Digestive System: General Anatomy, Oral Cavity, Salivary Glands, and Esophagu s

Organs of the alimentary Canal

• composed of:

  • lips

  • oral cavity

  • oropharynx

  • laryngopharynx

  • esophagus

  • stomach

  • small intestines

  • large intestines

  • anus

• Glands and organs involved in producing and secreting substances into the alimentary canal

  • salivary glands

  • pancreas

  • liver and gall bladder

Functions of the Alimentary canal + Associated Organs

• Ingestion

  • lips and oral cavity/teeth

• Mechanical Breakdown

  • Oral Cavity/teeth

  • Stomach

• Propulsion

  • oral cavity → anus

• Digestion

  • stomach

  • small intestine

• Absorption

  • Small Intestine

  • Large Intestine

• Defecation

  • Large Intestine

  • Anus

Peritoneum layers

• The peritoneum is the overall membrane with two parts:

  • Parietal Peritoneum → lines the abdominal wall

  • Visceral → wraps around organs

  • Peritoneal cavity → space in between them filled with serous fluid to reduce friction

Mesentery

• double layer of peritoneum (the serous membrane in teh abdominal cavity)

• Functions

  • hold organs in place → keep intestines from tangling or collapsing

  • allow organs movement → organs can still shift sightly when you move or digest food

  • provides pathway → BV, lymphatics and nerves run through the mesentery to reach digestive organs

  • stores fat

• Dorsal → attaches most digestive organs to the posterior abdominal wall

• Ventral → attaches stomach and liver to anterior wall

• Intraperitoneal organs → suspended within the mesentery and surrounded by peritoneum

  • includes most of digestive tract → Small Intestine + Stomach

• Retroperitoneal organs → behind the peritoneum and not completely covered

  • includes pancreas, duodenum (part of the small intestine), and parts of the large intestine

  • lost mesentery during development

• Infection of peritoneum → peritonitis

Four Layers of Alimentary Canal (Mucosa):

• Epithelium

  • stratified squamous → protection

  • simple columnar → secretion/absorption

• Lamina propria

  • Loose connective tissue

    • fibroblasts

    • immune cells and aggregates

    • collagen fibrils

  • Capillaries

    • fenestrated blood

    • lymphatic

  • Smooth Muscle

• Muscularis mucosa

  • Smooth Muscle

    • orthogonal layers (inner circular and outer longitudinal)

Four Layers of Alimentary Canal (Submucosa)

• Loose Connective Tissue

  • fibroblasts

  • immune cells

  • collagen fibers

  • elastic fibers

• Seromucous glands

• Nerve Plexi

• Vasculature

  • blood arterioles and venules

  • lymphatic

Four Layers of Alimentary Canal (Muscularis)

• inner circular layer

• nerve plexi

• outer longitudinal layer

Four Layers of Alimentary Canal (adventita)

• visceral peritoneum for serosa

• connective tissue for adventitia

Muscle control throughout the digestive tract

Skeletal muscle (voluntary control → you can consciously control this)

• Found in:

  • oral cavity

  • upper esophagus

  • anus

• Used for:

  • Mastication (chewing)

  • Swallowing

  • Defecation (pooping)

Smooth Muscle

• involuntary

• works automatically

• found from the lower esophagus to the rectum

• controlled in two ways:

  • Intrinsic → by the enteric nervous system (nerves built into the gut wall)

  • Extrinsic → by the autonomic nervous system (brain and spinal cord via sympathetic and parasympathetic nervous system)

• cells are connected by gap junctions so they contract together like a team → this is called a syncytium

Two layers of smooth muscle → arranged at 90 degrees to each other

Orthogonal Layers (perpendicular)

• Circular Layer → wraps around the tube

  • squeezes the tube to narrow its diameter

• Longitudinal Layer → runs along the tube’s length

  • shortens the tube

Peristalsis (moves food around like a wave)

• a coordinated movement:

  • circular muscles contract behind the food → squeezing it forward

  • Longitudinal muscles contract ahead of the food → shortening the path

  • together they both push food along the GI tract

Segmentation (mixes food in place)

• Circular Muscles contract in alternating sections

• churns food in place to help with mixing and absorption, especially in small intestine

