ISAP DIGESTIVE SYSTEM I – FROM PLATE TO BOWL

ISAP DIGESTIVE SYSTEM I: "FROM PLATE TO BOWL" Part 1

Learning Outcomes

By the end of this module, you should be able to:

  • Locate and describe the component organs and regions of the alimentary tract.

  • Explain the role of the mesenteries in supporting the gut and linking it with the vascular, lymphatic, and nervous systems.

  • Describe the general structural plan of the alimentary tract.

  • Describe how regional variations in the plan relate to function.

  • Describe the location and structure of the accessory organs of digestion and locate the entry points of their secretions.

  • Explain the interaction of reflex and voluntary control in swallowing, gut movements, and defecation.

Organisation of the Digestive System

Digestive Tract/Alimentary Canal (GIT - Gastro-Intestinal Tract)

  • Mouth (oral cavity)

  • Pharynx (throat)

  • Oesophagus (gullet)

  • Stomach

  • Small Intestine

    • Duodenum

    • Jejunum

    • Ileum

  • Large Intestine

    • Caecum

    • Appendix

    • Colon: ascending, transverse, descending, & sigmoid

    • Rectum & anal canal

    • Anus

Accessory Structures

  • Teeth

  • Tongue

  • Salivary glands

  • Liver

  • Gallbladder

  • Pancreas

Major Functions of the Digestive System

  1. Ingestion: Intake of food.

  2. Mastication (chewing).

  3. Propulsion:

    • Deglutition (swallowing).

    • Peristalsis: rhythmic, wave-like contractions that move food along.

    • Mass movements (specifically in the large intestine).

  4. Mixing: Segmental contractions for thorough mixing of contents.

  5. Secretion: Release of substances like mucus, H_2O, bile, enzymes, and HCl (hydrochloric acid).

  6. Digestion:

    • Mechanical: physical breakdown of food.

    • Chemical: enzymatic breakdown of food.

  7. Absorption: Movement of nutrients from the lumen of the tract into the circulation (blood/lymph).

  8. Elimination (egestion): Defaecation to remove undigested waste.

Histology - Layers of the GI Tract

The alimentary tract is generally composed of four distinct layers:

  1. Mucosal layer

  2. Submucosal layer

  3. Muscularis externa layer

  4. Serosa layer

Layer 1: Mucosa (Consists of 3 Sublayers)

a) Epithelium:
* Non-keratinised stratified squamous: Found in the mouth, oesophagus, & anus, providing protection.
* Simple columnar (secretory): Lines the remainder of the tract, responsible for:
* Secreting enzymes, acid.
* Absorbing nutrients.
* Contains specialised goblet cells that secrete mucus.
* Houses entero-endocrine cells involved in hormonal regulation.
b) Lamina Propria:
* A thin layer of loose connective tissue.
* Contains blood & lymphatic vessels and lymphoid tissue (e.g., MALT for immune function).
c) Muscularis Mucosae:
* A thin layer of smooth muscle.
* Responsible for 'twitching' movements.
* Its tone contributes to the formation of folds in the small intestine, increasing surface area.

Layer 2: Submucosa

  • Composed of loose connective tissue.

  • Contains blood vessels (BVs), lymphatics, lymphoid follicles, and glands.

  • Meissner's plexus (submucosal plexus): Part of the autonomic nervous system (ANS) and enteric nervous system.

    • Controls vasoconstriction.

    • Regulates local movement by the smooth muscle of the muscularis mucosae.

    • Promotes glandular secretions.

Layer 3: Muscularis Externa

  • Generally consists of two primary layers of muscle fibres:

    1. Inner circular layer.

    2. Outer longitudinal layer.

  • Innermost oblique fibers: A unique third layer found specifically in the stomach, aiding in churning.

  • Skeletal muscle: Provides voluntary control in certain regions:

    • Found in the mouth, pharynx, upper oesophagus, & anus.

    • Controls voluntary actions like swallowing & defaecation.

  • Smooth muscle: Provides involuntary control for gut motility:

    • Propels food along by peristalsis.

    • Mixes food by segmentation.

  • Auerbach's plexus (myenteric plexus):

    • Innervates both the circular & longitudinal smooth muscle layers.

    • Receives innervation from both the parasympathetic (PNS) & sympathetic (SNS) nervous systems, as well as the enteric nervous system.

    • Primarily controls gut movements (motility).

Peristalsis & Segmentation

  • Peristalsis: Coordinated waves of contraction and relaxation of the muscularis externa that propel food unidirectionally along the tract.

  • Segmentation: Alternating contractions and relaxations of non-adjacent segments of the intestine, primarily for mixing food with digestive juices and facilitating absorption, rather than propulsion.

Layer 4: Serosa (Visceral Peritoneum)

  • Forms part of the abdominal serous membrane.

