Download Digestive System Summary

Enteric Nervous System (ENS): 1) Myenteric (Auerbach) plexus & 2) Submucosal (Meissner’s) plexus à both triggered by
sensory neurons with chemo- and mechanoreceptors in the mucosal epithelium; effector motors neurons of the myenteric plexus
control contraction/motility of the GI tract, and effector motor neurons of the submucosal plexus control secretion of GI mucosa
& organs.
Although ENS neurons can function independently, they are subject to regulation by ANS.
Autonomic Nervous System (ANS):
1) parasympathetic (rest & digest) – can innervate the GI tract and form connections with ENS neurons that promote motility
and secretion, enhancing/speeding up the process of digestion
2) sympathetic (fight or flight) – can innervate the GI tract and inhibit motility & secretion by inhibiting neurons of the ENS
Sections and dimensions of the GI tract (alimentary canal):
Esophagus à ~ 10 inches
Stomach à ~ 12 inches and holds ~ 1-2 L (full) up to ~ 3-4 L (distended)
Duodenum à first 10 inches of the small intestine
Jejunum à next 3 feet of small intestine (when smooth muscle tone is lost upon death, extends to 8 feet)
Ileum à final 6 feet of small intestine (when smooth muscle tone is lost upon death, extends to 12 feet)
Large intestine à 5 feet
General Histology of the GI Tract:
4 layers – Mucosa, Submucosa, Muscularis Externa, and Serosa
Mucosa à epithelium, lamina propria (areolar connective tissue), & muscularis mucosae
Submucosa à areolar connective tissue
Muscularis externa à skeletal muscle (in select parts of the tract); smooth muscle (at least 2 layers – inner layer of circular
muscle and outer layer of longitudinal muscle; stomach has a third layer of oblique muscle under the circular layer)
Serosa à superficial layer made of areolar connective tissue and simple squamous epithelium (a.k.a. visceral peritoneum); not
found in esophagus (replaced by superficial layer known as Adventitia)
Stomach Cells (little absorptive and little digestive capabilities; enteroendocrine activity):
Cell Type/Name
Surface Mucous Cell
Neck Cell
Parietal Cell
Chief Cell
G Cell
D Cell
Location
Surface epithelium
Epithelium lining gastric pit
Epithelium of gastric gland
Epithelium of gastric gland
Epithelium of gastric gland
Epithelium of gastric gland
Function
Secretes mucus
Secretes mucus (exocrine)
Secretes HCl & intrinsic factor (exocrine)
Secretes pepsinogen & gastric lipase (exocrine)
Secretes hormone gastrin (endocrine)
Secretes hormone somatostatin (endocrine)
Small Intestine Cells (high absorptive and high chemical/enzymatic digestive capabilities; enteroendocrine activity):
Cell Type/Name
Absorptive Cell
Goblet Cell
S Cell
CCK Cell
K Cell
Paneth Cell
Location
Surface epithelium
Surface epithelium
Epithelium of intestinal gland
Epithelium of intestinal gland
Epithelium of intestinal gland
Epithelium of intestinal gland
Function
Brush-border digestive enzymes & absorbs nutrients
Secretes mucus (exocrine)
Secretes hormone secretin (endocrine)
Secretes hormone cholecystokinin (endocrine)
Secretes glucose-dependent insulinotropic peptide (endo)
Secretes lysozyme (bactericidal activity)
Large Intestine Cells* (moderate absorptive capabilities; no enteroendocrine activity):
Cell Type/Name
Location
Absorptive Cell Surface epithelium
& Epithelium of intestinal gland
Goblet Cell
Epithelium of intestinal gland
Function
Absorbs primarily water; no enzymes secreted
Secretes mucus (exocrine)
* - Chemical digestion (fermentation and amino acid conversion) is carried out by bacteria that inhabit the lumen; bacteria also
synthesize vitamins (B’s and K) that are absorbed
Liver Cells (form clusters of exocrine glandular epithelium called acini)
Cell Type/Name
Hepatocyte
Kupffer Cell*
Location
Hepatic laminae
Hepatic sinusoid
Function
Secretes bile (exocrine) that is stored in the gallbladder
Phagocytic breakdown of worn out blood cells & bacteria
* - also go by the name of hepatic macrophages or stellate reticuloendothelial cells; breakdown hemoglobin from RBCs into
heme and globin components à globin is broken into amino acids that are recycled for use in new protein synthesis, and heme is
further processed to remove Fe (for storage in the liver and eventual transfer to red bone marrow as new RBCs are generated
during erythropoiesis) from heme ring which is then converted to bilirubin (bile component) and ultimately converted from
bilirubin to stercobilin by bacteria in the large intestines (gives feces its pigmentation).
Pancreas Cells (form clusters of exocrine glandular epithelium called acini)
Cell Type/Name
Pancreatic Cell
Location
Pancreatic acini
Function
Secretes pancreatic juice (containing NaHCO3 and digestive enzymes)
Digestive Enzymes (see Table 24.5)
Secreted Enzyme
Salivary amylase
Lingual lipase
Pepsin*
Gastric Lipase
Pancreatic amylase
Trypsin
Chymotrypsin
Elastase
Carboxypeptidase
Pancreatic lipase
Nucleases
Brush-border Enzyme
α-Dextrinase
Maltase
Sucrase
Lactase
Enterokinase
Aminopeptidase
Dipeptidase
Nucleosidase
Phosphatase
Origin
Salivary glands
Lingual glands (tongue)
Gastric glands (chief cells)
Gastric glands (chief cells)
Pancreatic acini
Pancreatic acini
Pancreatic acini
Pancreatic acini
Pancreatic acini
Pancreatic acini
Pancreatic acini
Origin
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Function
Chemical digestion of polysaccharides (starches & glycogen)
Chemical digestion of lipids (triglycerides)
Chemical digestion of protein (* - pepsinogen becomes pepsin in acid)
Chemical digestion of lipids (triglycerides)
Chemical digestion of polysaccharides (starches & glycogen)
Chemical digestion of proteins
Chemical digestion of proteins
Chemical digestion of proteins
Chemical digestion of peptides
Chemical digestion of lipids (triglycerides)
Chemical digestion of nucleic acids (RNA & DNA)
Function
Chemical digestion of starch & glycogen fragments
Chemical digestion of maltose
Chemical digestion of sucrose
Chemical digestion of lactose
Chemical activation of trypsinogen to trypsin
Chemical digestion of peptides
Chemical digestion of dipeptides
Chemical digestion of nucleotides
Chemical digestion of nucleotides
Digestive Hormones (see Table 24.8)
Hormone
Origin
Gastrin
G cells (stomach)
Somatostatin
D cells (stomach)
Secretin
S cells (small intestine)
Cholecystokinin (CCK)
CCK cells (s. intestine)
Glucose-dependent Insulinotropic Peptide (GIP) K cells (s. intestine)
Function
Promotes gastric juice secretion
Inhibits release of gastrin
Promotes pancreatic juice secretion
Promotes pancreatic juice secretion & bile release
Stimulates beta cells of pancreas to release insulin