphatic capillary
(lacteal) (fig. 62 – 8, ppts. 13 & 14)
- venous drainage all of abdominal GI tract + pancreas and spleen drain
into hepatic portal system, which delivers nutrient-rich blood directly to liver (fig. 62
– 6 & ppt. 8)
- liver processing (storage, metabolic conversion, etc.) regulates blood
levels of absorbed substances
- liver drains via hepatic veins to inferior vena cava
3.
Motility and Secretory Activity:
• electrical activity of unitary smooth muscle
- organized as sheets of cells interconnected by gap junctions allowing
electrical disturbances to pass unimpeded throughout muscle mass (functional
syncytium)
- types of electrical activity (fig. 62 – 3 & ppt. 15):
- slow waves – graded potentials, usually not strong enough to elicit
contraction; slow waves that exceed threshold cause spike potentials
- spike potentials are volleys of action potentials; action potentials are of
long duration (10 – 40 times duration of a nerve AP); tonic contractions result from spike
potentials & also from hormonal stimulation
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- p. 5 -
- depolarization – factors such as stretching, acetylcholine or
parasympathetic stimulation or certain GI hormones may raise the resting
membrane potential, making the muscle more excitable
- hyperpolarization – factors such as epinephrine or norepinephrine
or sympathetic stimulation may drive the resting potential down, making the
muscle less excitable
• enteric nervous system (fig. 62 – 4 & ppt. 16)
- nerve plexuses (myenteric and submucosal) extend entire length of GI tract
from esophagus to anus
- myenteric regulates motility; submucosal regulates glandular secretions
- local reflexes or autonomic reflexes modulate enteric nervous system
- enteric nervous system secretes excitatory (e.g. acetylcholine) or
inhibitory (e.g. norepinephrine) neurotransmitters
- parasympathetic innervation – fibers of cranial nerves VII (facial) & IX
(glossopharyngeal) supply major salivary glands
- most of the remaining alimentary canal is supplied by cranial nerve X
(vagus)
- sacral nerves (pelvic nn.) from S2 – S4 supply lower large intestine,
rectum & anus (fig. 60 - 3 & ppt. 17)
- parasympathetic signals excite the enteric nervous system, leading to
enhanced motility & secretions
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- p. 6 -
- sympathetic innervation – fibers of spinal nerves from T5 to L2 supply
most of the alimentary canal via prevertebral ganglia (e.g. celiac) (fig. 60 - 1 & ppt. 18)
- sympathetic signals inhibit the enteric nervous system & inhibit smooth muscle
directly
• hormonal reflexes (table 62 - 1 & ppt. 19)
- gastrin is produced by G cells of stomach mucosa & triggers gastric acid
secretion & cell renewal
- cholecystokinin is produced by I cells of duodenum and jejunum mucosa; it
excites gall bladder contraction, pancreatic secretion of enzymes and inhibits stomach
emptying
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- p. 7 -
- secretin is produced by S cells of duodenum mucosa; it triggers release of
bicarbonate-rich juice by the pancreas & inhibits gastric secretion
• patterns of contraction
- propulsive – peristalsis (fig. 62 – 5 & ppt. 20) involves formation of ringlike constriction that advances slowly along the gut toward the anus, pushing
contents along (mediated by myenteric plexus)
- mixing – intermittent constrictions (segmentation contractions) produce
chopping or shearing of contents (assist peristaltic mixing) (ppt. 21)
4.
Regional Physiology (chapters 63 & 64)
• oral cavity, pharynx, esophagus
- mastication – breaking up of food by teeth & jaw muscles (trigeminal n.),
controlled by chewing reflex
- creation of a bolus: broken up food, mixed with saliva, facilitates
swallowing and renders food more accessible to enzymes
- deglutition (fig. 63 – 1 & ppt. 22) – swallowing consists of three stages:
- voluntary: bolus voluntarily propelled backward from oral cavity to
pharynx by elevation & retraction of tongue (hypoglossal n.)
- pharyngeal: entry of bolus into posterior oral cavity/oropharynx
triggers an autonomic reflex that promotes pharyngeal deglutition (trigeminal,
glossopharyngeal, vagus & hypoglossal nn.)
- effects include narrowing of oropharyngeal boundary (admits only
properly chewed food to pharynx), elevation of soft palate (occludes nasopharynx),
elevation and anterior displacement of larynx (closes glottis & depresses epiglottis,
obstructing entry into larynx), peristaltic contractions of pharyngeal muscles &
relaxation of upper esophageal sphincter
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- p. 8 -
- esophageal: stimulated by distension of esophagus, reflex involves
signals carried by vagus and glossopharyngeal nerves that stimulate primary and
secondary peristaltic waves; lower esophageal sphincter relaxes to admit food to stomach
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- p. 9 -
- secretions: (fig. 64 - 1 & ppt. 23)
- triggered by: 1) contact with mucosa; 2) chemical irritation of mucosa; 3)
distension of alimentary canal
- local reflexes (enteric nervous system), autonomic reflexes (mostly
parasympathetic) and some hormonal reflexes are involved
- salivary gland secretions range from watery (serous) to thick and sticky
(mucous)
- saliva also contains numerous ions (bicarbonate, potassium, sodium,
chloride) & enzymes (salivary amylase or ptyalin - commences chemical digestion &
lysozyme - combats bacteria in oral cavity)
- cranial nerves VII and IX (facial & glossopharyngeal) carry sensory limb
of reflex via solitary tract to salivatory nuclei, which send parasympathetic fibers
(motor fibers in nerves VII & IX) to major salivary glands (fig. 64 – 3 & ppt. 24)
• stomach (fig. 63 – 2 & ppt. 25):
- a relaxation reflex in response to initial distension facilitates storage
- mixing is accomplished by weak peristaltic (constrictor) waves
accompanied by tonic contraction of pyloric sphincter (may cause retropulsion)
- emptying – more intense peristaltic waves ‘pump’ chyme into duodenum
- controlled by gastrin (+), enterogastric reflex (-) & cholecystokinin (-)
- secretions – tubular glands (fig. 64 – 4 & ppt. 26) of the stomach mucosa
include mucous neck cells (mucus), parietal cells (HCl & intrinsic factor), chief cells
(pepsinogen) & G cells (gastrin)
- most of the surface mucosal cells secrete mucus
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- p. 10 -
- acid secretion (fig. 64 – 5) appears to be stimulated by gastrin and by postganglionic parasympathetic neurons (secrete acetylcholine)
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- p. 11 -
- pepsinogen secretion - stimulated by acid & by parasympathetic fibers
- mucus secretion (alkaline) is stimulated by contact of mucosa with food or
irritation of the mucosa
- phases of gastric secretion (fig. 64 – 7 & ppt. 27) – cephalic: sight, smell,
taste or even thought of food initiates parasympathetic signals via vagus nerve to
stimulate gastric secretion
- gastric: entry of food into stomach stimulates a vagovagal reflex and
gastrin secretion, which stimulates gastric juice secretion
- intestinal: enterogastric signals (via enteric nervous system), secretin &
other GI hormones (e.g. gastric inhibitory peptide) all inhibit secretion of gastric juice