Anatomy of Oesophagus and Swallowing

Oesophagus

• The oesophagus is a muscular tube with outer longitudinal and inner circular muscle layers, which are striated in the upper portion and smooth in the lower portion.
• Peristalsis in the upper portion depends on the vagovagal reflex, while in the lower portion it depends on the local enteric reflex.

Swallowing (Deglutition)

• Swallowing is the propelling of food bolus from the mouth to the stomach, controlled by the swallowing center in the medulla.
• It is divided into three phases:
  • Buccal phase: voluntary elevation and retraction of the tongue against the hard palate, propelling the bolus to the pharynx.
  • Pharyngeal phase: involuntary, rapid (1 second), and occurs reflexly via the swallowing reflex, which involves:
    • Receptor: in the oropharynx (tonsillar pillars).
    • Afferent: glossopharyngeal nerves.
    • Center: medulla oblongata (swallowing center).
    • Efferent: motor fibers of cranial nerves V, IX, X, and XI.
    • Response: series of reflexes to prevent entry of food into air passages, including:
      • Elevation of the soft palate, closing the nasal cavity.
      • Approximation of palatopharyngeal folds, forming a sagittal slit.
      • Closure of the glottis, and elevation of the larynx and folding of the epiglottis.
      • Inhibition of breathing (swallowing apnea).
      • Relaxation of the pharyngoesophageal sphincter and contraction of the superior pharyngeal muscle, resulting in rapid pharyngeal peristalsis.

Saliva

• Saliva has various functions, including:
  • Facilitating speech and deglutition by lubricating food with mucin.
  • Cleaning the mouth by washing and antibacterial effects of lysozymes, thiothianate ions, and immunoglobulins A.
  • Buffering function: maintaining pH at about 7.0 by bicarbonate and phosphate systems, preventing decalcification of teeth.
  • Digestive function: containing ptyalin (salivary α-amylase) that digests starch to maltose, maltotriose, and alpha-limit dextrin.
  • Excretory function: excreting lead, mercury, iodides, fluoride, and some drugs.
  • Facilitating taste sensation: serving as a solvent for molecules that stimulate taste receptors.
  • Containing kallikrein enzyme that produces bradykinin, which acts as a vasodilator during salivary secretion.
  • Regulating water balance: decreasing in dehydration and giving a thirst sensation.

Stages of Salivary Secretion

• I) Salivary acini (primary) → saliva similar in composition to plasma (isotonic).

Gastric Motility

• Filling and storage of food in the stomach:
  • Accommodating up to one liter of food without increasing intragastric pressure due to plasticity of the gastric wall, receptive relaxation, and the law of LaPlace.
• Gastric basic electrical rhythm (BER) (gastric slow waves):
  • 3-5 cycles/min, due to partial depolarization of circular smooth muscle cells in the stomach wall.
  • Some lead to spike potential → peristalsis.
  • Frequency of slow waves determines the rate at which action potentials and contractions occur.
  • Start at the midpoint of the greater curvature (pacemaker of the stomach).
  • Vagal stimulation, gastrin, and motilin increase spike potential rate, while sympathetic stimulation, secretin, and GIP decrease spike potential rate.

Types of Movements of the Stomach

• Tonic gastric waves:
  • Regular weak contractions (3 waves/min) that maintain intragastric pressure and mix gastric secretion with food.
• Receptive relaxation:
  • Reflex relaxation of the fundus and body to receive the bolus of food.

Physiology of Pancreas

• Pancreatic secretion:
  • The pancreas has both endocrine and exocrine glands.
  • Exocrine pancreatic secretion:
    • Volume: 1-1.5 L/day.
    • pH: 7.8-8.3.
    • Osmolarity: iso-osmotic with plasma.
    • Ions: Na+, K+, HCO3-, and Cl-.
    • Types: aqueous alkaline juice and enzyme-rich juice.

Bile Secretion

• Mechanism of secretion:
  • Cholagogue: factors evacuate the bile from the gallbladder and increase flow of bile.
  • Secretin hormone stimulates and sympathetic stimulation inhibits secretion.
• Mechanism of bile secretion:
  • Between meals: the sphincter of Oddi is closed, and the hepatic bile is stored in the bladder to be concentrated and acidified.
  • During food intake: the sphincter of Oddi is relaxed, and the bile is evacuated into the intestine.
  • After a meal: 90% to 95% of bile salts are actively reabsorbed from the terminal ileum back to the liver via the portal vein and re-excreted in the bile, stimulating more bile secretion (enterohepatic circulation).