GI tract Flashcards
What gives the gut motility?
Voluntary motility- skeletal muscle e.g. chewing, swallowing, defecation
Involuntary contractions- smooth muscle e.g. mix and move forward contents of the gut
What are the types of action of the smooth muscle in the GI tract?
TONIC- constant low level of smooth muscle contraction to maintain a steady pressure, prevents gut wall from being permanently streched
PHASIC- Generated via action potentials causing induced bursts of contraction in presence of food = propulsive and mixing movements
What is digestion?
The biochemical breakdown of complex proteins, carbohydrates and fats by enzymes
e.g. Maltose –> glucose via maltase
requires hydrolysis
Discuss protein digestion?
This begins in the stomach
- Pepsinogen is a proenzyme found in gastric juice secreted by gastric chief cells. It is converted to the active enzyme pepsin buy gastric acid in the low pH
- Pepsin then breaks down proteins into smaller peptides
Then in the small intestine:
- The Trypsinogen (made in pancreas) in pancreatic juice is converted to trypsin in basic pH
- Trypsin breaks down different bonds in peptides to continue digestion from the stomach
- Peptidases found in the epithelial cells that line the small intestine then break down these peptides into
individual amino acids. - These amino acids are absorbed via active transport through the gut wall and into the capillaries to be transported around the body to where they are needed e.g. heavily in the liver
Discuss lipid digestion?
The fat triglycerides travel to the duodenum (part of the small intestine)
- Lipids are combined with bile salts (made in the liver and stored and secreted from the gall bladder) to emulsify the fats with water. This increases the surface area for action of pancreatic lipase.
- Lipase (enzyme generated in the pancreas) then hydrolyses these triglycerides to monoglycerides and free fatty acids
- In order to move out into the water based environment of the GI tract, the bile then encompasses the monoglycerides and fatty acids to form micelles.
- These micelles, then diffuse into epithelial cells where they recombine and join with proteins to form lipoproteins known as chylomicrons (Chylomicrons are large triglyceride-rich lipoproteins produced in enterocytes from dietary lipids)
- Chylomicrons then travel to the liver and other tissues to be used
Discuss carbohydrate digestion?
- Polysaccharides, ogliosaccharides and starches are broken down by salivary amylase in the mouth- breaks the bond between the sugar units
- Amylase no longer has an effect in the stomach due to the acidity. But the carbohydrates are being mixed and propulsed mechanically to form chyme.
- Once the chyme from the stomach reaches the duodenum (small intestine), it is further broken down by pancreatic enzymes:
- Enzymes in the small intestine break down the simpler sugars:
Sucrase breaks sucrose into glucose and fructose molecules.
Maltase breaks the bond between the two glucose units of maltose lactase breaks the bond between galactose and glucose. - Once carbohydrates are chemically broken down into single sugar units they are actively transported through epithelial cells into blood capillaries and then to the rest of the body
What are the 4 layers of the gut wall?
- Mucosa
- Sub-mucosa
- Muscularis
- Serosa
Describe the 4 layers of the gut wall
MUCOSA:
The innermost layer, next to the lumen.
It is further divided into 3 layers:
Epithelium: contains epithelial cells and goblet cells (secret mucus to a aid passage of food) and is where most absorption occurs
lamina propria- consists of subepithelial connective tissue and lymph nodes
Muscularis mucosa- This is a continuous sheet of smooth muscle cells that lies at the base of the lamina propria
SUB-MUCOSA:
- Made up of connective tissue and contains blood vessels, lymph vessels and nerves. It has elasticity that stretches with increased capacity but maintains the shape of the intestine
MUSCULARIS:
Has 2 layers- inner-circular which contracts the diameter of the lumen and longitudinal which contracts the length of the lumen. The myenteric plexus is between the two muscle layers.
SEROSA:
Contains serous fluid which keeps the outer layer moist to prevent it sticking to other tissues
How does the structure of the mucosa vary throughout the gi tract?
- Eosophagus- is flat, no absorption occurs here so surface area doesn’t need to be increased
- Stomach- mucosa has big pits which contain acid, pepsin and mucus is produced to protect cells from acid
- Small intestine- has large folds in the surface which increases the SA- also have microvilli as a brush border which further increases surface area for increased absorption
- Large intestine- unlike small intestine, it has no folds. Has crips containing stem cells that are needed to differentiate into needed cells.
What is found between the inner circular and outer longitudinal layer of the muscularis?
The myenteric nerve plexus
What regulates the digestive function of the GI tract?
AUTONOMIC SMOOTH MUSCLE FUNCTION:
Self-induced electrical activity- causes slow wave potentials initiated by the interstitial cells of cajal. These act as pacemaker cells and contract sheets of smooth muscle cells when threshold is reached- e.g. enough food, or neural and hormonal factors
INTRINSIC NERVE PLEXUSES;
involves input neurone which respond to local stimuli
Output neurons- modulate motility or secretion of enzymes or hormones
Acetylcholine stimulates smooth muscle contraction
NO relaxes the smooth muscle
EXTRINSIC NERVES:
The sympathetic and parasympathetic ns influence motility and secretion by modifying activity of intrinsic nerve plexuses:
They alter hormone secretion
They act on smooth muscle and glands
Sypathetic- slows digestion as eating is not important in fight or flight
Parasympathetic- vagus nerve- increases SM motility and hormone and enzyme production
RECEPTORS IN THE TRACT:
Chemoreceptors - senses chemical changes in blood
Mechanoreceptors- senses mechanical pressure or distortion
Osmoreceptors- regulate sodium and water retention- prevent diarrhoea
GI hormones are secreted into the tract by endocrine glands
e.g.
gastrin- Gastrin is a peptide hormone primarily responsible for enhancing gastric mucosal growth, gastric motility, and secretion of hydrochloric acid (HCl) into the stomach
insulin and glucagon - oth hormones come from your pancreas — alpha cells in your pancreas make and release glucagon, and beta cells in your pancreas make and release insulin. The difference is in how these hormones contribute to blood sugar regulation. Glucagon increases blood sugar levels, whereas insulin decreases blood sugar levels§
What formulations of drugs can be absorbed in the mouth?
Sublingual
buccal