Enteric Nervous System (ENS) = the gut’s built-in nervous system

• works on its own (intrinsic control system)

  • gut moves and responds without conscious input

  • can be influenced by the brain and spinal cord (extrinsic control)

• ENS structure

  • made of ganglia (cluster of nerve cell bodies) and plexi (nerve networks)

    • Submucosal (Meissner’s Plexus)

      • found in the submucosa layer

      • helps controls secretion and blood flow

    • Myenteric (Auerbach’s Plexus)

      • found between muscle layers (in the muscularis extrema)

      • Helps control muscle contractions (peristalsis and segmentation)

• Sensory Receptors in the gut wall

  • Chemoreceptors: detect what’s in the food (acidity, nutrients, etc.)

    • found in mucosa (epithelium + lamina propria)

  • Mechanoreceptors: detect stretching of the gut wall when food enters

    • found in the muscle layers

Reflexes: How Signals Travel

• Short Reflexes (local control)

  • stay entirely within the gut

  • no brain involvement

  • controlled by enteric plexuses

  • Used for

    • segmentation

    • peristalsis

  • Happen in:

    • Esophagus

    • Stomach

    • Small and large intestine

• Long Reflexes

  • involves the brain/spinal cord

  • begin with sensory nerves in the gut

  • Signal → CNS → gut

  • CNS sends motor signals via

    • Parasympathetic nerves → stimulate digestion

    • Sympathetic nerves → inhibit digestion

• Brain can override or alter enteric activity as needed.

General Controls of Digestive Activity

• Neural and Hormonal Control

  • Neurons inside (ENS) and outside (CNS)

  • Also uses:

    • Hormones (like gastric, secretin)

    • Paracrine signals (act nearby)

    • Neurocine signals (neurotransmitter - like)

• Effectors

  • Glands

    • secrete things like saliva, mucus, acid, enzymes and bile

    • stimulate secretion from epithelial cells and smooth muscle along/around ducts

  • Smooth Muscle

    • Contracts to move food and mix it

• Stimuli that trigger these responses:

  • Chemical Stimuli

    • detected by chemoreceptors in mucosa and lamina propria

    • Senses:

      • pH

      • Osmolarity → how concentrated something is

      • Food Components → amino acids, sugar, and fatty acids

  • Mechanical Stimuli

    • detected by mechanoreceptors in the muscularis externa

    • senses stretching when food enters

• Nerve Signals

  • Receptors send afferent signals to:

    • Enteric nervous systems (local reflexes)

    • Central nervous systems (long reflexes)

    • Travel via:

      • Vagus Nerve (parasympathetic)

      • Sympathetic nerves

Oral Cavity - Functional Anatomy (Boundaries and Lining)

• The oral cavity is bordered by:

  • lips in the front (anterior)

  • oropharynx in the back (posterior)

  • Hard and soft palate on top (superior)

  • tongue on the bottom (inferior)

  • cheeks on the sides (lateral)

• Lined mostly by stratified squamous epithelium

  • protects against abrasions from food

  • keratinized - like areas (tougher) in:

    • parts of the tongue + gum

Oral Cavity - Functional Anatomy (Lips and Cheeks)

• Made of skeletal muscle to help move and contain food

  • Orbicularis Oris muscle = controls the lips

  • Buccinator muscle = controls the cheeks

• Helps forms the oral vestibule (the space between the gums/teeth and lips/cheek) → like a pocket for holding food

Oral Cavity - Functional Anatomy (Roof of the Mouth)

• helps with chewing (mastication)

• hard palate

  • made of bone (palatine and maxilla)

  • forms the front, firm portion

• Soft Palate

  • made of skeletal muscle

  • moves to close off the nasopharynx during swallowing

• Anchors:

  • Palatoglossal arches: side connections

  • Palatopharyngeal arches: further back

  • Uvula: the small dangling structure

• Oropharynx → the throat is located behind these arches

Oral Cavity - Functional Anatomy (Tongue)

• made of interlacing skeletal muscles for movement

• Intrinsic Muscles

  • located within the tongue

  • change tongue shape (e.g. curling or flattening)

• Extrinsic Muscles

  • connected to bones (mandible, hyoid, styoid)

  • move the tongue in and out or side to side

  • named as glossus muscles (e.g. genioglossus)

