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• Function and processes of the digestive
system
• Anatomy of the digestive system
• Motility
• Secretion
• Regulation of GI function
• Digestion and absorption
Basic Processes of the
Digestive System
•Most food is taken in as solids and
as macromolecules that cannot be
readily transported across the
intestinal epithelial cells. Digestion
which involves physical and
chemical alterations of the food
has to first take place and this is
facilitated by secretions from the
G.I. tract and associated organs.
•The absorbed nutrients are then
metabolised through a series of
chemical processes making it
possible for cells to continue living.
• Waste substances from the
ingested food are stored and
excreted out of the G.I. tract.
Digestive System Anatomy
Oral cavity  esophagus  stomach  small
intestine  large intestine  rectum
•Each part is adapted to its specific
functions:
•some to simple passage of foode.g., esophagus,
•others to temporary storage of
food such as the stomach and
•others to digestion and
absorption, such as the small
intestine
The Wall of the Digestive Tract
• A typical section of the digestive tract reveals four main layers. From
inside (the lumen) to outside they are:
o Mucosa
o Submucosa
o Muscularis (externa)
o Serosa (a.k.a. visceral peritoneum)
Example: Four-layered Wall of the Small Intestine
absorption
Epithelium
secretion
endocrine
Lamina propria
Muscularis mucosae
Submucosal plexus
Circular muscle
Myenteric plexus
Longitudinal muscle
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The Upper Part of the GI TractMouth,esophagus,stomach.
a. Structure of the salivary glands
serous
Three major salivary glands:
Parotid glands
Submandibular glands
Sublingual glands
mixed
serous + mucous
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Unusual feature :
PSNS
SNS
both stimulate
Saliva production
Unusual feature :
unusual high blood flow, > 10 times the blood flow to exercising skeletal
muscle (when corrected for organ size)
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b. Formation of saliva
Step 1
- The acinar cells secrete the initial saliva.
- The initial saliva is isotonic.
- It has the same electrolyte composition as plasma.
Step 2
- The ductal cells modify the initial saliva.
- Absorption of Na+, CI- > secretion of K+ and HCO3- The final saliva is hypotonic.
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c. Regulation of salivary secretion
• Salivary secretion is exclusively under neural control.
• Both PSNS and SNS stimulate saliva production. PSNS is primary.
• Conditioning, food, thought, and nausea etc. also stimulate salivary secretion.
• Dehydration, fear, and sleep inhibit salivary secretion.
PSNS:
SNS:
Primary controller of salivation, large amount of watery saliva containing
enzymes
Small volume of saliva, thick with mucus
Because sympathetic stimulation accompanies frightening or stressful situations, the
mouth may feel dry at such times.
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Summary of Salivary Secretion
Characteristics of saliva secretion:
•
•
•
•
•
•
high volume (approx. 1 L/day)
high K+ and HCO3- concentrations
low Na+ and Cl- concentrations
hypotonicity
The composition of saliva varies with flow rate.
pH of 6.0 – 7.0
Functions of saliva:
• lubrication
• Protection
thiocyanate ions, proteolytic enzymes (lysozyme), IgA etc.
•
α-amylase, lingual lipase
• Kallikrein cleaves kininogen to produce bradykinin (a strong
vasodilator, accounts for high salivary blood flow)
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The Upper Part of the GI Tract- Stomach.
The Lower Part of the GI Tract
Small Intestine.
lymphoid
nodules
The Lower Part of the GI Tract
Large Intestine
Accessory Glands
The accessory glands that produce secretions
to aid in digestion are the salivary glands (3
pair), liver, and pancreas.
Salivary glands moisten food, cleanse and
protect the mouth, and produce amylase to
begin enzymatic digestion of starch
The liver produces bile, which emulsifies
fats to increase their surface area for
subsequent chemical digestion by lipases; bile
is stored in and released from the gall bladder
into the duodenum
The pancreas is the main digestive enzymeproducing exocrine organ in the body. It
releases a host of digestive enzymes into the
duodenum via the pancreatic duct; it also
produces bicarbonate to neutralize the chyme
from the stomach
Control of the Digestive System
The autonomic nervous
system, hormones, and
other chemicals control
motility and secretion in
the digestive system to
maximize digestion and
absorption.
Hormones modulate digestive activity
• GI hormones are
peptides (amino acid
based).
• GI hormones include
-gastrin,
-cholecystokinin (CCK),
- secretin,
-glucose dependent
insulinotropic peptide
(GIP) and
-motilin.
