THE DIGESTIVE SYSTEM I

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THE DIGESTIVE SYSTEM I

• D. C. Mikulecky
• Professor of Physiology
• Virginia Commonwealth University

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FUNCTIONS OF THE DIGESTIVE SYSTEM

• MOTILITY
• SECRETION
• DIGESTION
• ABSORPTION

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THE DIGESTIVE TRACT

• MOUTH AND PHARYNX
• ESOPHAGUS
• STOMACH
• SMALL INTESTINE
• LARGE INTESTINE
• ANUS

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THE DIGESTIVE TRACT
MUCOSA
LOGITUDINAL MUSCLE
SUBMUCOSAL PLEXUS
LUMEN
CIRCULAR MUSCLE
EXTRINSIC NERVES
MYENTERIC PLEXUS
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INNERVATION OF THE G.I. TRACT

• PARASYMPATHETIC
• SYMPATHETIC
• INTRINSIC

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PARASYMPATHETIC

• VAGUS DOWN TO TRANSVERSE COLON. PREGANGLIONIC
  CHOLINERGIC NEURONS ORIGNINATE IN MEDULLA
• PELVIC BELOW THAT. PREGANGLIONIC CHOLINERGIC
  NEURONS ORIGNINATE IN SACRAL S.C.

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SYMPATHETIC

• GANGLIA OUTSDE G. I. TRACT CELIAC, SUPERIOR AND
  INFERIOR MESENTERIC
• POSTGANGLIONIC ADRENERGIC FIBERS INNERVATE
  MYENTERIC AND SUBMUCOSAL PLEXUS

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INTRINSIC INNERVATION

• SUBMUCOSAL PLEXUS
• MYENTERIC PLEXUS
• DIFFERENT MEDIATORS ACh, Nitric Oxide, VIP,
  Enkephalins, Serotonin, Substance P

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G. I. PEPTIDES

• HORMONES
• NEUROCRINES
• PARACRINES

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G. I. HORMONES

• GASTRIN
• CHOLECYSTOKININ (CCK)
• SECRETIN
• GIP
• MOTILIN

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GASTRIN

• ENDOCRINE CELLS IN PYLORIC STOMACH
• STIMULATED BY PROTEIN IN STOMACH
• STIMULATES SECRETION BY PARIETAL AND CHIEF CELLS
• STIMULATES ILEAL MOTILITY
• RELAXES ILEOCECAL SPHINCTER
• INDUCES COLONIC MASS MOVEMENTS

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SECRETIN

• ENDOCRINE CELLS IN DUODENAL MUCOSA
• ACID IN DUODENAL LUMEN
• INHIBITS GASTRIC EMPTYING
• INHIBITS GASTRIC SECRETION
• STIMULATES AQUEOUS BICARBONATE SECRETION BY
  PANCREAS
• STIMULATES BICARBONATE RICH BILE SECRETION
  BYLIVER

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CHOLECYSTOKININ

• ENDOCRINE CELLS IN DUODENAL MUCOSA
• FAT AND PROTEIN IN DUODENAL LUMEN
• INHIBITS GASTRIC EMPTYING
• INHIBITS GASTRIC SECRETION
• CAUSES GALL BLADDER CONTRACTION
• CAUSES RELAXATION OF THE SPHINCTER OF ODDI
• CONTRIBUTES TO SATIETY

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GASRTIC INHIBITORY PEPTIDE (GIP)

• STRUCTURALLY RELATED TO SECRETIN AND GLUCAGON
• INHIBITS GASTRIC ACIS SECRETION (PHARMACOLOGICAL
  DOSES)
• STIMULATES INSULIN RELEASE

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MOTILITY

• THE DIGESTIVE TRACT IS SURROUNDED BY LAYERS OF
  SMOOTH MUSCLE
• THESE MUSCLES ENABLE MIXING AND PROPULSIVE
  MOVEMENT TO BE CARRIED OUT BY THE DIGESTIVE TRACT

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SECRETION

• DIGESTION REQUIRES THAT ENZYMES BE SECRETED BY
  THE PANCREAS AND OTHER ORGANS
• MUCOUS SECRETIONS PROTECT THE DIGESTIVE TRACT
• ACID IS SECRETED IN THE STOMACH
• THERE ARE OTHER SECRETIONS OF IMPORTANCE

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DIGESTION

• BREAKING DOWN COMPLEX FOODSTUFFS INTO ABSORBABLE
  UNITS BY ENZYMES PRODUCED IN THE DIGESTIVE SYSTEM
• INVOLVES THE BREAKDOWN OF CARBOHYDRATES, PROTEINS
  FATS, AND OTHER FOODS

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ABSORPTION

• ALL OTHER FUNCTIONS SUPPORT THIS ONE
• THIS IS HOW WE OBTAIN THE NECESSARY FUEL FOR OUR
  CELLS

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ACCESSORY DIGESTIVE ORGANS

• SALIVARY GLANDS
• EXOCRINE PANCREAS
• BILLIARY SYSTEM (LIVER AND GALLBLADDER)

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CONTROL OF DIGESTIVE SYSTEM
External Influences
Local Influences
Receptors in Digestive Tract
Intrinsic Nerve Plexuses
Extrinsic Autonomic Nerves
GI Hormones
Smooth muscle Exocrine Gland Cells Endocrine
Gland Cells
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THE MOUTH