  • Visceral peritoneum: Covers organs (e.g., much of the small intestine, stomach).

  • Parietal peritoneum: Lines the interior surface of the body wall.

  • Consists of loose connective tissue covered with simple squamous epithelium.

  • Covers most organs and lines the walls of the abdominal cavity.

  • Secretes serous (serum-like) fluid, which lubricates surfaces and reduces friction.

  • Forms folds that suspend organs within the abdominal cavity.

  • Retroperitoneal organs: Certain organs are covered by peritoneum on only one surface and are considered to be positioned behind the peritoneum. Examples include the kidneys, pancreas, and duodenum.

Peritoneum and Associated Structures

Mesenteries

  • Double layers of peritoneum with a thin layer of loose connective tissue in between.

  • Serve as important routes by which vessels and nerves pass from the body wall to the organs.

  • Greater omentum: Connects the greater curvature of the stomach to the transverse colon.

  • Lesser omentum: Connects the lesser curvature of the stomach & proximal part of the duodenum to the liver and diaphragm.

  • Other named mesenteries include the transverse mesocolon, sigmoid mesocolon, and mesoappendix.

Ligaments

  • Coronary ligament: Connects the liver and diaphragm.

  • Falciform ligament: Connects the liver and the anterior abdominal wall.

The Oral Cavity

  • Boundaries:

    • Anteriorly: Lips.

    • Posteriorly: Fauces (the opening into the pharynx).

  • Divisions:

    • Vestibule: The space between the lips/cheeks and the alveolar processes (gums) with teeth.

    • Oral cavity proper: The area medial to the alveolar processes.

  • Lining: Lined with moist non-keratinised stratified squamous epithelium.

  • Primary Function: Ingestion of food.

Lips and Cheeks

  • Both structures are important in mastication and speech.

  • Lips (labia):

    • Contain the orbicularis oris muscle.

    • Have a thin keratinised stratified squamous exterior; the red/pink colour is due to blood in the dermis.

    • Labial frenula: Mucous folds that extend from the alveolar processes of the maxilla and mandible to the upper and lower lips, respectively.

    • Many facial muscles act to move the lips.

  • Cheeks: Form the lateral walls of the oral cavity.

    • Primarily composed of the buccinator muscle.

    • Contain a buccal fat pad.

Palate and Palatine Tonsils

  • Palate: Forms the roof of the oral cavity.

    • Hard palate: Anterior portion, supported by the maxilla and palatine bone.

    • Soft palate: Posterior portion, composed of skeletal muscle and connective tissue.

    • Uvula: A small, fleshy projection from the posterior of the soft palate.

  • Palatine tonsils: Located on the lateral walls of the fauces, part of the immune system.

Tongue

  • A muscular organ with a free anterior surface and an attached posterior surface.

  • Covering: Moist stratified squamous epithelium.

  • Muscles:

    • Intrinsic muscles: Change the shape of the tongue.

    • Extrinsic muscles: Protrude/retract the tongue and move it side to side.

  • Lingual frenulum: Attaches the tongue inferiorly to the floor of the oral cavity.

  • Anterior part: Contains papillae, some of which house taste buds.

  • Posterior part: Lacks papillae, but has a few scattered taste buds and embedded lymphoid tissue known as the lingual tonsil.

  • Functions: Moves food in the mouth during mastication, crucial for speech, and aids in swallowing.

Mastication (Chewing)

  • Action: Incisors and canines bite or cut off food, while molar-type teeth grind food.

  • Muscles involved: Masseter, temporalis, medial and lateral pterygoids.

  • Mandibular movements:

    • Elevation: Primarily by the temporalis, masseter, and medial pterygoids.

    • Depression: Primarily by the lateral pterygoids.

    • Protraction, lateral and medial excursion: Primarily by the pterygoids & masseter.

    • Retraction: Primarily by the temporalis.

  • Mastication reflex:

    • Controlled by the medulla oblongata.

    • Descending pathways from the cerebrum provide conscious (voluntary) control over chewing.

    • Controls the basic, rhythmic movements involved in chewing.

Salivary Glands

There are three pairs of multicellular salivary glands:

  1. Parotid Gland:

    • The largest salivary gland, secretes primarily serous (watery, enzyme-rich) saliva.

    • Located anterior to the ear.

    • The Parotid duct enters the oral cavity adjacent to the 2^{nd} upper molar.

  2. Submandibular Gland:

    • A mixed gland, secreting more serous than mucous saliva.

    • Located along the inferior border of the mandible.

    • Its duct enters the oral cavity on either side of the lingual frenulum.

  3. Sublingual Gland:

    • The smallest salivary gland, a mixed gland but primarily secretes mucous saliva.

    • Each gland has 10-12 ducts that enter the floor of the oral cavity.

  • Lingual glands: Small, coiled tubular glands distributed over the surface of the tongue, contributing to salivary secretions.