Hormone – Gastrin
Stimuli for Secretion –
Small peptides and amino acids
Distention of the stomach
Vagal stimulation (GRP)
Site of Secretion -G cells of the antrum, duodenum, and
jejunum
Actions –
↑ Gastric H+ secretion
Stimulates growth of gastric mucosa
Zollinger-Ellison syndrome
Hormone – Cholecystokinin
Stimuli for Secretion –
Small peptides and amino acids
Fatty acids
Site of Secretion - I cells of the, duodenum, jejunum and Ileum
Actions –
↑ Pancreatic enzyme secretion(major)
↑ Pancreatic HCO3- secretion(minor)
Stimulates contraction of the gallbladder and relaxation of the
sphincter of Oddi
Stimulates growth of the exocrine pancreas and gallbladder
Inhibits gastric emptying
Hormone – Secretin
Stimuli for Secretion –
H+ in the duodenum
Fatty acids in the duodenum
Site of Secretion - S cells of the duodenum
Actions –
↑ Pancreatic HCO3- secretion
↑ Biliary HCO3- secretion
Inhibits gastric emptying
Hormone – Glucose-Dependent Insulinotropic Peptide
(GIP)
Stimuli for Secretion –
Fatty acids
Amino acids
Oral glucose
Site of Secretion - K cells of duodenum and jejunum
Actions –
↑ Insulin secretion from pancreatic β cells
↓ Gastric H+ secretion
Other Hormones
Motilin
• Secreted by cells of the stomach, small intestine and colon
• Major stimulator of the GIT motility during fasting
Substance P
• Neurocrine agent which stimulates the motility of the GIT
Gastrin-releasing peptide, a.k.a. bombezin
• Act as vagally produced neurocrine, which stimulates
secretion of gastrin
Somatostatin
• Is secreted by D-cells of the pancreas (act as a hormone) and
D-cells of the gastrointestinal mucosa (paracrine action)
• Release is stimulated by acids and inhibited by the vagus
• Acts as an universal inhibitor, which inhibits release of
peptide hormones, pancreatic exocrine secretion, gastric
secretion and motility, gallbladder contraction and absorption
of the products of digestion
Neurotensin
• Secreted by neurons and mucosa cells in the ileum in response to FA
• Inhibits GIT motility
Pancreatic polypeptide (PP)
• Inhibits pancreatic and billiary secretion and delays the absorption of nutrients
Enkephalins
• Neurocrines, which cause contraction of GIT sphincters and reduce intestinal
secretion of water and electrolytes (some opiates are used for treatment of
diarrhea)
Histamine
• Is secreted by GI mucosa, particularly in the H+-secreting region of the stomach.
• Increases gastric H+ secretion directly.
• Potentiates the effects of gastrin and vagal stimulation.
Innervation of The Gastrointestinal Tract
A Comparison
In most body systems:
PSNS
Muscle, glands,
blood vessels
SNS
In GI tract:
PSNS
SNS
Muscle, glands,
blood vessels
Enteric nervous system (ENS)
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Innervation of the GI Tract
Extrinsic nervous
system
- Parasympathetic
(vagus nerve, pelvic
nerve)
- Sympathetic (T5 –
L2)
- Neurotransmitters
- Efferent fibers and
afferent fibers
Intrinsic/Enteric
nervous system
- Submucosal plexus
- Myenteric plexus
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"Little Brain" Model of the Enteric Nervous
System
PC-program circuits
IC-integration circuits
Parasympathetic Nervous System (PSNS)
•
•
•
•
divided into cranial and sacral divisions.
excitatory for the functions of the GI tract.
usually ____________
is carried via the vagus and pelvic splanchnic nerves.
Preganglionic fibers synapse in the myenteric and
submucosal plexuses.
• Cell bodies in ganglia of the plexuses then send
information to the smooth muscle, secretory cells, and
endocrine cells of the GI tract.
• The vagus nerve carries PSNS information to the esophagus,
stomach, pancreas, and upper large intestine.
• The pelvic splanchnic nerves carry information to the lower
large intestine, rectum, and anus.
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Sympathetic Nervous System (SNS)
inhibitory on the functions of the GI tract.
• is usually _________
• Fibers originate in the spinal cord between T5 and L2.
• Preganglionic fibers travel in the Greater, Lesser and least
Splanchnic nerves.
• Preganglionic cholinergic fibers synapse in prevertebral ganglia.
• Postganglionic fibers leave the prevertebral ganglia, travel
with arteries and synapse in the myenteric and submucosal
plexuses.
• Direct postganglionic adrenergic innervation of blood vessels
and some smooth muscle cells also occurs.
