GI anatomy

Gastrointestinal Physiology

General histological organization of the GI tract

• 5 layers (from inner most to outermost layers)

  • Mucosa

    • mucous epithelium

    • lamina propia

    • muscularis mucosae

  • Submucosa

  • Muscularis

    • circular muscle layer

    • longitudinal muscle layer

  • Serosa

    • connective tissue layer

    • peritoneum

What modulates the GI track?

• Enteric nervous system (ENS) → nerve network located in the GI tract and controlls the motility and secretory functions of it

  • also known as the “little brain of the gut”

  • contains sensory mechanoreceptors and chemoreceptors

  • Composed of 2 plexuses:

    • Myenteric plexus (Auerbach’s plexus)→ found in the muscularis externa

      • controls motility function of the GI track

    • Submucosa plexus (Meissner’s plexus)→ found in the submucosa (duh)

      • controls glandular secretions

• Autonomic Nervous System (ANS) → composed of two systems

  • Parasympathetic nervous system → stimulates GI activity

  • Sympathetic nervous system → inhibits GI activity

• Diffuse neuroendocrine system (DNES) → composed gastroendocrine and enteroendocrine cells

  • secretes endocrine hormones that modulate GI activity

General functions of the GI tract and their locations

• Motility → esophagus, stomach, sphincters, small & large intestine

• Secretion → mouth, stomach

• Digestion → mouth, stomach & small intestine

• Absorption → small & large intestine

• Excretion

Motility

• Peristalsis → wave-like contractions that push the bolus down the esophagous to the large intestine

  • Sties → esophagus, distal stomach, small & large intestine

  • Main function → propulsion

    • driving or pushing forward

• Rhythmic segmentation → contractions on either side of the intestines that aid in mixing of foor

  • Sties → small & large intestine

  • Main function → mixing

• Tonic contractions → consists of prolonged contractions that segment off in the sphincters

  • Sties → sphincters & proximal stomach

  • Main function → propulsion

Digestion overview

Oral cavity

Anatomy

Mouth

Salivary glands

• innervated by the ANS

• largely induced by feedforward and visceral receptors

Function

• Ingestion → the concious placement of food in the oral cavity

• Deglutition → conscious process of swallowing.

  • The tongue pushes bolus against the soft plate and the back of mouth, triggering the swallowing reflex

Diggestion

• Mechanical

  • hewing (mastication) and smashing food against the hard palate with the tongue increases its surface area for digestive enzyme activity

• Chemical

  • paired salivary glands secrete the constituents of saliva (water, mucus, ions, buffers, antimicrobics (IgA, lysozyme), enzymes (lingual lipase, salivary amylase))

  • Oprimum pH in oral cavity → 7.4

• What is diggested in the oral cavity?

  • Carbohydrates with salivary amylase

Salivary glands

• gland → secretory product

  • parotid gland → serous fluid

  • sublingual gland → mucous

  • submandibular glands → serous & mucous

• Components of salive → water, mucus, buffers, antimicrobics, lysozymes. iodine, and enzymes

Pharynx & Esophagus

Pharynx

• What passes through it?

  • Food, liquid, and air

Esophagus

Anatomy

Functions

• Primary function is to move the bolus towards the stomach

• secretes mucous to lubricate the esophagus reducing friction to Protect the mucosa of the esophagus.

• Two esophageal sphincters

  • Upper esophageal sphincters (UES) → located in the uper part of the esophagus

    • In charge of preventing air from entering the stomach during respiration

  • Lower esophageal sphincters (LES) → located below the diaphragm

    • Prevents reflux of chum into the esophagus

Pathologies

• Gastroesophageal reflux disease (GERD) → chronic form of acid reflux caused by abnormal/weakened/relaxed LES or increased intra-abdominal pressure

  • Can develop in obesity, pregnancy, hiatal hernia*, gastroparesis

  • Can be treated with…

    • Anti-acids → TUMS

    • H2RAs → Pepcid, Zantac

    • PPIs → Proliosec

    • Surgery

      • MSAD (LINX system) → a small flexible band of interlinked titanium beads with magnetic cores designed to restore the body's natural barrier to reflux

      • Nissen Fundoplication → procedure, where a surgeon wraps the top of the stomach around the lower esophagus (LES)

        • this reinforces the LES sphincter, decreasing the possibility of acid reflux.

