DIGESTIVE SYSTEM LECTURE 1

Structure of a tooth

Superficial to deep: Enamel/Cementum → Dentin → Root canal/Pulp cavity → Pulp → BV

Superior to inferior: Enamel → Dentin → Pulp cavity → Pulp → BV → Cementum → Root canal

Crown → Neck → Root

Location of a tooth

Sockets of alveolar processes of mandible and maxillae
Alveolar process covered by gingivae (gums) extending slightly into the socket
Sockets are lined w/ periodontal ligaments/membrane

Periodontal ligaments/membranes

Dense fibrous CT that anchors teeth to socket walls (alveolar bone) and shock absorber during chewing
Rapid turnover

Dentin

Calcified CT which gives the tooth its basic shape and rigidity
Harder than bone due to higher content of hydroxyapatite (70% vs 55% DW)

Enamel

Covers dentin of crown
Primarily crystalline rods/prisms of calcium phosphate and carbonate
Hardest tissue of body (95% DW)
No cells & sensation 
Protects against acids and wear and tear

Cementum

Covers dentin of roots
Calcified CT which attaches roots to periodontal ligaments

Pulp cavity

Lies within the crown and filled with pulp
Narrow extensions called root canals run through roots

Pulp

Soft CT that contains BV, nerves, and lymphatics
Contains odontoblasts lining the pulp cavity and root canals

Root canals

Opening at its base called apical foramen for BV, nerves, and lymphatics 

Tongue

Skeletal muscle covered w/ mucous membrane
Divided into symmetrical lateral halves by medial septum
Contains extrinsic and intrinsic muscles, and covered w/ papillae on dorsal and lateral surfaces

Extrinsic muscles

Originate outside the tongue and insert into CT in tongue
Move tongue side to side, n’n’out to position food for chewing, shaping, and forcing food to the back for swallowing
Form the floor of the mouth and hold it in position 

Intrinsic muscles 

Originate in and insert into CT in tongue
Alter shape and size of tongue for speech and swallowing

Papillae

Projections of lamina propria covered w/ stratified squamous epithelium
Contains taste buds, receptors for touch, and increases friction for easier manoeuvrability of food

Major salivary glands

Parotid, sublingual, and submandibular 
Secrete saliva following parasympathetic stimulation induced by anything related to food

Parotid glands

Located inferior and anterior to the ears
Serous only containing salivary amylase

Submandibular glands

Medial, partly inferior to mandible, and in the floor of mouth
Mixed only containing amylase in a thicker fluid

Sublingual glands

Beneath the tongue and superior to submandibular glands
Mixed but mostly mucous containing a small amount of salivary amylase in a very thick fluid

Saliva

= H₂O + mucous + enzymes (amylase which breaks down starchy debris around teeth & lysozyme which is antibacterial)
99.5% H₂O & 0.5% solutes

Components of digestive system

Separated into the GI tract and accessory digestive organs

Gastrointestinal tract

Mouth → Most of pharynx → Oesophagus → Stomach → Small intestines → Large intestines

Continuous tube from the mouth to the anus

Accessory digestive organs

Teeth, tongue, salivary glands, liver, gall bladder, and pancreas
Only teeth and tongue physical touch food

Functions of digestive system

Digestion, absorption, secretion, and transportation

Digestion

Chemical and mechanical breakdown of food into absorbable molecules but requires secretion

Absorption

Fluids, ions, electrolytes, and products of digestion passing through epithelial cells lining lumen of GI tract
Absorbed substances pass into blood or lymph for circulation

Secretion

Walls of GI tracts and accessory digestive organs secrete total 7L of H₂O, acids, buffers, and enzymes into lumen to aid digestion

Transportation

Alternating contractions and relaxations of smooth muscle in walls of GI tract move contents towards anus

Motility

Ability of GI tract to mix and move material along its length

Layers of GI tract

Mucosa → Submucosa → Muscularis externa → Serosa/Adventitia

Lower oesophagus to anal canal has same basic four-layered arrangement of tissues
Other regions differ in mucosa, other layers less variable

Mucosa

Mucous membrane that lines the GI tract

3 layers; Epithelium → Lamina propria → Muscularis mucosae

Mucosa (Epithelium)

Layer in direct contact with contents of GI tract

Can serve as protection, absorption, secretion, or combinations

Mucosa (Lamina propria)

Soft fibrous bed of loose areolar CT containing BV, nerves, and LV

Provides routes for absorption
Supports and binds epithelium to muscularis mucosae
Populated w/ defensive cells (MALT)

Mucosa-associated lymphatic tissue

Prominent lymphatic nodules that contain immune system cells that protect against disease
Present all along GI tract

Mucosa (Muscularis mucosae)

Two layer of smooth muscle fibres; inner circular and outer longitudinal

Provides mucosa movement independent of muscularis external (squeeze secretions out of glands)
Increases surface area by creating many small folds

Submucosa

Thick bed of loose areolar CT containing BV, LV, nerves (Submucosal plexus), glands, and lymphatic tissues

Connects mucosa to muscularis external but allows movement between

Muscularis externa

Two layers to produce peristalsis; inner circular and outer longitudinal, and between are nerves (Myenteric plexus)

Serosa/adventitia

When in contact with body cavity → Serosa. If not → Adventitia

Serous membrane of outer mesothelium on a bed of areolar CT