D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
DIGESTIVE SYSTEM
Lecture 23: Motility & Secretion (continued)
• small intestine: (ppt. 1)
- motility: mixing and propulsive contractions; segmentation ‘chops’
chyme via kneading action (fig. 63 – 3 & ppt. 2); peristalsis moves contents toward
ileocecal valve and spreads chyme uniformly through small intestine (fig. 62 - 5 &
ppt. 3)
- contractions are stimulated by myenteric nerve net, essential for normal
contractile strength, e.g., from stomach via gastroenteric reflex
- hormonal reflexes are not clearly understood – some enhance motility,
some inhibit motility
- ileocecal valve & sphincter (fig. 63 – 4 & ppt. 4) – thickening of circular
muscle extending along last several centimeters of ileum, leading to cecum
- persistent state of constriction delays or arrests emptying into cecum;
distension or irritation of cecum strongly stimulates contractions of sphincter &
valve closed by cecal pressure, thus preventing emptying of ileum or reflux of cecal
contents into ileum; peristalsis & distension of ileum relaxes sphincter
- secretions of small intestine:
- enzymes that complete process of chemical digestion and promote
absorption of various foodstuffs, e.g., disaccharidases for completion of
carbohydrate digestion; peptidases for completion of protein conversion to amino
acids; lipases for completion of triglyceride breakdown
- serous (watery) secretions & mucous secretions that provide generous
aqueous milieu for action of enzymes secreted by pancreas, e. g., product resembling
interstitial fluid from crypts of Lieberkuhn, alkaline mucus from Brunner’s glands
of submucosa & mucus from goblet cells (ppt. 5)
- secretory activity regulated by local submucous plexus of enteric
nervous system
Bio 659 lec. 23
- p. 2 -
• liver & pancreas
- pancreatic secretions (fig. 64 – 10 & ppts. 6 - 8):
- bicarbonate-rich juice stimulated by secretin in response to acidic chyme
- enzyme-rich juice stimulated by cholecystokinin (response to arrival of
protein-rich &/or fat-rich chyme from stomach)
- enzymes of protein digestion – trypsinogen (activated by enterokinase
from intestinal mucosa), chymotrypsinogen (activated by trypsin) and procarboxypeptidase
(activated by trypsin); trypsin & chymotrypsin convert proteins to shorter polypeptides and
dipeptides; carboxypeptidase converts proteins to amino acids (ppt. 9)
- pancreatic amylase converts complex carbohydrates to maltose (a
dissacharide)
- lipases, including phospholipases, break down triglycerides and
phosphoglycerides, respectively; cholesterol esterases degrade esters of cholesterol
- neural reflex stimulation of pancreatic secretion involves cephalic,
gastric & intestinal phases similar to reflexes for stimulation of gastric secretion
- hepatic secretion: bile, stored and concentrated in gall bladder
- glandular parenchyma of liver organized into lobules (ppt. 10)
- bile release (gall bladder contractions) stimulated by cholecystokinin +
some stimulation by parasympathetic n.s. (fig. 64 – 11 & ppts. 11 & 12)
- bile contains bile salts (table 64 – 2 & ppt. 13) that emulsify fats,
facilitating digestion by lipases and promoting absorption; bile also contains bile pigments
(e.g. bilirubin), which are excreted
- bile salts are reabsorbed, returned to liver and resecreted in bile
(enterohepatic circulation)
Bio 659 lec. 23
- p. 3 -
• large intestine (fig. 63 – 5 & ppt. 14)
- motility & secretion – mixing contractions (haustral contractions) involve
very thick circular muscle and sparse longitudinal muscle (tenia coli) that create pouches
(haustrations) in wall of colon; kneading action mixes mucus (secreted in generous
amount by colonic mucosa) with undigested roughage, dead bacteria, sloughed
epithelial cells and pigments to form feces
- propulsive contractions consist of powerful peristalsis (mass movements)
occurring 3 – 4 times per day
- arrival of feces in rectum triggers defecation reflex (contraction of rectum &
relaxation of anal sphincters) to expel feces; regulated by myenteric plexus of rectal
wall (response to distension) and enhanced by pelvic nerves (parasympathetic
fibers) (fig. 63 - 6 & ppts. 15 & 16)