Upper digestive system

From mouth to stomach

Oral cavity (buccal cavity) → functions

• Functions:

  • Sensory analysis: taste, temperature, texture, touch

  • Mechanical processing: teeth, tongue, palate

  • Lubrication: mucous and saliva → helps with movement through alimentary canal

  • Chemical digestion (limited): salivary enzymes

Teeth and mastication → types of teeth and mastication

• Primary and secondary dentation

• Types of teeth:

  • Incisors (8): blades; clipping/cutting

  • Cuspids (4): cones; tearing/slashing

  • Bicuspids (8): bi-pronged; crush/grind/mash

  • Molars (12): flattened: crush and grind

• Mastication:

  • Breaks down tough connective tissues

  • Mixes food with saliva

  • Regulated by muscles of the jaw

  • Up/down and side to side

Salivary glands

• 2 parotid glands → largest salivary glands (production of amylase, an enzyme that breaks down carbohydrates, an example of chemical digestion)

• 2 sublingual glands (below tongue) → buffers and lubricant - mucin to mucous

• 2 submandibular glands → buffers, mucin, amylase

Lysozymes

• Innate immunity: antimicrobial, break down peptidoglycans (protein sugar coat on bacteria)

• Highest level in sublingual saliva

What are salivary glands regulated by

• ANS

  • Salivatory nuclei in the medulla oblongata

  • Each gland has PNS and SNS innervation

    • PNS activation leads to increase in saliva production

    • SNS activation - unclear roles, small amounts of very thick, less watery saliva

Salivatory nuclei influenced by…

• Cranial nerve sensory afferents

  • Any oral stimuli - increased production

• Higher centers and other brainstem nuclei

  • Smelling food, chewing motions, thinking about food, nausea

Esophagus

• How long

• Upper 1/3 is what type of muscle

• Carries…

• Muscular tube: 10 inches long; 0.8 inches in diameter

• Upper 1/3 of the esophagus → striated muscle

• Carries bolus from pharynx to stomach, has to pass through diaphragm

  • Bolus enters the peritoneum via the esophageal hiatus (an opening in the diaphragm)

Phases of swallowing (a reflex) - overview

• Oral (buccal phase): voluntary

• Pharyngeal phase: involuntary

• Esophageal phase: involuntary

Oral (buccal phase)

• Tongue against

• Bolus pushes towards

• Bolus enters… and stimulates

• Voluntary

• Tongue against hard palate - bolus pushed towards pharynx

• Bolus enters oropharynx and stimulates touch receptors

Pharyngeal phase

• Whats inhibited

• Activation of touch receptors send…

  • Closure of

  • What raises

  • Relaxation of

  • Contraction of

• Uvula

• Involuntary

• Respiration inhibited

• Activation of touch receptors sends afferent signals to swallowing center (medulla oblongata) to coordinate the following:

  • Closure of entrance to nasopharynx

  • Larynx raises and epiglottis classes off entrance to the trachea

  • Relaxation of UES → skeletal but involuntary

  • Contraction of pharyngeal muscles to move bolus through UES

• Uvula → flap to close nasopharynx

Esophageal phase

• Entrances of bolus into esophagus triggers…

• Bolus pushed against

• Involuntary

• Entrance of bolus into esophagus triggers primary peristaltic wave then pushes bolus towards stomach

• Bolus pushed against LES which triggers opening of LES

Gross anatomy of the stomach

• Cardia: near esophageal entrance; mucus cells

• Fundus: superior portion (rounded end)

• Body: major portion of stomach; mixing tank

• Pylorus: area adjacent to duodenum; mucus and hormones

Inner surface and extra muscle layers of stomach

• What do they assist with

• List them

• Inner surface has folds called rugae (when empty)

  • Can expand up to 50x

  • Empty → appears as a tube

  • Full → 1.5 L

• Extra muscle layers

  • Strengthen muscle wall, assist in secretion and mixing

  • Muscularis mucosae → Extra outer circular layer

    • Circular

    • Longitudinal

    • Circular (extra)

  • Muscularis externa → extra internal oblique layer

    • Circular

    • Oblique (extra)

    • Longitudinal

Histology of the stomach

• Simple columnar cells (mucus surface cells): life expectancy only 3 to 7 days (because they live in HCl)

  • Abundance of mucus cells

• Gastric pits: shallow depressions in surface of stomach lining

  • Basal cells at the base (neck) continually divide to replace damaged epithelial cells

• Gastric glands: glands extending off the neck of gastric pit (each pit communicates with several glands)

  • Houses important secretory cells

Gastric glands

• Parietal cell

• Chief cell

• Enteroendocrine cells (primary in pylorus)

  • G and D cells

Parietal cell

• Secrete

  • HCl - maintain balance of stomach pH 1.5-2.0

  • Intrinsic factor - absorption of vitamin B12 (how we get vitamin B12)

Chief cell

• Secretes pepsinogen

  • Inactive precursor of pepsin

  • Pepsinogen is cleaved into pepsin in low pH

  • Pepsin is a proteolytic enzyme that BEGINS the process of protein digestion

Enteroendocrine cells

• Primary in pylorus

• G cells: secrete the hormone gastrin → increases in stomach activity → on plexus → stimulating secretion and motility

  • Stimulates secretion of parietal and chief cells

  • Muscle contraction leading to mixing

• D cells: secrete the hormone somatostatin (growth hormone inhibiting hormone)

  • Inhibit the release of gastrin

Acid secretion by the stomach

• 1) hydrogen ions generated inside a parietal cell as carbonic anhydrase converts CO2 with H2O to carbonic acid (H2CO3) which then dissociates

• 2) counter-transport mechanism ejects the bicarbonate ions into the interstitial fluid and imports chloride ions into the cell

• 3) chloride ions then diffuse across the cell and exit through open chloride channels into the lumen of the gastric gland

• 4) hydrogen ions are actively transported into the lumen of the gastric gland

Function of gastric acid

• Destroys microorganisms

• Denatures proteins and inactivates any enzymes present in the food

• Breaks down plant cell walls - cellulose and connective tissues in meat

• Conversion of pepsinogen to pepsin

Phases of gastric secretion

• Cephalic

• Gastric

• Intestinal

Cephalic phase of secretion

• Initiation of

  • Regulated by

  • What plexus

  • Lasts for how long

• Initiation of gastric secretion prior to food entering

  • Regulated by PNS - vagus nerve

  • Submucosal plexus

  • Lasts only a few minutes

Gastric phase of secretion

• Distention

• Stimulate plexus…

• Lasts

• Begins with arrival of food into the stomach; further increases of gastric secretion

  • Distention - stretch detected by baroreceptors, increased pH, and undigested proteins

  • Stimulate submucosal plexus (secretion)

  • Stimulate myenteric plexus (mixing waves)

  • 3-4 hours

Intestinal phase of secretion

• Presence of chyme in small intestine trigger…

• Release of hormones

  • GIP

  • Secretin

  • CCK

• Overall effect

• Presence of partially digested food (chyme) in small intestine trigger:

  • Enterogastric reflex (neural): afferent nerves carry mechanical signal to ENS of the stomach to inhibit gastric motility and secretion

  • Release of hormones: decrease gastric secretion and motility

    • GIP → Minor role in inhibiting stomach activity, released by carbs to cause release of insulin

    • Secretin → released by duodenum as pH changes

    • CCK → released by cells of duodenum in presence of proteins/fats, tells stomach to decrease activity

  • Overall effect: slow the rate of gastric emptying - ensure the small intestine is not overwhelmed and can proceed efficiently