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The Digestive System

This article provides an overview of the histology of the alimentary canal wall, including the layers and components of the mucosa, submucosa, muscularis externa, and serosa. It also covers the smooth muscle structure and contraction, innervation of smooth muscle, nerve plexuses, and the peritoneal cavity and peritoneum. Additionally, it includes information on the mouth and associated organs, the tongue, teeth, salivary glands, pharynx, and esophagus.

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The Digestive System

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  1. PART 2 The Digestive System

  2. Histology of the Alimentary Canal Wall • Same four layers from esophagus to anus • The mucosa – innermost layer • Consists of • Epithelium • Lamina propria • Muscularis mucosae • The submucosa – external to the mucosa • Contains blood and lymphatic vessels, nerve fibers

  3. Histology of the Alimentary Canal Wall • The muscularis externa – external to the submucosa • Two layers • Circular muscularis – inner layer • Longitudinal muscularis – outer layer • The serosa – the outermost layer • Is the visceral peritoneum

  4. Histology of the Alimentary Canal Figure 22.5

  5. Smooth Muscle • Primarily found in walls of viscera • Fibers elongated • Have one centrally-located nucleus • Grouped into sheets • Longitudinal layer – parallel to long axis of organ • Circular layer – deeper layer, fibers run around circumference of organ

  6. Smooth Muscle Figure 22.6a, b

  7. Smooth Muscle Contraction • Myofilaments operate by interaction with cytoskeleton • Dense bodies – correspond to Z-discs of skeletal muscle Figure 22.7a–c

  8. Innervation of Smooth Muscle • Innervated by ANS • Few fibers per sheet innervated • Sheet of smooth muscle contracts as a unit • Exceptions • Iris of eye • Arrector pili muscles in skin

  9. Innervation of Smooth Muscle Figure 22.8

  10. Nerve Plexuses • Myenteric nerve plexus • Lies between circular and longitudinal muscularis • Controls peristalsis and segmentation • Submucosal nerve plexus • Lies in submucosa • Signals glands to secrete • Innervation • Sympathetic and parasympathetic motor fibers • Visceral sensory fibers

  11. The Peritoneal Cavity and Peritoneum • Peritoneum – a serous membrane • Visceral peritoneum – surrounds digestive organs • Parietal peritoneum – lines the body wall • Peritoneal cavity – a slit-like potential space

  12. The Peritoneal Cavity and Peritoneum • Mesentery – a double layer of peritoneum • Holds organs in place • Sites of fat storage • Provides a route for circulatory vessels and nerves Figure 22.9a

  13. The Peritoneal Cavity and Peritoneum • Retroperitoneal organs • Behind the peritoneum • Peritoneal organs • Digestive organs that keep their mesentery Figure 22.9b

  14. Mesenteries • Superficial view of abdominal organs Figure 22.10a

  15. Mesenteries • Lesser omentum attaches to lesser curvature of stomach Figure 22.10b

  16. Mesenteries • Greater omentum – a “fatty apron” of peritoneum • Greater omentum and transverse colon reflected Figure 22.10c

  17. Mesenteries • Sagittal section through the abdominopelvic cavity • Mesenteries attach to posterior abdominal wall Figure 22.10d

  18. Secondarily Retroperitoneal Organs • Initially formed within peritoneum • Become retroperitoneal • Fuse to posterior abdominal wall Figure 22.11

  19. Summary of Intraperitoneal and Secondarily Retroperitoneal Organs Table 22.1

  20. The Mouth and Associated Organs • The mouth – oral cavity • Mucosal layer • Stratified squamous epithelium • Lamina propria • The lips and cheeks • Formed from orbicularis oris and buccinator muscles, respectively

  21. Anatomy of the Mouth Figure 22.12a

  22. Anatomy of the Mouth • The labial frenulum • Connects lips to gum • The palate • Forms the roof of the mouth Figure 22.12b

