Welcome to Ingestion and Oral Health!
Welcome to your study notes for Ingestion, an essential part of Unit A2 7: Oral Health and Dentistry in CCEA Life and Health Sciences. Ingestion is where human digestion begins. We will explore how our teeth and oral cavity physically and chemically process food, examine the detailed anatomy of a tooth, learn the dental formula, and discover the chemical processes behind dental decay and gum disease.
Don't worry if tooth anatomy or chemical formulas feel a bit overwhelming at first! We have broken down each concept into clear, digestible steps with simple analogies and memory aids to guide you through your coursework and revision.
---1. The Process of Ingestion and the Oral Cavity
Ingestion is defined as the consumption and taking of substances (food and drink) into the body through the mouth cavity.
Mechanical Digestion (Physical Breakdown)
Mechanical digestion begins immediately via mastication (chewing):
• Chewing cuts, crushes, and grinds food into smaller pieces.
• Why is this important? Breaking food into smaller pieces increases its surface area-to-volume ratio, which allows digestive enzymes to work much more efficiently.
• It also mixes the food thoroughly with saliva for lubrication.
Chemical Breakdown (Enzymatic Digestion)
While food is being chewed, salivary glands secrete saliva into the oral cavity. Saliva has two key components:
• Salivary Amylase: An enzyme that initiates the chemical breakdown (hydrolysis) of starch into maltose. It works optimally at a neutral to slightly alkaline pH.
• Water and Mucus: Lubricates and binds the masticated food particles together into a smooth, cohesive ball known as a bolus, making swallowing (deglutition) safe and easy.
Common Pitfall to Avoid: Salivary amylase does not digest all carbohydrates or proteins! It only begins breaking down starch into maltose. Once the bolus enters the acidic environment of the stomach, salivary amylase is denatured and stops working.
Key Takeaway: Ingestion combines mastication (increasing surface area) and salivation (starting starch breakdown with amylase and forming a lubricated bolus for swallowing).
---2. Tooth Anatomy and Tissue Structure
A human tooth is divided anatomically into three main regions:
• Crown: The visible portion of the tooth situated above the gumline.
• Neck: The narrow junction where the crown meets the root at the gumline.
• Root: The lower portion anchored firmly into the alveolar bone socket of the jaw.
The Microscopic and Histological Layers of a Tooth
1. Enamel
• The outermost, extremely hard layer covering the anatomical crown.
• It is acellular (contains no living cells) and highly calcified.
• Its primary chemical constituent is hydroxyapatite, a crystalline calcium phosphate mineral with the chemical formula: \(\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2\).
• Important Fact: Enamel cannot biologically heal or regenerate if it is physically damaged or lost because the cells that create it (ameloblasts) degenerate once the tooth erupts!
2. Dentine
• A bone-like, mineralised matrix located directly beneath the enamel and cementum.
• It is permeated by microscopic channels called dentinal tubules.
• Dentine is continuously formed and maintained by specialised living cells called odontoblasts, which line the outer perimeter of the pulp cavity.
3. Pulp Cavity (and Root Canal)
• The living, unmineralised central core of the tooth.
• Contains connective tissue (fibroblasts), blood vessels (supplying oxygen and nutrients to keep the tooth alive), lymph vessels, and sensory nerve endings (which transmit pain signals when the tooth is injured or exposed to extreme temperatures).
4. Cementum
• A specialised, calcified layer of connective tissue covering the outer surface of the tooth root.
5. Periodontal Ligament (PDL)
• A network of tough, fibrous connective tissue fibers that anchors the cementum of the root into the surrounding alveolar bone socket.
• Analogy: Think of the periodontal ligament as a natural "shock absorber" or hammock for the tooth, cushioning the enormous forces generated during biting and chewing.
Quick Review Box:
• Enamel: Hardest outer layer, acellular, made of \(\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2\).
• Dentine: Contains tubules, made by odontoblasts.
• Pulp: Living core with blood vessels and nerves.
• Cementum & PDL: Secure the root into the jaw bone socket.
3. Classification of Human Teeth and Dentition
Humans have heterodont dentition, which means we possess different types of teeth specialised for specific mechanical tasks during ingestion.
The Four Types of Human Teeth
• Incisors (I): Chisel-shaped front teeth with sharp edges, designed for cutting, shearing, and biting off pieces of food.
• Canines (C): Pointed teeth with a single sharp cusp, specialized for gripping, puncturing, and tearing food.
• Premolars / Bicuspids (PM): Teeth with flat occlusal (biting) surfaces and two cusps, designed for crushing and grinding food.
• Molars (M): Large teeth with broad occlusal surfaces and 4 to 5 cusps, designed for fine mastication and thorough grinding before swallowing.