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Intrinsic / Enteric Nervous System
The submucosal and
myenteric plexuses
comprise the enteric
nervous system (ENS).
Submucosal plexus:
• controlling inner wall
functions
- secretion
- local blood flow
- mucosal movement
Myenteric plexus:
• controlling muscle activity
along the gut
Intrinsic and independent
- lies entirely in the wall of the gut.
- can function on its own in the absence of extrinsic
innervation.
- controls most functions of the GI tract, especially
motility and secretion.
Integrated
- stimulation by the parasympathetic and sympathetic
systems can greatly enhance or inhibit its functions.
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Reflexes coordinate and modulate digestive activity
• Reflexes (stimulus, integration, and response) that are totally controlled by the
enteric nervous system are termed short reflexes.
• Reflexes that involve the CNS as an integration center are called long reflexes.
• Short and long reflexes can occur simultaneously
• Larger volumes of food in the stomach produce stronger gastric
contractions than smaller volumes of food.
• Chemical composition of food influences the rate of digestion:
lipid rich meals take longer to digest than carbohydrate rich meals
• Emotional stressors cause the brain to
override the intrinsic controls of
digestion and subsequently cause problems
such as constipation,
diarrhea, and stomachaches.
There are many neurotransmitters in the GI tract
• All preganglionic fibers of the ANS
release acetylcholine (ACh).
• Parasympathetic postganglionic
fibers also release ACh
• Sympathetic postganglionic fibers
release norepinephrine
• Other enteric nervous system
neurotransmitters include serotonin,
vasoactive intestinal peptide (VIP), nitric
oxide (NO), and somatostatin (SST).
• ACh and substance P stimulate smooth
muscle of the digestive tract.
• Smooth muscle of the digestive tract is
inhibited by norepinephrine, VIP, and NO
• Enkephalins released in the submucosal
and myenteric plexuses slow intestinal
motility, inhibit intestinal secretion, and
contract the LES, pyloric and ileocecal
sphincters.
Division of GIT into Functional Segments by Sphincters
Sphincters regulate the passage of
food from one region of the
digestive tract to the next, and
finally, out of the body as feces
The sphincters of the digestive tract,
from mouth to anus, are the:
o Upper esophageal sphincter or UES
(circular skeletal muscle – an
anatomical sphincter)
o Lower esophageal sphincter or LES (a
physiological sphincter)
o Pyloric sphincter (circular smooth
muscle)
o Ileocecal sphincter or valve (circular
smooth muscle)
o Internal anal sphincter or IAS(circular
smooth muscle)
o External anal sphincter or EAS
(circular skeletal muscle)
THE DIGESTIVE SYSTEM
Motility
Chewing occurs in the mouth
Swallowing initiates primary
peristalsis in the esophagus
Stretch of the esophageal wall
initiates secondary peristalsis
Motility in esophagus :
-Primary and secondary peristalsis.
-Function of peristalsis is to
Propel a bolus of food to stomach.
Clinical correlaton-Peptic Ulcer.
-Gastroesophageal reflux disease(GERD)
-AchalasiaChagas disease
Barium swallow showing bird's beak“
or "rat's tail" appearance in achalasia.
Motility
• Different types of contraction• Tonic contractions
– Sustained
– Smooth muscle sphincters and stomach
• Phasic contractions
– Last a few seconds
– Peristalsis moves bolus forward
– Segmentation mixes
GI Smooth muscle contract spontaneously• The smooth muscle of the gastrointestinal tract is excited by
almost continual slow, intrinsic electrical activity along the
membranes of the muscle fibers.
• This activity is simillar to what happens in myocardial muscle
and the electrical wave is referred to as -slow waves
Patterns of Contractions in the GI Tract
•The basic propulsive movement of the
gastrointestinaltract is peristalsis,
illustrated in Figure.
•A contractile ring appears around the gut(in
the circular muscle layer) and then moves
forward.
•The usual stimulus for intestinal peristalsis is
distention of the gut. That is, if a large amount
of food collects at any point in the gut, the
stretching of the gut wall stimulates the
enteric nervous system to contract
the gut wall 2 to 3 centimeters behind this
point, and a contractile ring appears that
initiates a peristaltic movement.
Motility
•local intermittent constrictive
contractions occur every few
centimeters in the gut wall.
•These constrictions usually last only 5
to 30 seconds, then new constrictions
occur at other points in the gut, thus
“chopping” and “shearing” the
contents first here and
then there.
Clinical correlation-esophageal spasm,
Delayed gastric emptying,
constipation, diarrhea,
Irritable bowel syndrome.