• INGESTION OF FOOD
• CHEWING AND SWALLOWING
• SALIVARY SECRETION

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CHEWING

• CHEWING IS THE FIRST STEP IN THE DIGESTIVE
  PROCESS
• FOOD IS MIXED WITH SALIVA AND BROKEN UP INTO
  SMALLER PIECES
• TASTE BUDS ARE STIMULATED

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SALIVARY SECRETION

• SALIVARY AMYLASE DIGESTS CARBOHYDRATE
• MOISTENING AND LUBRICATING AGENT
• ANTIBACTERIAL ACTION LYSOZYME AND RINSING
• SOLVENT FOR TASTE STIMULI
• SPEECH FACILITATION
• ORAL HYGENE
• BICARBONATE BUFFERS NEUTRALIZE ACIDS

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Swallowing

• A PROGRAMMED ALL-OR-NONE REFLEX
• CHEWING AND MOVING THE BOLUS OF FOOD BACK IS
  MANLY VOLUNTARY (STRIATED MUSCLE)
• PRESSURE OF BOLUS ON PHARYNX TRIGGERS INVOLUNTARY
  REFLEX (SMOOTH MUSCLE)
• TONGUE PREVENTS FOOD FROM MOVING BACK
• UVULA ELEVATED, SEALING NASAL PASSAGE
• LARYNX ELEVATES AND CLOSURE OF GLOTTIS
• RESPIRATION BRIEFLY INHIBITED
• PHARYNGEAL MUSCLES FORCE BOLUS BACK
• PERISTALTIC WAVES MOVE BOLUS THROUGH ESOPHAGUS

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PERISTALSIS
RINGLIKE CONTRACTION SWEEPS DOWN THE ESOPHAGUS
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THE STOMACH
ESOPHAGUS
GASTROESOPHAGEAL SPHINCTER
FUNDUS
BODY
OXYNTIC MUCOSA
ANTRUM
PYLORIC SPHINCTER
PYLORIC GLAND AREA
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THE STOMACH

• MIXING AND STORAGE ORGAN
• HAS ITS OWN PACEMAKER SETS FREQUENCY OF RHYTHMIC
  PERISTALTIC CONTRACTIONS
• PERISTALSIS BECOMES STRONGER IN ANTRUM
• MOTILITY IS HIGHLY CONTROLLED
• NO ABSORPTION OF FOOD, ONLY ASPIRIN AND ALCOHOL

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FACTORS INFLUENCING GASTRIC MOTILITY

• DISTENSION OF STOMACH INCREASES
• FEEDBACK FROM THE SMALL INTESTINE DECREASES
• CONTROL FROM CNS
• GASTRIN INCREASES

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MIXING MOTIONS IN THE STOMACH
PYLORIC SPHINCTER CLOSED
PERISTALSIS BEGINS
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MIXING MOTIONS IN THE STOMACH
PYLORIC SPHINCTER CLOSED
PERISTALTIC WAVE TRAVELS DOWNWARD
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STOMACH EMPTYING
PERISTALTIC WAVE TRAVELS DOWNWARD
PYLORIC SPHINCTER OPENS
CHYME SQUIRTED INTO DUODENUM
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FACTORS CONTROLLING STOMAC EMPTYING

• GASTRIC MOTILITY
• ENTEROGASTRIC REFLEX VIA INTRINSIC AND AUTONOMIC
  NERVES
• ENTEROGASTRONES SECRETIN, CHOLECYSTOKININ (CCK),
  GASTRIC INHIBITORY PEPTIDE

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FACTORS IN SMALL INTESTINE CONTROLLING STOMAC
EMPTYING

• FAT
• ACID
• HYPERTONICITY
• DISTENSION

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GASTRIC SECRETIONS

• MUCOUS SURFACE CELLS
• MUCOUS MUCOUS NECK CELLS
• HYDROCHLORIC ACID PARIETAL CELLS
• INTRINSIC FACTOR PARIETAL CELLS
• PEPSINOGEN CHIEF CELLS
• GASTRIN ONLY IN THE PYLORIC GLAND AREA

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ACTIVATION OF PEPSIN
CHIEF CELL
PEPSIN-OGEN
PEPSIN
HCL
DIGESTS PROTEIN
PARIETAL CELL
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STIMULATION OF GASTRIC SECRETION CEPHALIC PHASE
SEEING, SMELLING, TASTING FOOD
INTRINSIC NERVES PYLORIC AREA
VAGUS
PARIETAL CHIEF CELLS
INCREASED GASTRIC SECRETION
GASTRIN
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STIMULATION OF GASTRIC SECRETION GASTRIC PHASE
STIMULI IN STOMACH PROTEIN, DISTENSION, CAFFEINE
, ALCOHOL
INTRINSIC NERVES PYLORIC AREA
PARIETAL CHIEF CELLS
VAGUS
INCREASED GASTRIC SECRETION
GASTRIN
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THE GASTRIC MUCOSAL BARRIER

• PROTECTS THE CELLS FROM CONTENTS OF STOMACH
• LUMINAL MEMBRANES OF CELLS ARE IMPERMEABLE TO
  PROTONS
• CELLS ARE TIGHTLY ADJOINED
• RAPID TURNOVER
• IF BROKEN, PEPTIC ULCER MAY RESULT POSITIVE
  FEEDBACK INVOLVING HISTAMINE

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GASTRIN SECRETION INHIBITION

• ACID IN ANTRUM
• REMOVAL OF PROTEIN AS STOMACH EMPTIES