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Physiology of Digestive System
Lecture 23 Part 2
Terms in this set (53)
Digestive activities of GI tract occur in what 3 overlapping phases?
1) The cephalic phase
2) The gastric phase
3) The intestinal phase
smell, sight, thought, or initial taste of food activates neural centers in cerebral cortex, hypothalamus, and brain stem to prepare for digestion
what stimulates secretion of saliva?
what relays info to stimulate secretion of gastric juice?
vagus nerve (CN X)
- begins when food reaches stomach
- Neural and hormonal mechanisms promote secretion of gastric juice and increase gastric motility.
- begins once food enters the small intestine
- Neural and hormonal responses promote the continued digestion of foods that have reached the small intestine.
oral (buccal) cavity
- formed by cheeks, hard and soft palates, and tongue.
- Mechanical digestion of food via mastication
- enables it to be mixed w/ saliva to form a bolus that can be easily swallowed
what initiates chemical digestion?
Enzymes present in saliva
a digestive enzyme that acts on starch and glycogen
a digestive enzyme that acts on lipids - fats
what else does saliva functions in?
- Lubricate food for esophageal propulsion
- Prevent bacterial growth
- Prevent desiccation (drying out) of oral cavity
how is salivary regulated?
under the control of ANS
how does Parasympathetic stimulation regulate saliva?
- Promotes secretion of a moderate amount of saliva
- Salivary centers are located in brain stem and efferent nerve impulses are transmitted by facial (VII) and glossopharyngeal (IX) nerves
- Touch (pressoreceptors), smell, taste (taste buds), and psychological factors are also salivary stimulators.
how does sympathetic stimulation regulate saliva?
increase saliva secretions but product is thicker and more viscous
- Used to cut, tear, and mash food in mouth
- located in sockets of alveolar processes of mandible and maxillae.
- line the teeth sockets
- A dense fibrous CT that anchors teeth to socket walls
- acts as a shock absorber during chewing.
- composed of skeletal muscle under voluntary somatic motor control
- forces bolus into position for swallowing (deglutition) and places bolus in contact w/ teeth for chewing.
- act of swallowing food
- involves many muscles that are subject to a number of voluntary and involuntary controls
what has digestive as well as respiratory functions?
oropharynx and laryngopharynx
- Swallowed food must transit them both on the way to esophagus
- It must do so while not going into nasal cavity or airway.
Deglutition has what 3 stages?
- Voluntary (buccal)
- 1st stage of Deglutition
- voluntary act of tongue forcing bolus to back of oral cavity and into oropharynx
- 2nd stage of Deglutition
- begins as bolus passes into oropharynx
what happens during the pharyngeal stage?
- Receptors send impulses to deglutition center in medulla and pons.
- Returning impulses cause contractions of muscle necessary to move bolus toward esophagus
- soft palate moves superiorly and posteriorly to close nasopharynx.
- epiglottis moves slightly inferiorly to close glottis
- begins once bolus enters esophagus
progression of coordinated contractions and relaxations of circular and longitudinal layers of muscularis, push bolus down toward stomach.
- function: propulsion
- a muscular tube that begins inferior to laryngopharynx, and positioned posterior to trachea.
- Leaving the neck, it traverses the thoracic cavity down the posterior mediastinum before piercing the diaphragm thru the esophageal hiatus.
What epithelial tissues line the mucosa of the esophagus?
nonkeratinzed stratified squamous epithelium
how does the type of muscle in muscularis externa of esophagus vary by region?
- superior 1/3 is skeletal muscle
- intermediate 1/3 is skeletal and smooth muscle
- inferior 1/3 is smooth muscle
Upper esophageal sphincter (UES)
- aka pharyngoesophageal sphincter
- Refers to superior portion of esophagus.
- consists of striated muscle; However, not under conscious control.
- Opening is triggered by swallow reflex.
Lower esophageal sphincter (LES), or cardiac sphincter)
- at inferior portion of esophagus
- regulates mvmt of food from esophagus into stomach.
- Disfunction or damage to it may result in Gastroesophageal Reflux Disease (GERD), which manifests as chronic "heart burn"
protrusion (or herniation) of upper part of stomach into thorax viaa tear or weakness in hiatus of diaphragm.
- ~60% of people over age of 50 have a hiatal hernia
- Only ~9% are actually symptomatic
what are the main types of Hiatal hernia?
- mix of two
Sliding type of Hiatal hernia
- most common (95%)
- gastroesophageal junction moves above diaphragm together w/ some of stomach.
Rolling (or paraesophageal) type of Hiatal hernia
- accounts for the remaining 5%
- part of stomach herniates thru the esophageal hiatus and lies beside the esophagus, w/o mvmt of gastroesophageal junction.
J-shaped enlargement of GI tract situated btwn esophagus and first part of small intestine (duodenum).
- b/c a meal can be eaten much more quickly than the intestines can digest and absorb it, one of the functions of stomach is to
serve as a mixing chamber and holding reservoir
how does position and size of stomach continually vary?
the diaphragm pushes it inferiorly w/ each inhalation and pulls it superiorly w/ each exhalation
- large folds in mucosa of empty stomach
- enable gastric distension (stretching) as it accommodates food
how does size of stomach vary?
- Empty, stomach is ~ size of a large sausage
- but it is the most distensible (stretchable) part of GI tract and can expand to accommodate a large quantity of food.
As a functional adaptation, the gastric muscularis externa contains what?
additional 3rd inner oblique layer of muscle to facilitate the churning (mixing) action of mechanical digestion.
- This aids in mixing contents w/ gastric juice allowing greater available surface area for gastric enzymatic activity (chemical digestion)
stomach has what 4 main regions?
Simple columnar epithelial cells
- line mucosal epithelial surface
- secrete a protective mucous layer in addition to various other substances.
Columns of secretory cells extend down into the lamina propria forming "these"
Several gastric glands open into narrow channels
what populates the gastric glands and pits?
A variety of specialized exocrine and endocrine cell types
mucous neck cells
produce alkaline mucus
Enteroendocrine G cells
- located mainly in pyloric antrum
- secrete the hormone gastrin into bloodstream.
secrete the protease pepsinogen and gastric lipase
produce intrinsic factor and HCl (hydrochloric acid)
- strongly acidic nature of "this" kills many microbes, partially denatures (disrupts the 3D shape) proteins in food, and converts pepsinogen into pepsin.
- contents: pepsin, gastric lipase, intrinsic factor (IF)
Why is the alkaline mucous layer important in the stomach?
only active proteolytic (breaks proteins) enzyme in the stomach.
Breaks down triglycerides (lipids) into fatty acids, monoglycerides, and glycerol.
Intrinsic factor (IF)
- needed for absorption of vit B12 in terminal ileum.
- Vit B12 is needed for RBC production.
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