The monogastric stomach and oesophagus

Which main food group digested in the mouth and oesophagus?

Starch - carbohydrate (salivary amylase)

How does the process of swallowing occur?

Voluntary control:

• Food molded into a bolus by tongue

• Bolus moved upwards and backwards to pharynx

• Forces soft palate to seal off nasal cavity Involuntary control:

• Pressure-sensitive sensory cells stimulated

• Swallowing centre in medulla stimulates swallowing reflex

• epiglottis closes off trachea Contraction & Relaxation forces bolus into oesophagus

How can the involuntary swallowing reflex be used to administer tablet-form medication (specifically to cats)?

If a tablet is placed at the back of the cat's mouth

What are the 4 layers of tissue in the GI tract?

mucosa submucosa muscularis serosa (true serosa in thorax

How is the oesophagus sympathetically innervated?

via the cervical sympathetic chain

How is the oesophagus parasympathetically innervated?

How is food transported down the oesophagus?

• Upper oesophageal sphincter closes

• Epiglottis opens to allow respiration

• Peristaltic contractions

• Lower oesophageal sphincter opens

• Food moves into the stomach

What is the alternative name for the lower oesophageal sphincter?

Cardiac sphincter (physiological sphincter not anatomical)

At what stages is the cardiac sphincter open/closed?

Always closed

How is the cardiac sphincter closed?

Pressure in abdomen > pressure in thorax -> stomach exerts pressure on diaphragm -> closes sphincter

How does the process of vomiting/emesis occur?

• Deep inspiration (trachea and nasal cavity close) increases abdominal pressure

• Forceful contraction of abdominal muscles

• Cardiac sphincter opens

• Antiperistalsis -> food forced back up towards mouth (oral direction)

• Upper oesophageal sphincter opens

How is vomiting/emesis controlled and stimulated?

Stimulated by gastric or pharyngeal distension/irritation Controlled by vomiting centre in medulla

What occurs in gastric torsion

• Stomach rotates 90-360 degrees

• Seals off cardiac sphincter

What impact can gastric torsion have on the gastric tissue and heart?

• Blood supply to gastric tissue may be compromised

Hypoxia

deficiency in the amount of oxygen reaching the tissues

What is the ruminant equivalent of the simple stomach?

Abomasum

What are the 4 functions of the simple/monogastric stomach?

• Digestion -> Continuation of starch digestion

initiation of protein digestion

• Mechanical breakdown -> breaks down food and mixes with gastric juices to form chyme

• Protection -> Stomach acid kills many bacteria

• Storage -> reaches small intestine at a controlled rate

Chyme

A mixture of food and gastric juices

What are the 4 cell types in the siumple/monogastric stomach?

Mucous/goblet cells Parietal/oxyntic cells Chief/peptic cells Entero-endocrine cells

Goblet cells

Mucous-secreting cells that protect against stomach acid

Parietal/oxyntic cells

Secret HCl to digest proteins

Chief/peptic cells

Secrete pepsinogen (inactive pepsin) to digest proteins

Entero-endocrine cells

Secrete hormones

What is the function of the motility of the stomach?

• Mix and mechanically break down chyme

• Empty stomach contents into small intestine

• Prevent regurgitation of stomach contents into oesophagus

What happens to stomach smooth muscle when an animal starts eating?

Initial period of relaxation This is regulated by the swallowing centre

Describe motility of the stomach

as the chyme moves through it

How is stomach emptying stimulated neurally and hormonally?

Dilation of stomach causes stimulation of stretch-sensitive sensory cells This causes contraction of smooth muscle and increased gastrin secretion (hormonal)

What does the release of gastrin do?

• Increases strength of contraction

• Dilates pyloric sphincter

How is stomach emptying inhibited?

Duodenum inhibits gastric contractions

• Increased pressure in duodenal walls

osmolarity

A measure of total concentration of all solute particles in a solution -> determines osmotic pressure

What conditions does amylase favour (pH)?

pH > 6

Which enzyme acts on starch?

Amylase

Which enzyme acts on protein?

Pepsin

What is the inactive form of pepsin?

Pepsinogen

What are the componenets of starch?

Amylose and amylopectin Basic unit -> maltose (glucose) Joined by α-glycosidic bonds

Can α-glycosidic bonds be broken down by amylase?

Yes NOT β-glycosidic bonds can't - found in cellulose

What is the structure of amylose?

Double helix of maltoses joined by α-glycosidic bonds Unbranched

What is the structure of amylopectin?

Long

What is the first stage in carbohydrate/starch digestion?

Salivary amylase (pigs

Amylase favours a more alkaline pH (>6). How does some salivary amylase continue to act in the stomach?

Acid is secreted from the walls of the stomach. This means there is a gradual decline in pH from the centre to the periphery of the stomach

Which type of diet contains high levels of starch?

Herbivorous -> Low Omnivorous -> High (e.g. pigs) Carnivorous -> Low

Working horses have higher levels in starch in their diets so have more salivary amylase

Why is pepsinogen (inactive) secreted instead of pepsin?

Organs are made of protein

What activates pepsin?

HCl or pepsin (autocatalysis)

NB: can also act on itself as it is a protein

What stimulates pepsin secretion in the stomach?

• Presence of peptides in the stomach

• Neural stimulation via the parasympathetic nervous system via the vagus nerve

• Gastrin

Secretin

Digestive hormone that stimulates the pancreas to release bicarbonate (HCO3-) to neutralize acid in duodenum.

Stomach ulceration

Breach of the mucosal barrier

What is the function of HCl in the stomach?

• Activates pepsinogen

• Provides acidic environment for protein digestion by pepsin

• Prevents fermentation by killing microorganisms

How is HCl transported into the lumen using parietal cells?

H+/K+ transporter actively transports H+ into lumen from parietal cell Cl- diffuses into lumen using secondary active transport (using HCO3-)

(the opposite process happens in the pancreas to reduce acidity and protect the duodenum from acid)

How much HCl is secreted and for how long?

Duration and volume depend on species

Alkaline tide

When urine pH increases after a meal due to the delay between food passing from the stomach to the pancreas

What effect does pepsin have on protein?

Degrades protein and collagen by breaking peptide links between amino acids

What stimulates H+ secretion in the stomach?

Gastrin

• Activation of ECL cells to secrete histamine Acetylcholine and histamine

• Direct stimulation using receptors Reflex arcs

• Long via vagus

• Short locally

What stimulates mucin secretion by goblet cells in the stomach?

Goblet cells in the stomach continuously secrete mucin Goblet cells in the intestines secrete mucin upon stimulation

ECL cell

Histamine-secreting cell

What are the non-specific mechanisms of protein digestion in the stomach?

HCl and pepsin (broad specificity)

What is the cephalic phase of stimulation?

Associated with the head Neural stimulation Caused by sight

What is the gastric phase of stimulation?

Associated with the stomach Neural stimulation Stomach expansion (detected by sensory cells in stomach) and peptides in lumen cause secretion of HCl and pepsinogen via:

• acetylcholine (direct)

• gastrin in blood (indirect)

What is the intestinal phase of stimulation?

Associated with the intestines Neural AND Hormonal stimulation/inhibition Once chyme has entered the duodenum

Is it mostly inhibition or stimulation of secretion in the intestines?

Inhibition Reduces release of gastric juices

CCK

cholecystokinin Breaks down fats and proteins into smaller parts Stimulates release of bile and pancreatic enzymes