overview of the gastrointestinal tract

what is the gut lumen external to

the body

why is the gut lumen termed external to the body

• the guts environment is hostile

• acids in the gut would destroy the tissue if I went through the gut wall

when do food components enter the body

after absorption

where are conditions for digestion tolerated

• in the gut

• but not in the body

what are the conditions for digestion that are tolerated in the gut

• pH of stomach- as low as 2.0, in body pH 6.8-8.0

• digestive enzymes- would destroy body’s own tissue

• microorganisms- in lower intestine but destructive or lethal if entered body

• foods are foreign particles- would be attacked by immune system if entered body as eaten

what are the 4 basic processes of the digestive system

• motility

• secretion

• digestion

• absorption

how is motility in the gut controlled

by voluntary and involuntary control

what mediates voluntary control in the gut

• skeletal muscle

• responsible for chewing, swallowing and defaecation

what mediates involuntary control in the gut

• smooth muscle

• contractions that mix and move forward contents of gut

what are the 2 types of smooth muscles in the gut

• phasic

• tonic

describe phasic smooth muscles in the gut

• action-potential induced bursts of contraction

• propulsive and mixing movements

describe tonic smooth muscles in the gut

• constant low level of contraction

• tone maintains a steady pressure on contents of gut and prevents wall from being permanently stretched

what are 2 things secreted by the gut

• digestive juices

• GI hormones

what are digestive juices secreted by

exocrine glands

what do digestive juices contain

• water

• electrolytes

• specific products for digestion and absorption e.g. mucus, enzymes, and biles salts

what do secretory cells extract for secretion

large volumes of water and raw materials for secretion

what does secretion require

energy

why does secretion require energy

• active transport of raw materials into the cell

• synthesis of secretory products by ER

what does neural or hormonal stimulation release

secretion

what normally happens to secretions

reabsorbed into blood

what are GI hormones secreted by

endocrine glands

what are GI hormones secreted into

into blood by endocrine cells along tract wall

what do GI hormones regulate

motility and exocrine gland secretion

what is digestion

the biochemical breakdown of complex proteins, carbohydrates, and fats by enzymes

what is the mechanism enzymes in the gut digest by

• hydrolysis

• they are specific in the bonds they hydrolyse

what are larger molecules in the gut broken down into and how

• broken down into smaller units

• in a stepwise manner through the gut that are eventually absorbed

where does protein digestion begin

in the stomach

what happens in the stomach during protein digestion

• pepsinogen in gastric juice is converted to pepsin at low pH

• pepsin breaks down proteins to peptides

what is pepsinogen produced by

cells in the stomach wall

where does protein digestion continue to

small intestine

what happens in the small intestine during protein digestion

• trypsinogen in pancreatic juice is converted to trypsin at basic pH

• trypsin breaks down proteins to peptides

• peptidases are produced by the small intestine and breaks down peptides to amino acids

where are peptidases present

in the brush border of the epithelial cells that line the small intestine

where are amino acids actively transported into

epithelial cells of villi and from there to blood

where does lipid digestion occur

in the duodenum

what happens in the duodenum in lipid digestion

• lipids combine with bile salts to form fat droplets

• lipase, from pancreas, digests triglyceride to monoglycerides and fatty acids

what do monoglycerides and fatty acids diffuse into

• epithelial cells

• where they recombine and join with proteins to form lipoproteins, called chylomicrons

• these enter a lacteal

what are polysaccharides broken down into in carbohydrate digestion

dissacharides

what are examples of polysaccharides

• starch

• glycogen

• cellulose

what is starch broken down by

salivary amylase and then pancreatic amylase to maltose

what is glycogen broken down to

glucose

what happens to cellulose and other indigestible CHO in the gut

not broken down by gut enzymes

what are different enzymes in the small intestine that break down simpler sugars

• maltase

• sucrase

• lactase

what does maltase break down

maltose to glucose

what does sucrase break down

sucrose to glucose and fructose

what does lactase break down

lactose to glucose and galactose

how is glucose transported to epithelial cells of villi

actively transported to epithelial cells of villi and then into blood

what are the 4 main layers of the digestive tract wall

• mucosa

• submucosa

• muscularis

• serosa

what does the mucosa line

the lumen

what are the 3 layers of the mucosa

• mucous membrane

• laminate propria

• muscularis mucosa

why is the mucosa in the oesophagus white flat

you dont really need to absorb anything here

mucosa layer of the stomach

contains ‘pits’ where acid, pepsins, and hormones are produced

mucosa layer of the small intestine

• huge folds (villi) that have microvilli- which are the brush border

• increased surface area to allow absorption in the gut

mucosa layer of the large intestine

crips- at the bottom you have stem cells which differentiate into cells important in the area

