Human Bio - Exam 3

Digestive System Functions

• Absorbs nutrients, electrolytes, and water

• Converts food to ATP and products for new and existing tissues

Digestive Processes

• Motility - Muscles move contents (via mixing and propulsion) in digestive tract. Longer tract (typically 4.5 m or 15 ft long) allows more digestion and absorption

• Secretion - Includes endocrine (for integrated function), paracrine (local changes), neurocrine (nerve endings), and juxtacrine (for pathological circumstances)

• Digestion - Breaking complex molecules to simple absorbable ones

• Absorption - Absorbable units transferred from lumen of tract to blood/lymph

Digestive Tract Wall Layers (outer to inner)

(Ducts of accessory glands like liver and pancreas empty into tract lumen)

• Body wall

• Peritoneum

• Mesentery

• Serosa

• Muscularis externa

  • Outer longitudinal muscles

  • Myenteric Plexus

  • Inner circular muscles

• Submucosa

  • Submucosal plexus

• Mucosa

  • Mucus Membrane

  • Lamin Propria

  • Muscularis Mucosa

Neurohumoral System

Nutrients trigger two paths:

• Special senses → Dorsal Vagal Complex → Vagus Nerve

• Chemo/mechanosensitive nerve endings → ENS

These paths trigger stomach, intestines, pancreas, gallbladder, and sphincter to change secretion and motility

Intrinsic Nerves (Enteric Nervous System)

• Consists of sensory neurons, interneurons, and efferent neurons (to smooth muscle, exocrine, and endocrine cells)

Myenteric Plexus

• Control muscularis externa along tract

• Stimulatory neurons - ACh

• Inhibitory neurons - Nitric oxide

• Ascending/descending interneurons - Ach and Serotonin (5-hydroxytryptamine)

• Sensory neurons - Substance P

Submucosal Plexus

• Regulate secretion of fluids/electrolytes and muscularis mucosa

• Promotes vasodilation

• Noncholinergic neurons - VIP

• Cholinergic neurons - ACh

• Sensory neurons - Substance P

Extrinsic Nerves

Sympathetic Innervation (slows contraction/secretion)

• Postganglionic adrenergic nerves (prevertebral ganglion)

• Synapse enteric nervous system, glands and smooth muscles

Parasympathetic Innervation (enhances digestion)

• Preganglionic CN X fibers

• Synapse myenteric plexus of stomach, small intestine, cecum, ascending, and transverse colon

• Rest of colon innervated by pelvic nerve

Autonomous Smooth Muscle Function

• Pacemaker cells (Cajal) and gap junctions of muscularis externa

• Slow waves (cyclic membrane potential fluctuations) do not trigger action potential normally

• Presence of food moves starting point of slow waves near threshold and causes rhythmic cycles of contraction

GI Hormones

• Gastrin (G cells) target CCK-B receptors (G-protein receptor → calcium release)

• CCK (I cells) target CCK-A and CCK-B receptors (G-protein receptor → calcium release)

• Secretin (S cells) target secretin receptors (G_s-protein receptor → cAMP release)

• GIP (K cells) target GIP receptors (G_s-protein receptor → cAMP release)

• Motilin (M cells) controls motor complex in tract

• Other hormones include enteroglucagon, pancreatic peptide, and pYY

• Only gastrin produced in antrum; all hormones made in duodenum and jejunum; only CCK and secretin produced in ileum

Paracrine and Immune Mediators of GI Tract

• Made from non-excitable cells in lamina propria to fine-tune humoral and neural regulation

• Histamine (ECL and mast cells) secretes gastric acid and intestinal chloride

• Serotonin (enterochromaffin cells) responds to nutrients

• Somatostatin (D cells) is stored in nerve cells and has inhibitory effects in tract

• Prostaglandins (subepithelial myofibroblasts) and adenosine (various cells) regulates vascularity and causes intestinal secretion

Short vs Long Reflex

Short Reflex

• Influences motility and secretion in response to change

• In digestive tract walls

• In stomach, stretch receptors trigger ENS for acid secretion

Long Reflex

• Extrinsic nerve system superimposes local controls

• Modifies smooth muscle and glandular activity

• Stomach uses vagovagal reflex

Oral Cavity Components

• Lips - Tactile sensation, guides food to mouth, speech

• Palate - Breathing, chewing, sucking occur simultaneously

• Uvula - Seals off nasal passage when swallowing

• Tongue - Taste and tactile sensation, guides food, speech

• Teeth - Grind/break down food, stimulate taste buds

Saliva

1-2 liters produced per day (0.5-5 mL/min)

Components:

• Water - Helps with dissolving food, speech, and swallowing

• Bicarbonate - Neutralizes gastric acid reflux

• Mucins - Lubrication

• Amylase - Starch Digestion

• Lipase - Lipid Digestion

• Lysozyme, lactoferrin, IgA - Innate and acquired immune protection

• Epidermal and nerve growth factors - Mucosal growth and protection

Salivary Gland Cells

• Acinar Cells - Produces saliva, chloride, and sodium ions

  • Parotid glands - Serous (and protein content of saliva)

  • Submandibular glands - Serous and mucous mix

  • Sublingual glands - Mucous (and a small amount of serous)

• Ductular Cells - Sends saliva out of acini, modifies ionic composition (slow rates is hypotonic with plasma; high rates resemble plasma) and prevents backflow

