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Chapter 25

BISC 3221 Chapter Notes - Chapter 25: Sternocleidomastoid Muscle, Chyme, Vagus Nerve

Biological Sciences
Course Code
BISC 3221
Craig Frank

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Chapter 25: The Digestive System
Digestive system = digestive tract + accessory organs
o Tract: oral cavity + esophagus + stomach + small intestine + large intestine
o Accessory: teeth + tongue + salivary glands + liver + pancreas
o Ingestion: food and liquids enter digestive tract at mouth
o Mechanical processing: squashing with tongue, crushing with teeth, movement in
o Digestion: chemical and enzymatic breakdown of sugars/lipids/proteins into smaller
o Secretion: secrete acids, enzymes, buffers by tract or accessory organs
o Absorption: movement of organic molecules, electrolytes, vitamins, water across
digestive epithelium and into interstitial fluid
o Excretion: waste products secreted into digestive tracts
o Compaction: dehydration of indigestible materials/wastes before defecation of feces
Lining of tract protects against corrosive effects of acids/enzymes, mechanical stresses, pathogens
swallowed/living in digestive tract
25.1 An Overview of the Digestive System
Histological Organization of the Digestive Tract
Major layers: mucosa, submucosa, muscularis externa, serosa
Variations are related to specific organ/region
The Mucosa
o Inner lining, example of mucous membrane; loose connective tissue covered by
epithelium moistened by secretions
o Mucosal epithelium can be stratified (mouth) or simple (small intestine) (depends on
o Mucosa of tract is organized in folds (plicae) that increase surface area for absorption; in
some areas they are permanent, in others they disappear as lumen fills
o Lamina propria: areolar tissue under epithelium; contains blood vessels, sensory nerve
endings, lymphatic vessels, smooth muscle fibers, areas of lymphatic tissue (part of
o Muscularis Mucosae
Band of smooth muscle and elastic fibers external to lamina propria
Inner circular layer, outer longitudinal layer
The Submucosa
o Dense, irregular CT outside muscularis mucosae
o Contains large blood vessels and lymphatics, exocrine glands that secrete
o Submucosal plexus: network of nerve fibers/neuron bodies along outer margin;
innervates mucosa; contains sensory neurons, parasymp. ganglia, symp. ganglia
The Muscularis Externa
o Surround submucosa, dominated by smooth muscle; inner circular, outer longitudinal
o Plays important role in mechanical processing, moving material through tract

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o Myenteric plexus: network of parasymp. ganglia and symp. postganglionic fibers
between 2 muscle layers; coordinate movements of 2 layers
o Forms sphincters at certain points along tracts; thickened areas of circular smooth muscle
The Serosa
o Outer serous membrane
o NONE at pharynx, esophagus, rectum; instead adventitia (thick, fibrous sheet)
Muscularis Layers and the Movement of Digestive Materials
Smooth muscle, nonstriated, involuntary, surrounded by CT, most have no motor innervation,
arranged in sheets, gap junctions electrically connect adjacent cells; when on contracts, the
contraction spreads like a wave throughout tissue
Can contract in response to neurons, chemicals, hormones, concentrations of O2 or CO2, physical
factors (stretching/irritation)
Plasticity: ability for muscle cell to adapt to stretched length and retain ability to contract on
Pacemaker cells: undergo spontaneous depolarization, causing contractions that lead to peristalsis
and segmentation; maintains rhythmic cycles in smooth muscle
o Waves of muscular contractions that move a bolus along length of tract
o Peristaltic wave: circular muscles contract behind digestive contents, then longitudinal
muscles contract, shortening adjacent segments, then circular contraction pushes bolus
o Occurs in small intestine and large intestine
o Movements churn and fragment digestive materials, don’t produce net movement in a
Segmentation and peristalsis can be triggered by pacemaker cells, hormones, chemicals, physical
Peristalsis can be triggered by afferent and efferent fibers in glossopharyngeal, vagus, pelvic
Myenteric reflexes: short reflexes that don’t involve CNS; can control local contractions
Long reflexes: involve CNS interneurons and motor neurons; control large scale peristaltic waves
that move stuff from one region of tract to another
The Peritoneum
Visceral peritoneum (serosa) is continuous with parietal peritoneum (lines inner body wall)
Intraperitoneal organs: in peritoneal cavity, covered on all sides by visceral peritoneum (stomach,
liver, ileum)
Retroperitoneal organs: covered by visceral peritoneum on anterior surface, organ lies outside
peritoneal cavity (kidneys, ureters, abdominal aorta)
Secondary retroperitoneal organs: form as intraperitoneal but become retroperitoneal (pancreas,
Peritoneum produces peritoneal fluid that lubricate surfaces
o Fused double sheets of peritoneal membrane
o Stabilize relative positions of organs, prevent entanglement of intestines
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