what is the submucosa

connective tissue

what does the submucosa being connective tissue allow

tract to distend and be elastic

what does the submucosa contain

• larger blood vessels and lymph vessels, sends branches to mucosa and muscular external

• submucosa nerve plexus

what is the muscularis externa

major smooth muscle

what are the 2 layers of the muscularis externa

• inner circular layer

• outer longitudinal layer

what is the muscularis externa responsible for

mixing and propulsive movements

where is the myenteric nerve plexus located

between the inner circular layer and outer longitudinal layer of the muscularis externa

what is the serosa

• outermost layer

• connective tissue

what does the serosa secrete

serous fluid which lubricates and prevents friction between tract and surrounding tissues and organs

what is the serosa continuous with

continuous with the mesentery- suspends digestive organs from inner wall of abdominal cavity like a sling

how is digestive function regulated

• autonomic smooth muscle function

• intrinsic nerve plexuses

• extrinsic nerves

• gastrointestinal hormones

what is the electrical activity in digestive smooth muscles

• self-induced electrical activity

• slow wave potentials (basal electrical rhythm)

what instigates the cyclic slow-wave activity in digestive autonomic smooth muscles

pacemaker cells which are called interstitial cells of cajal

where are interstitial cells of cajal located and what do they allow

• between circular and longitudinal muscle

• allow whole sheets of smooth muscle cells to contract together when threshold reached (when there is enough food present to contract)

what does reaching threshold to induce contraction in the digestive smooth muscles depend on

mechanical, neural, and hormonal factors e.g. food present or absent

is rate of contraction in the digestive smooth muscles constant

no it varies depending on conditions e.g. peristalsis in stomach or segmentation in small intestine

what does intensity of contraction in the digestive smooth muscle depend on

• number of action potentials once slow-wave threshold potential reached

• also depend son the amount of food present

what is involved in the intrinsic nerve plexuses

submucosa and myenteric nerve plexuses

enteric nervous system

entirely and throughout digestive tract wall- 100 million neurons

what doe intrinsic nerve plexuses primarily coordinate

local activity in the tract

what are the various types of neurons in the intrinsic nerve plexuses

• input neurons (sensory) respond to stimuli

• excitatory and inhibitory output neurns- moderate motility or secretion of hormones or enzymes

• Ach promotes smooth muscle contraction

• NO and vasoactive intestinal peptides act together to relax smooth muscle

what nerves influence motility and secretion and how do they do it

• sympathetic and parasympathetic nerves

• modify activity if intrinsic nerve plexuses

• alter secretion of GI hormones, directly act on smooth muscles and glands

what is involved in extrinsic nerves

sympathetic and parasympathetic nerves

what does sympathetic activity do to digestion

slows digestion

what does parasympathetic (vagus nerve) activity do to digestion

• increases smooth muscle contractility

• promote GI hormone and enzyme secretion

what do the extrinsic nerves do in digestion

coordinate activity between different regions of the tract

what are the 3 types of sensory receptors

• chemoreceptors

• mechanoreceptors

• osmoreceptors

what does stimulation of receptors in the tract result in

neural reflexes or secretion of hormones which alter activity of effector cells

what are receptors on effector cells directly stimulated by

GI hormones, neurotransmitters and local chemical mediators

what doe endocrine glands in mucosa of specific regions of tract release

hormones into blood upon stimulation

where are GI hormones transported to

other parts of digestive tract where stimulate or inhibit other exocrine gland cells or smooth muscle

what is cholecystokinin

• GI hormone

• involved in gall bladder contraction, GI motility, and pancreatic exocrine secretion