• Lingual Frenulum

  • a fold of mucosa that anchors the tongue to the floor of the mouth

• Papillae

  • small structures on the tongue’s surface that help move food and house taste buds

  • Filiform:

    • most numerous

    • NO taste buds

    • help create friction

  • Fungiform:

    • scattered across the tongue

    • contain taste buds on top

    • appear as red dots

  • Foliate

    • located on posterior-lateral edges

    • taste buds on the sides

  • Circumvalliate

    • largest

    • found on the back of the tongue

    • have taste buds along the sides

  • Posterior Tongue

    • ends at the terminal sulcus

    • contains lingual tonsils

    • no papillae at this region

    • marks the boundary between oral cavity and the oropharynx

Oral Cavity - Functional Anatomy (Teeth)

• Structure

  • Teeth are made of mineralized bone-like tissue

  • anchored into sockets in the jawbone by connective tissue

• Tooth Layers

  • Enamel (outer layer of the crown):

    • hardest material in the body

    • no cell (acellular)

    • formed during development and protection with saliva

  • Cementum (outer layer of the tooth):

    • cellular

    • bone-like but lacks blood vessels

  • Dentin (under enamel and cementum)

    • has cells (odontoblasts) and tubules for flexibility

    • maintained throughout life

Tooth and Socket Anatomy

• Clinical Crown → visible portion of tooth

• Anatomical Crown → includes part below the gum line, down to where enamel meets cementum

• Cementoenamel junction → where crown and root meet

• Neck → area covered by gum

• Gingival Sulcus → space between tooth and gum

• Gingiva → the gums

• Root → embedded in bone (alveolus in maxilla or mandible)

• Periodontal Ligament → fibers that hold the tooth in place (allows for slight movement)

• Pulp Cavity → center of the tooth with BV and nerves

• Apical foramen → small hole at the root’s tip for nerves/blood vessels to enter

Dentition

• Incisors: sharp front teeth → cutting

• Canines: pointed → tearing

• Molars: flat and broad → grinding/mashing

• Maxillary → upper jaw

• Mandibular → lower jaw

• Tooth Development

  • begin forming in sockets (alveolae)

  • Eruption Timeline:

    • Primary Teeth (deciduous/milk teeth)

    • appears from 6 months to 2 years

    • Order:

      • 4 incisors (2 central, 2 lateral)

      • 2 canines

      • 2 molars

    • Permanent Order (replace primary teeth used in childhood) :

      • 4 incisors

      • 2 canines

      • 4 pre-molars (only in permanent teeth)

      • 6 molars

Saliva

• Cleans the mouth

• maintain tooth enamel

• inhibits microbial growth

• dissolves tastants

• moistens food

• provides proteins for partial digestion and protection

Structure of Salivary Glands

• Secretory Cells (make the saliva)

  • Mucous Cells

    • secrete glycoproteins (like mucin)

    • forms tube-like structures

  • Serous Cells

    • secrete digestive enzymes and proteins

    • form acini (rounded cluster shapes)

• Leaky Tight Junctions between Secretory Cells:

  • let plasma ultrafiltrate pass between them

  • More blood flow = more saliva

• Duct Lining Cells

  • create a passage from secretory cells to the mouth

  • modify electrolytes in saliva

    • reabsorb Na+

    • Secrete K+

What’s in Saliva? (Composition)

• Saliva is a complex mix of water, ions, proteins and enzymes

• Water (main component):

  • Electrolytes

    • Potassium (K+)

    • Sodium (Na+)

    • Calcium (Ca2+)

    • Chloride (CI-)

    • Phosphate (PO43-)

    • Bicarbonate (HCO3-)

  • Metabolic Wastes

    • Urea and Uric Acid

  • Enzymes

    • Amylase: breaks down starches

    • Lipase: begins fat digestion (and helps release flavor)

  • Glycoproteins (mucins)

    • make saliva thick and slippery

    • protect and lubricate the oral cavity

  • Antibacterial proteins

    • Lysozyme: breaks bacterial walls

    • IgA: immunoglobulin antibody

    • Defensins: natural antibacterial peptides

Types of Salivary Glands (Extrinsic and Intrinsic)

Intrinsic Glands

• located inside the oral cavity (e.g in the tongue and soft palate)