  • Na/K ATPase in basolateral membrane for sodium secretion

  • HCO₃^- for bicarbonate secretion

  • No water reabsorption makes hypotonic saliva

• Myofibroblasts (contractile cells) - Hydrostatic force expels saliva from salivary glands

Salivary Secretion Mechanism Pathway

• Pressure and chemoreceptors (simple reflex; pressure in mouth) or cerebral cortex (conditioned reflex; thinking, seeing, smelling food) →

• Salivary centers in medulla* →

• Autonomic parasympathetic nerves →

  • Otic ganglion → parotid gland (via ACh and VIP on muscarinic receptors)

  • Submandibular ganglion → submandibular gland (via ACh and VIP on muscarinic receptors)

*Inhibited by fatigue, sleep, fear, and dehydration

• Sympathetic innervation from superior cervical ganglion causes more protein release

Pharynx

• Cavity at rear end of throat

• Common passageway for digestive and respiratory tract separating air and food

• Tonsils - Lymphoid tissue in oropharynx walls

Anatomy and Innervation of Esophagus

• Information from pharynx and esophagus relayed via vagus nerve to medulla (nucleus tractus solitarius and dorsal vagal complex)

• Skeletal muscle (upper half) and smooth muscle (lower half) innervated by vagus nerves

• UES (innervated by nucleus retrofacialis and ambiguus of medulla) and LES

• Prevents backflow into esophagus

• Mucosa has stratified squamous epithelium. Lubricates and protects tract

• Retrograde movement (belching and vomiting) can occur because air presence opens UES

Swallowing Proces

• Food is chewed till suitable to swallow

• Tongue pushes bolus to back of tongue

• Soft palate elevates and epiglottis covers glottis

• Cricopharyngeus muscle relaxes to open UES and let bolus descend in esophagus

• Pressure gradient of tongue and muscles of pharynx push bolus down UES

• Longitudinal contractions bring UES to tongue

• Transverse contractions of pharynx sweep remaining food

Peristalsis

Primary Peristalsis

• Moves bolus along esophagus with help of gravity

• Each wave is ~ 10 seconds long

• Contraction varies along esophagus

• Vagovagal reflex (vagus nerve to dorsal vagal complex) activates enteric neurons above bolus to release ACh and contract muscle and enteric neurons below bolus to release NO and relax muscle

Secondary Peristalsis

• For smooth muscles to clear bolus not expelled from primary wave or to clear acid

• Also uses vagovagal reflex

LES Relaxation

• Normally contracted

• Tone is increased by neurohumoral agents (ACh and gastrin)

• Relaxation occurs by vagus nerve through release of NO and VIP

GERD

• Caused by retrograde flow from inappropriate relaxation of LES or gastroparesis

  • Obesity (high BMI) also relaxes LES

• Barrett’s esophagus (damage from acid reflux) can cause cancer

• Manifestations include heartburn, regurgitation, and dysphagia

• Symptoms are damage to mucosal lining of esophagus, water brash, a globus, odynophagia, and nausea

• Treated with diet changes, no lying down after eating, antacid to neutralize acids, histamin-2 receptor agonists and proton pump inhibitors

• Surgically can narrow LES diameter or alter gastroesophageal junction

• GERD can cause erosion of enamel and dentin lingual surfaces and incisal edges of maxillary anterior teeth; can be fixed by placing lots of crowns

Esophageal Varices

• Caused by cirrhosis and portal hypertension (high blood resistance)

• Variceal hemorrhage can occur

• Blood diversion due to resistance of collateral vessels (thin walled and supported by connective tissue)

• Pressure dilates collateral vessels causing mucosa protrusion

• Swallowing and acid reflux causes trauma

Treatments

• Insert balloon device in esophagus

• Rubber rings around dilated vessels

• Surgical bypass

Stomach Functions

• Stores food and empties into intestine for digestion and absorption

• Secretes HCl and digestion proteins

• Food is pulverized to liquid as chyme

Gastric Cells

Fundus Region (Parietal Glands)

• Parietal cells - HCl and intrinsic factor (binds to Vitamin B12 allowing it to be absorbed; lack of it causes pernicious anemia). HCl cleaves pepsinogen to pepsin (pepsin also converts pepsinogen to pepsin; auto-catalytic), denatures proteins, and kills microorganisms

• Chief cells - Inactive pepsinogen (exocytosis of zymogen granules)

• Enterochromaffin (ECL) - Histamine

• Surface mucous cells - Mucous (alkaline lubricant protecting stomach lining from mechanical harm, pepsin, and acid)

• Anchored stem cells - Parietal, chief, ECL, and mucous cells

Antral Region (Pyloric Gland)

• G cells - Gastrin (released from peptide in diet with help of Gastrin Releasing Peptide or GRP)

• D cells - Somatostatin (released when pH is below 3 to stop acid secretion)

• Mucous secreting cells - Mucous

Stomach Innervation

• Vagovagal Reflex - Afferent sent through vagus nerve to dorsal vagal complex

• Processed in hypothalamus and sent back through vagus nerve

• Visceral input from nucleus tractus solitarius

• ENS controls secretion activity

Postprandial Secretion

• Cephalic

  • Preparation for receiving food from sight, smell, and taste

  • Vagovagal reflex releases GRP and ACh to activate gastrin, which activates parietal and chief cells

• Gastric

  • Vagal activity

• Intestinal