Lecture 5 -- Functional Anatomy and Motility of the Stomach

Describe the process of swallowing

→ Food are moulded as bolus by tongue + Pushed down the pharynx via voluntary control
→ The soft palate cover the nasal cavity
→ Pressure sensitive sensory cells in the pharynx is simulated
→ Send signal to the swallowing centre in the medulla oblongata → Trigger the swallowing reflex
→ Epiglottis closes the trachea
→ Contraction and relaxation of smooth muscle forces food goes into the oeosphagus
→ Upper oesophageal sphincter close
→ Epiglottis open to allow breathing
→ Peristaltic contraction causes food moves down the oesophagus
→ Lower oesophageal sphincter open to allow the food to go down stomach
→ When the stomach expands, pressure in the abdomen is greater than thoracic pressure → Closure of esophagus (Close immediately to prevent reflux of stomach contents + acid back into the esophagus).

What triggers the swallowing reflex?

Pressure-sensitive sensory cells stimulated in the pharynx lead to activation of the swallowing centre in the medulla.

What can cause swallowing disorders?

1. Failure of soft palette to close off the nasal cavity

2. Failure of epiglottis to close off trachea

3. Botulism → Clostridial toxin block the release of ACh

4. Myaestheania gravis → Antibodies formed against ACh receptors

5. Anaesthesia → Cause inhalational pneumonia

What are the histological anatomy of oesophagus?

1. Mucosal

2. Submucosal

   → Contain blood vessels

3. Muscular

   → Outer: Longitudinal; Inner: Circular
   → Contain striated and smooth muscle

4. Serosa
   → Adventitia in the neck
   → True serosal layer in thorax

Describe the process of vomiting

→ Simultaneous closure of nasal cavity and trachea
→ Deep inspiration → Airs in lungs increase pressure down diaphragm → Increase intra-abdominal pressure
→ Force contraction of abdominal muscle
→ Cardiac sphincter opens
→ Food propelled up oesophagus
→ Upper oesophageal sphincter opens

What initiates the vomiting process?

Stimulation of the vomiting centre in the medulla by pharyngeal or gastric distension/ irritation

Why can horses not vomit?

They have a very developed cardiac sphincter and have lost the reflex to induce vomiting
→ Their stomach rupture before vomitting occur

What does gastric torsion causes?

→ Stomach rotates 90-360 degree → Seal off cardiac sphincter → Prevent vomiting → Stomach dilatation → Impair venous return to heart from caudal vena cava → Hypovolemic shock
→ Rotation compromise blood supply → Oedema + Hypotoxic + Necrosis

Which embryological regions is the horse's corpus derived from?

Fundic region

Which embryological regions is the horse's fundus derived from?

Oesophageal region

Which part of the ruminant stomach is derived from the oesophageal region embryologically?

Foregut

What are the anatomical regions of stomach?

1. Carida

2. Fundus

3. Corpus

4. Pylorus

Which part of the dogs and cats’ stomach is derived from the oesophageal region?

None

Which part of the dogs and cats’ stomach is derived from the fundic region?

Fundus and corpus

What are the embryological regions of stomach?

1. Oesophageal region: Produce nothing but stratified squamous epithelium

2. Cardiac region: Produce mucus

3. Fundic region: Produces mucus, HCl and pepsinogen

4. Pyloric regions: Produce mucus + Regulate stomach emptying

What are the four types of cells in stomach?

1. Parietal cells: Secrete HCl → Provide acidic environment for pepsinogen to convert to pepsin

2. Entero-endocrine cells: Secrete hormones

3. Chief cells: Secrete pepsinogen (Inactive form)

4. Goblet cells: Secrete mucus to protect the stomach lining

Why is pepsinogen secreted instead of pepsin in the first place?

Most of the cells in GI tract proteins → Prevent self-digestion

Describes the motility of stomach

→ Weak contraction from the fundus
→ Propagate down the corpus
→ Pyloric sphincter open to allow food enter the duodenum
→ When contraction reach pylorus, pyloric sphincter close
→ Food forces back to corpus for mixing

What stimulates stomach emptying?

1. Distention of stomach → Increase activity of stretch-sensitive sensory cells → Increase contraction of smooth muscle cells → Increase stomach emptying

2. Increase peptide in stomach → Increase gastrin released by enter-endocrine cells → Increase contraction of smooth muscle cells + Dilation of pyloric sphincter→ Increase stomach emptying

What factors inhibit stomach emptying?

High fat/ peptide concentration + Low pH + High osmolarity + High pressure (Too much food coming through)

Describe the process of inhibition of stomach emptying

High fat/ peptide content + Low pH + high osmolarity + high pressure stimulate…
1. Increase sensory cells in duodenum → Increase sympathetic neural fibre activity to stomach + Decrease parasympathetic neural fibre activity to stomach via vagus nerve → Decrease stomach emptying
2. Increase release of hormone from duodenal epithelium e.g. CCK + GIP and secretin