• continuously secrete small amounts of saliva

Extrinsic Glands

• Larger, paired glands located outside the oral cavity but connected to it by ducts:

  • Parotid Glands

  • Sublingual Glands

  • Submandibular Glands

3 Major Extrinsic Glands

Parotid Glands

• Location: in front of the ear, between the masseter muscle and facial skin

• Ducts: Opens near the 2nd upper molar

• Secretion: Mostly serous

Submandibular Glands

• Location: Below the jaw (mandible)

• Duct: Opens beneath the tongue

• Secretion: mixed (both serous and mucous)

Sublingual Glands

• Location: Under the tongue, above the submandibular gland

• Ducts: Multiple small ducts open under the tongue

• Secretion: Mostly mucous

Control of Salivation

• controlled by the nervous system + it changes based on activity

• Always a little active

• Parasympathetic Nervous System

  • increases saliva output

  • triggers serous, watery, enzyme-rich saliva

  • Does this by:

    • increasing blood flow to glands

    • stimulating secretory glands

• How it gets triggered

  • Chemoreceptors and mechanoreceptors in ur mouth send signals to the salivatory nuclei in your brainstem

  • Also stimulated by higher brain input like

    • thinking about food

    • seeing or smelling food

• Sympathetic Nervous System

  • Decreases saliva volume

  • produces thicker, mucous-rich saliva

  • reduces blood flow to glands

Esophagus: Location and Path

• esophagus is a muscular tube that connects the throat to the stomach

• Position: runs behind the larynx and trachea (windpipe)

• travels through inside the mediastinum (central area of the chest cavity

• passes through the diaphragm at a hole called esophageal hiatus

• enters the abdomen and bends slightly to join the stomach at the cardiac orifice

Gastroesophageal Junction

• sphincter-like area that:

  • allows food into the stomach

  • acts more like a valve than a tight ring → prevents backflow (acid reflux)

Wall Structure of the Esophagus

• same 4 basic layers as the GI tract

• Mucosa (Innermost Layer)

  • Stratified squamous epithelia: thick protective lining to resist abrasion against food

  • Lamina propria: connective tissue below the epithelium

  • Muscularis Mucosa: a thin layer of smooth muscle

• Submucosa

  • contains mucosa glands to lubricate the esophagus and ease passage of food

• Muscularis externa

  • middle muscle layer

  • two layers:

    • inner circular

    • outer longitudinal

  • Muscle type changes as you move down:

    • Upper Third: Skeletal Muscle (voluntary control)

    • Middle Third: Mixed

    • Lower Third: Smooth Muscle (involuntary)

  • Includes sphincters:

    • Upper esophageal sphincter: between throat and esophagus

    • Lower esophageal sphincter: between esophagus and stomach

• Adventitia

  • outer layer

  • a layer of connective tissue that anchors the esophagus to surrounding structures → no serosa here because it’s in the thorax, not the abdominal cavity

Swallowing

Deglutitution

• coordinated action involving more than 20 muscles and

Buccal Phase of Swallowing

• voluntary - happens in the mouth

• Steps:

  • food is chewed, moistened and shaped into a bolus (soft mass)

  • Tongue pushes the bolus towards the back of the mouth (oropharynx)

  • Upper Esophageal Sphincter closes to prevent air from entering the esophagus too early

Paryngeal - Esophagus Phase (Involuntary → happens in throat/esophagus)

• automatic and controlled by swallowing centers in the brainstem

• Involves both skeletal and smooth muscle:

  • Skeletal: in the tongue, oral cavity, and pharynx

  • Smooth and Skeletal: in the esophagus

• Steps

  • Tongue blocks the mouth to keep food from re-entering

  • The soft palate and uvula rise to seal off the nasopharynx (no food in the nose!)

  • larynx (voice box) rises and the epiglottis covers the glottis → protecting the airway

  • Upper esophageal sphincter opens to let food into the esophagus

  • Pharyngeal muscles contract → pushing food into the esophagus

  • Upper Esophageal Sphincter → closes again to prevent backflow into the throat

  • Peristalsis (wave-like contractions) moves the bolus down the esophagus

  • Gastroesophageal sphincter opens → allowing food into the stomach

  • once the food is in the stomach, the sphincter closes to prevent regurgitation.