  23. The Tongue • Interlacing fascicles of skeletal muscle • Grips food and repositions it • Helps form some consonants • Intrinsic muscles – within the tongue • Extrinsic muscles – external to the tongue • Lingual frenulum • Secures tongue to floor of mouth

  24. The Superior Surface of the Tongue • Tongue papillae • Filiform papillae – no taste buds • Fungiform papillae • Circumvallate papillae • Sulcus terminalis • Marks border between mouth and pharynx • Posterior 1/3 of tongue lies in oropharynx • Lined with lingual tonsil

  25. Superior Surface of the Tongue Figure 22.13

  26. The Teeth • Deciduous teeth – 20 teeth • First appear at 6 months of age • Permanent teeth – 32 teeth • Most erupt by the end of adolescence • Dental formula – shorthand • Way to indicate number and position of teeth • 2I, 1C, 2P, 3M

  27. The Teeth Figure 22.14

  28. Tooth Structure • Longitudinal section of tooth in alveolus Figure 22.15

  29. The Salivary Glands • Produce saliva • Compound tubuloalveolar glands • Parotid glands • Parotid duct – parallel to zygomatic arch • Contains only serous cells • Submandibular glands • Lies along medial surface of mandible • Sublingual glands • Lies in floor of oral cavity • Contains primarily mucous cells

  30. The Salivary Glands Figure 22.16

  31. The Pharynx • Oropharynx and laryngopharynx • Passages for air and food • Lined with stratified squamous epithelium • External muscle layer • Consists of superior, middle, and inferior pharyngeal constrictors

  32. The Esophagus • Gross anatomy – muscular tube • Begins as a continuation of the pharynx • Joins the stomach inferior to the diaphragm • Cardiac sphincter – closes lumen to prevent stomach acid from entering esophagus

  33. The Esophagus • Microscopic anatomy • Epithelium is stratified squamous epithelium • When empty – mucosa and submucosa in longitudinal folds • Mucous glands – primarily compound tubuloalveolar glands • Muscularis externa • Skeletal muscle first third of length • Adventitia – most external layer

  34. Microscopic Structure of the Esophagus Figure 22.17a, b

  35. The Stomach • Site where food is churned into chyme • Secretion of pepsin begins protein digestion • Functions under acidic conditions • Food remains in stomach approximately 4 hours

  36. The Stomach Figure 22.18a

  37. The Stomach Figure 22.18b

  38. Microscopic Anatomy of the Stomach • Muscularis has three layers • Circular and longitudinal layers and oblique layer • Epithelium is simple columnar epithelium • Mucosa dotted with gastric pits • Gastric glands – deep to gastric pits

  39. Microscopic Anatomy of the Stomach • Gastric glands of fundus and body • Mucous neck cells • Secrete a special mucus • Parietal (oxyntic) cells • Secrete hydrochloric acid and gastric intrinsic factor • Chief (zymogenic) cells • Secrete pepsinogen

  40. The Stomach – Microscopic Anatomy Figure 22.19a–d

  41. The Small Intestine – Gross Anatomy • Longest portion of the alimentary canal • Site of most enzymatic digestion and absorption • Three subdivisions • Duodenum • Jejunum • Ileum

  42. The Duodenum • Receives digestive enzymes and bile • Main pancreatic duct and common bile duct enter duodenum • Sphincters control entry of bile and pancreatic juices

  43. The Duodenum and Related Organs Figure 22.20

  44. The Small Intestine – Microscopic Anatomy • Modifications for absorption • Circular folds (plicae circulares) • Transverse ridges of mucosa and submucosa • Villi • Finger-like projections of the mucosa • Covered with simple columnar epithelium • Microvilli • Further increase surface area for absorption

  45. Histology of the Intestinal Wall • Absorptive cells • Uptake digested nutrients • Goblet cells • Secrete mucus that lubricates chyme • Enteroendocrine cells • Secrete hormones • Intestinal crypts • Epithelial cells secrete intestinal juice

  46. The Small Intestine – Structural Features Figure 22.21a–d

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