The Human Dental Formula
The dental formula represents the number and type of teeth in one quadrant (half of one jaw) in an adult with a full set of permanent teeth:
\(\text{I}\frac{2}{2}, \text{C}\frac{1}{1}, \text{PM}\frac{2}{2}, \text{M}\frac{3}{3}\)
• Upper quadrant: 2 Incisors, 1 Canine, 2 Premolars, 3 Molars = 8 teeth.
• Lower quadrant: 2 Incisors, 1 Canine, 2 Premolars, 3 Molars = 8 teeth.
• Across all four quadrants (upper right, upper left, lower right, lower left):
\(8 \times 4 = 32\text{ teeth in total}\).
Memory Trick: Remember the order from front to back: Interesting Chefs Prepare Meals (Incisors, Canines, Premolars, Molars) with the numbers 2 - 1 - 2 - 3.
---4. Oral Pathology and Dental Chemistry
Dental Plaque and Biofilm Formation
• After brushing, a thin layer of salivary proteins called the salivary pellicle forms on the teeth.
• Bacteria naturally present in the mouth—predominantly Streptococcus mutans—adhere to this pellicle, multiplying to form a sticky biofilm known as dental plaque.
• When you eat dietary carbohydrates (sugars), these bacteria ferment the sugars through anaerobic respiration, producing lactic acid as a metabolic byproduct.
Demineralisation vs. Remineralisation and the Critical pH
• Demineralisation: When bacterial acid lowers the oral pH, calcium and phosphate ions dissolve out of the tooth enamel mineral (\(\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2\)).
• The Critical pH Threshold: For enamel hydroxyapatite, the critical pH is 5.5. Whenever plaque pH falls below 5.5, demineralisation exceeds remineralisation, leading to enamel breakdown (dental caries).
• Remineralisation: When the acid is neutralised, calcium and phosphate ions naturally present in saliva deposit back into the enamel structure.
The Stephan Curve
The Stephan Curve is a classic graph that shows how plaque pH changes over time after eating sugary foods:
1. Rapid Drop: Within minutes of consuming fermentable carbohydrates, bacteria produce lactic acid, causing the plaque pH to plummet rapidly below the critical pH of 5.5.
2. Demineralisation Phase: While the curve stays below pH 5.5, enamel actively dissolves.
3. Slow Recovery: Over the next 30 to 60 minutes, plaque pH gradually climbs back up to resting levels (around pH 7). This recovery is driven by salivary bicarbonate ions, which act as a natural chemical buffer to neutralise the acid.
Caries vs. Erosion (Crucial Distinction):
• Dental Caries: Tooth decay caused by bacterial fermentation of sugars producing organic acids in plaque.
• Acid Erosion: Enamel loss caused by direct contact with external dietary acids (such as citric acid in fruit juices or phosphoric acid in carbonated soft drinks) without involving bacterial plaque.
Fluoride Chemistry: Why Fluoride Protects Teeth
Fluoride ions (\(\text{F}^-\)) in drinking water and toothpaste play a major protective role against decay:
• Fluoride ions replace the hydroxide ions (\(\text{OH}^-\)) in the hydroxyapatite crystal lattice.
• This forms a new mineral called fluorapatite:
\(\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2 + 2\text{F}^- \longrightarrow \text{Ca}_{10}(\text{PO}_4)_6\text{F}_2 + 2\text{OH}^-\)
• Lower Critical pH: Fluorapatite is far more resistant to acid attack than normal hydroxyapatite. It lowers the critical demineralisation threshold from pH 5.5 down to approximately pH 4.5, significantly protecting teeth from decay.
5. Periodontal Diseases
Periodontal diseases affect the supporting tissues (the periodontium) that surround and anchor the teeth.
1. Gingivitis
• What is it? A mild, reversible inflammation of the superficial gingiva (gums).
• Cause: Plaque accumulation along the gingival margin.
• Symptoms: Redness (erythema), swelling (oedema), and bleeding on probing or brushing.
• Reversibility: Completely reversible with effective oral hygiene practices (e.g., proper brushing and flossing) to remove plaque.
2. Periodontitis
• What is it? A chronic, irreversible inflammatory destruction of the deeper periodontal structures.
• Progression: If gingivitis is left untreated, the bacterial infection spreads deeper.
• Consequences: Leads to the permanent breakdown of the periodontal ligament and destruction of the surrounding alveolar bone.
• Clinical Signs: Deepening periodontal pockets form between the tooth and gum, leading to gum recession, tooth mobility (loose teeth), and eventual tooth loss.
Key Takeaway: Gingivitis is reversible inflammation confined to the gums; periodontitis is irreversible damage involving bone and periodontal ligament destruction.