D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
DIGESTIVE SYSTEM
Lecture 21: Motility, Secretion, Blood supply
1.
Organization: (chapter 62)
• 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
• accessory glands
- major salivary glands (three pairs: sublingual, submandibular, parotid) –
secretion of saliva to aid oral digestion; numerous minor salivary glands in tongue &
oral mucosa
- liver – metabolic regulation of absorbed nutrients, secretion of bile
- pancreas – secretion of buffering and enzyme – rich juices
• wall layers (fig. 62 – 2 & ppt. 3)
- mucosa – terminal digestion (secretion), absorption, barrier to infection,
protection from chemical digestion
- submucosa – blood supply, glands, nerve net (submucosal plexus)
- muscularis – motility; inner circular & outer longitudinal smooth muscle,
nerve net (myenteric plexus)
- serosa – outer limiting serous membrane, continuous with peritoneum
(visceral peritoneum)
Bio 659
2.
- p. 2 -
Motility and Secretory Activity:
• organization of smooth muscle: (fig. 8 – 1 & ppt. 4)
- a sheet-like population of fibers (unitary smooth muscle); nerve supply less
discrete, but stimuli pass readily throughout population by means of gap junctions
- contractions are slower, more sustained (often hours in duration (= tonic
contractions)
• electrical activity of unitary smooth muscle
- spontaneous depolarization (autostimulation); sufficient numbers of sodiumcalcium (slow) channels are open at resting potential to provoke depolarization (fig. 8
– 5 & ppt. 5)
- electrical disturbances pass unimpeded throughout muscle mass
(functional syncytium)
- two types of electrical activity (fig.62 – 3 & ppt. 6):
- 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 (associated with
opening of slow sodium-calcium channels); action potentials are of long duration (10 – 40
times duration of nerve AP)
- depolarization – factors such as stretching, acetylcholine or
parasympathetic stimulation 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
- tonic contractions (prolonged contractions lasting several minutes up to
several hours) result from spike potentials (recall calcium-calmodulin mechanism for
switching on contraction) &/or from hormonal stimulation
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- p. 3 -
• enteric nervous system (fig. 62 –4 & ppt. 7)
- nerve plexuses (myenteric and submucosal) extend entire length of GI
tract from esophagus to anus; myenteric regulates motility; submucosal regulates
glandular secretions & some absorption
- sensory cells can elicit local reflexes by synapsing with local nerve
plexuses or other reflexes via autonomic innervation, which modulates enteric
nervous system
- neurons of the enteric nervous system secrete excitatory (e.g.
acetylcholine) or inhibitory (e.g. norepinephrine, vasoactive intestinal polypeptide)
neurotransmitters
- parasympathetic innervation – fibers of cranial nerves VII (facial) & IX
(glossopharyngeal) supply major salivary glands, whereas 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
- parasympathetic signals excite the enteric nervous system, leading to
enhanced motility & secretions
- sympathetic innervation – fibers of spinal nerves from T5 to L2 supply
most of the alimentary canal via prevertebral ganglia (e.g. celiac and mesenteric)
- sympathetic signals inhibit the enteric nervous system & inhibit smooth
muscle directly
- hormonal reflexes – gastrin is produced by G cells of stomach mucosa
(antrum); it triggers gastric acid secretion & cell renewal
- cholecystokinin is produced by I cells of duodenum and jejunum
mucosa; it excites gall bladder contraction, triggers release of enzyme- rich juice by the
pancreas and inhibits stomach emptying
- secretin is produced by S cells of duodenum mucosa & triggers release of
bicarbonate- rich juice by the pancreas
- gastric inhibitory peptide is produced by the duodenum and jejunum
& slows stomach emptying
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- p. 4 -
• patterns of contraction:
- propulsive – peristalsis (fig. 62 – 5 & ppt. 8) involves formation of ringlike constriction that advances slowly along the gut toward the anus, pushing
contents along (mediated by myenteric plexus)
- mixing – intermittent constrictive contractions (segmentation contractions)
produce chopping or shearing of contents (assist peristaltic mixing) (fig. 63 - 3 & ppt. 9)
3.
Circulation: (figs 62 – 6, 62 – 7 & ppts. 10 - 16)
• arterial supply – celiac trunk (from aorta) supplies liver, stomach & spleen
- superior mesenteric artery (from aorta) supplies the small intestine, pancreas
& the right half of large intestine
- inferior mesenteric artery (from aorta) supplies left half of large intestine &
rectum
- increased intestinal activity stimulates vasodilatation of arterioles supplying
active area; several hormones released by the active GI tract also promote vasodilatation
• circulation to villi: capillary circulation + central lymphatic (lacteal)
• venous drainage: all of abdominal GI tract + pancreas and spleen drain into
hepatic portal system, which delivers nutrient-rich blood directly to liver; liver
processing (storage, metabolic conversion, etc.) regulates levels of absorbed substances in
the blood