Neuron Structure and FunctionCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PowerPoint® Lecture Slides prepared by Stephen Gehnrich, Salisbury University
11 C H A P T E R
Digestion
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Alimentary systems fall into one of three categories

Coelenteron
Composition of food varies according to its position
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Gut Formation
Gut is derived from endoderm Three regions
Foregut Esophagus, stomach, and the anterior section of the duodenum Forms buds that become the pancreas and liver
Midgut Posterior part of duodenum, jejunum, ileum, and large intestine
Hindgut Colon and rectum
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Gastrointestinal Tract
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Figure 11.22
Stomach
Tight junctions Prevent leakage across epithelium
Mucous neck cells Secrete mucus
Parietal cells () Secrete hydrochloric acid (HCl)
Chief cells () Secrete the protease pepsin
Enteroendrocrine cells Secrete hormones into the blood
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gastric secretions
Parietal cell
The secretion of HCl by gastric parietal cells employs primary active transport of H produced from the breakdown of H2CO3 and electrochemical transport of Cl acquired in exchange for HCO3-
The powerful proteolytic enzyme pepsin is secreted in an inactive form (pepsinogen), which is then activated by HCl.
H/K-ATPase
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digastric
Digestive enzyme

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Intestines
Most nutrients are absorbed in the intestines Four main layers
Mucosa Submucosa
Blood and lymphatic vessels, nerves Circular smooth muscle Longitudinal smooth muscle
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Intestinal epithelium 1
1mm
Lumen of gut Mucosa (epithelium) Submucosa (connective tissue) Circular muscle Longitudinal muscle Serosa (connective tissue)
(Nervous plexus)
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Villi of the Intestine
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Motility of the alimentary canal: Propulsion of food along the alimentary canal Mix food with digestive juices Renewal of material in contact with the absorbing and secreting surface
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--- Intrinsic control
Local influences: Stretch (food) Chemical (substance in chyme)
Control of motility Intrinsic control Extrinsic control
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Extrinsic control
Inhibitory (norepinephrine)
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Intrinsic pacemaker activity Neuronal and endocrine control
Different rates of contractile activity in different regions
Slow powerful Churning action
Frequent shorter Kneading, transporting
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Gastrointestinal secretions
Exocrine tissues
Intestine epithelium
Acinar lumen
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Exocrine Secretions into the Intestine
Bile Solution of digestive chemicals and liver waste products Produced in the liver and stored in the gallbladder Phospholipids
Aid in the uptake of lipids
Bile salts Emulsify fats
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Digestive Enzymes
Enzymes convert macromolecules to forms that can be absorbed and processed
Lipases Break down triglycerides and phospholipids into fatty acids
Proteases Break down proteins into shorter polypeptides
Amylases Break down polysaccharides into oligosaccharides
Nucleases Break down DNA into nucleotides
Most digestion takes place extracellularly For example, in the lumen of the GI tract
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Exocrine secretions of the alimentary canal
Mixtures of water, ions, mucus, enzyme
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Activation of Proenzymes
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Regulating Feeding and Digestion
Nerve signals From central nervous system (CNS) Within the gastrointestinal tract
Hormone signals Wide variety of endocrine tissues and hormones
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Hormonal Control of Digestion
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Control of Appetite
Three hormones control appetite by binding to receptors in the hypothalamus
Leptin Secreted by white adipose tissue when lipid content is high Suppresses appetite
Ghrelin Secreted by stomach when empty Stimulates appetite
Peptide YY Secreted by colon when full Suppresses appetite
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Control of Appetite
Hypothalamic neurons release neurotransmitters in response to hormones from the gut
Some neurotransmitters stimulate appetite Neuropeptide Y (NPY) Agouti-related peptide Gamma amino butyric acid (GABA)
Some neurotransmitters inhibit appetite Proopiomelanocortin (POMC)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Figure 11.26
Hormonal Control of Appetite
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Cephalic phase
The present of food Chemoreceptor, mechanoreceptor Gastrin (endocrine cell) muscle, HCl+pepsin
Vasoactive intestinal peptide (VIP) Gastric inhibitory peptide (GIP)
Fats, sugar in duodenum GIP (endocrine cell)
Gastric secretion in mammals occurs in three distinct phases
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Gastrointestinal hormones influence the secretory and mechanical activity of the digestic tract
CCK
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Control of Gastric Secretions
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Control of Intestinal Secretions
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Neurohormonal Effectors of GI Motility
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Nutrient Transport Across Plasma Membranes
Some nutrients are transported by protein carriers Polar molecules require specific protein transporters Transport down a concentration gradient
Facilitated diffusion Transport against a concentration gradient
Active transport Via Na+-dependent cotransporters
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Nutrient Transport Across Plasma Membranes
Some nutrients are transported in vesicles Uptake
Pinocytosis Nutrients in solution
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Carbohydrate Breakdown
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Carbohydrate Transport
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Proteins
Amino acids are absorbed by epithelial cells Amino acid-Na+ cotransporters
Some proteins may be transported across the epithelial cells via endocytosis/exocytosis
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Proteins
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Lipids
Digestion and import of lipids is complicated by their hydrophobicity
GI tract secretes bile that emulsifies lipids into small droplets (micelles) Dietary fats are broken down into fatty acids and monoglycerides Lipids diffuse across cell membrane into enterocyte
Transport of lipids depend on physical properties Short chain fatty acids and glycerol in blood Triglycerides in the lymph as chylomicrons
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Special handling of lipids
Emulsion droplets
Pancreatic lipase
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Lipoproteins in the Blood
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Chylomicrons and Lipoproteins in the Blood
Digestion
Digestive Enzymes
19
Carbohydrate Breakdown
Carbohydrate Transport