• Nissen Fudoplication:

• MSAD (LINX system)

• Hiatal Hernia* → occurs when part of the stomach protrudes up into the chest through the diaphragm (LES).

  • The hernia itself can play a role in the development of both acid reflux and GERD.

  • Weak supportive tissues and increased abdominal pressure can contribute to the condition.

• Chronic Heart burn → chronic erosion of the esophagous and can lead to esophageal cancer

  • It can be treated with anti-acids and proton pump inhibitors

• Esophagitis → inflammation of esophageal lining

  • Caused by GERD, infection, certain medications (e.g., NSAIDs)

  • “Pill-induced” Esophagitis → occurs when capsules or tablets get stuck in the esophagus and cause tissue damage (resulting in inflammation)

• Barrett’s esophagus → metaplasia of the glandular cells of the lower 1/3 of esophageal lining to a simple columnar epithelium w/goblet cells

  • Caused by chronic Esophagitis (normally due to GERD)

  • Cells can become dysplastic and eventually precancerous

    • Dysplastic → term used to describe the presence of abnormal cells within a tissue or organ

      • Dysplastic ≠ Cancerous

      • can be mild, moderate, or severe, depending on how abnormal the cells look under a microscope and how much of the tissue/organ is affected

• Esophageal cancer → 6th most common cancer related rate, only a 15% 5-year survival rate

  • Two forms…

    • esophageal adenocarcinoma → occurs in the esophageal glands

    • esophageal squamous cell carcinoma → occurs in the squamous epithelium

  • May be caused by tabacco (any form), GERD, poor diet, and obesity

The stomach

Anatomy

Why is the histological organization of the stomach unique from the rest of the GI tract?

• stomach has 3 layers of smooth muscle in the muscularis externa

  • inner oblique

  • middle circular

  • outer longitudinal layer

• this allows the stomach to contract in an additional plane and therefore impart a churning effect on the food

4 regions of the stomach

• cardiac (cardia), corpus (body), pyloric (pylorus), fundic (fundus)

Histology

• Gastric glands → located at the bottom of the gastric pits

  • composed of…

    • Parietal cells

      • Pumps H into the lumen & Cl follows (HCl)

      • pH of stomach → ~2-5

        • the acidity of the stomach lills microbes, denatures protein and activates pepsinogen

      • Secretes gastric intrinsic factor (GIF)

        • essential of vitamin B12 absorption

    

    • Chief cells

      • Secretes pepsinogen and gastric lipase

      • pH optima → ~2 and ~5, respectively

        • pepsinogen autoclaves to pepsin (peptidase) at low pH

    • Gastroendocrine cells

      • G cells

        • ↑ gastric activity (mechanical and chemical)

        • induced by AAs, distension, and vagal stimulation

      • D cells (somatostatin)

        • inhibits parietal cels

        • induced by low luminal pH

      • Gr cells (ghrelin)

        • ↑ neuropeptide Y (NPY) in hypothalamus, stimulating appetite

• Surface lining cells

Function

• Digestion

  • Mechanical → peristaltic wave contractions mix the bolus with digestive juices and periodically “squirt” chime into the duodenum

    • Steps of mechanical digestion:

      • Propulsion → peristaltic waves move from the fundus toward the pylorus

      • Grinding → the most vigorous peristalsis and mixing action occur close to the pylorus

      • Retropulsion → the pyloric valve* act as a pump that delivers small amounts of chyme into the duodenum. simultaneously forcing most of its contained material backward into the stomach

    • Pyloric valve* → located at the end of the stomach

      • opens during retropulsion

      • closes during propulsion and grinding

  

  • Chemical → gastric glands produce digestive enzymes and
    gastroendocrine hormones

    • Enzymes (and other gastric juices) are mixed with food forming chyme