lecture 1 Flashcards
4 tunics:
deep - superficial: mucosa, submucosa, muscularis, serosa
GI tract=
more neurons than spinal cord
largest lymphoid organ
harbors largest genome
largest endocrine organ in body
mucosa:
ole: absorption and secretion; villi increase absorptive area. Hosts specialized EE, goblet cells.
contains:
* lamina propria = layer of connective tissue under the epithelium, containing blood vessels, nerves
and lymphatic vessels (Peyer’s patches). Rich in immune cells; role in host defense.
* muscularis mucosae: thin layer of smooth muscles; provides villus movement.
submucosa role:
thick, highly vascular layer of connective elastic tissue where absorbed molecules enter blood and lymphatic vessels. Contains glands and submucosal plexus (Meissner’s plexus) that carries ANS activity to muscularis mucosae of small and large intestines.
serosa role:
role: binds and protects. Connective tissue continuing through the
mesenteries (a thin membrane rich in blood and lymphatic capillaries) and the peritoneum (a double
layer membrane surrounding the abdominal organs)
Muscularis externa:
radial (inner) and longitudinal (outer) smooth muscles + Auerbach plexus (in
contact with Meissner’s). Myenteric plexus lies in between. Controls segmental and peristaltic
movements along GI. Includes both sympathetic and parasympathetic nerve fibers and ganglia
Different Muscle for Different GI Functions
- Longitudinal smooth muscle has very few electrical junctions; contracts in response to excitatory
innervation. It contracts for intestinal
shortening. - Circular smooth muscle has many
electrical junctions and contracts
spontaneously unless inhibited by
its innervation. Reduces diameter
of the lumen when it contracts. - Esophagus –largely striated
muscle with both longitudinal
and circular muscle. - Proximal Stomach – lacks
distinct layering; smooth
muscle lacks action
potentials; slow changes in
tone in proportion to changes
in Vm. - Distal Stomach (antrum) –
rhythmically active;
contraction triggered by
regular action potentials.
sphincters (GI)
control of urine
they are always contracting
* Most under involuntary control, no conscious input (ONE
exception) = external anal sphincter
* Autonomous of CNS, controlled by ENS.
* Retain content in specific sites for optimal digestion&absorption
GI sphincters: (pressure)
- resting state (pressure) > adjacent segments = inhibits movement entre segments
- relaxation = adjacent segments
allows forward flow - constriction»_space; adjacent segments
prevents retrograde flow
main functions of GI tube:
ingestion
digestion
absorption (diffusion&active transport)
storage
elimination (Temporary storage and
subsequent elimination of
indigestible components of
food (but also cholesterol,
steroids, drug metabolites)
immune barrier (* Includes physical barrier formed by tight junctions
between cells of small intestine
* Cells of the immune system
that reside in connective tissue just below epithelium
* Commensal bacteria)
this all = via motility + secretion
Motility actions:
- digestion ((motility, pH changes, biological detergents, enzymes), mastication, deglutition, peristalsis ( contractions that make food move through GI
secretion actions:
release of exocrine and endocrine products into GIT
- exocrine secretions: HCI, H2O, HCO2, bile, lipase, pepsin, amylase, trypsin, elastase, histamine
- endocrine secretions: hormones secreted into stomach + small intestin that help regulate GIT functions;
Oral cavity and esophagus process:
mastication: lubrication, food bolus, salivary amylase in saliva
catalyzes partial digestion of
starch.
medulla oblongata is important for swallowing
deglutition: begins as
voluntary activity
* Based on pressure gradient
role of peristalsis =
move food from mouth to stomach
fct = independant of gravity
GI sphincter: (LES)
constricts after food passes into stomach, relaxes on swallowing, tonically active under neural control, AcH contracts, NO VIP relaxes, prevents backflow/reflux (GERD)
stomach:
fct = reservoir
controls rate of meal delivery
partial digestion (proteins + sterilization), low pH kills pathogens
stomach contains gastric pits that increases surface area
exocrine glands = cells secrete gastric juice (goblet cells=secrete mucus, parietal cells= HCI + intrinsic factor (B12 absorption in intestine) (Children born to women who have low blood levels of vitamin B12 shortly before and after conception have an increased risk of neural tube defects such as spina bifida)
Contractions – churns chyme mixing it with gastric secretions
Eventually propelling food into small intestine
!! RECEPTIVE RELAXATION!!—controls pressure, accommodates volume
NOT essential to digestion – allows resections!!
chief cells = secrete pepsinogen, enterochromaffin = histamine + serotonin
G cells = gastrin
D cells = somatostatin
stomach divisions:
- Divided anatomically into:
- Cardia (secretes mucus and bicarbonate
to protect) - Fundus (specialized secretory cells)
- Body
- Antrum (pyloric zone, motility, mixing
grinding)
SI:
- duodenum: after pyloric sphincter: has chemo + mechanosensitive nerve endings + EE cells, regulates digestion + absorption, receives pancreas + gallbladder secretions, coordinates food intake.
- jejunum = abundant w villi
- ileum: contains fewer folds, empties into large intestine, can be called for absorption if duodenum can’t do it.
SI functions:
- foldings and projections: plicae circulares: large folds
villi: fingerlike folds of mucosa, 15x more surface area
microvilli: apical hair-like projection, 300x more surface area
folds, villi, microvilli = increase surface area for exchange
epithelial cells of GI mucosa are continuously replaced (billionth/day)
they’re the ones that breakdown food when it comes
microvilli = Brush border enzymes – attached to microvilli but not secreted into
the lumen
o Enzyme active sites are exposed to chyme in the lumen helping
to complete digestion of food molecules
LI
fcts: reservoir for waste storage + indigestible material, reabsorption of fluid, solutes biles and salts
bacterial metabolism, production + absorption of vitamins ?
has no villi
does not express absorptive transporters for conventional nutrients
descending colon = primary fecal waste reservoir
contains large population of microflora
ferment indigestible food to produce short chain fatty acids
produce folic acid + vitaminK
reduce ability of pathogenic bacteria to invade
antibiotics can neg affect commensals
individuaal microbiome: role in obesity?
◦ Absorbs 90% of the remaining
water coming from the small
intestine
◦ Begins with osmotic gradient set up
by Na+
/K+ pumps
Water follows by osmosis
* Digestive secretions are mostly water.
* Only 100 ml are excreted in feces, so
the mechanisms for water absorption
are efficient
colon fluid and electrolyte absorption:
absorbs 90% of water from SI
water follows by osmosis
splanchnic circulation:
Blood flows from the intestine first
to the liver via portal vein
Fasting state: 65 % of blood flows
to the liver.
Post-prandial period: 5x increase
in perfusion and 85% to the liver.
Ensures detoxification, if needed
Can reduce oral drug
bioavailability (“first pass”)
Most lipids enter the lymphatic
drainage of the gut, packaged as
chylomicrons. Thereafter is
emptied into the thoracic duct.
- Blood supply to the
gastrointestinal tract is under
sympathetic control.- Sympathetic activation triggers
constriction of arteries and
large veins, shunting blood to
systemic circulation. - Local blood flow is primarily
under local control by GI
hormones and
neurotransmitters. For
example CCK, gastrin and
substance P all increase gastric
and intestinal blood flow.
- Sympathetic activation triggers
- Blood supply to the
