1.1 Head & Neck Anatomy and Underpinning Basic Sciences
Key Takeaways
- The maxillary division of the trigeminal nerve supplies the upper teeth through the anterior, middle and posterior superior alveolar nerves, with palatal mucosa supplied by the greater palatine nerve posteriorly and the nasopalatine nerve from canine to canine
- The marginal mandibular branch of the facial nerve runs close to the lower border of the mandible and is the branch most often injured in submandibular surgery, producing an asymmetrical lower lip on smiling
- Tongue-tip lymphatics drain to the submental (level Ia) nodes and can drain bilaterally, which is why midline and near-midline tongue cancers carry a risk of contralateral nodal disease
- Enamel is about 96% mineral by weight and acellular, dentine about 70% mineral with a vital tubular structure, so only dentine and pulp can mount a biological response to injury
- Hyposalivation is defined by measured flow: unstimulated whole saliva below 0.1 mL/min or stimulated whole saliva below 0.7 mL/min
Why the Basic Sciences Are Examined
Syllabus outcome B1.1 asks candidates to demonstrate and apply knowledge of basic science relevant to dentistry — anatomy, biology, physiology, pathology and microbiology, and C7.1 adds normal head and neck, oral and dental anatomy, terminology for macroscopy and orientation of specimens. In an SBA paper these outcomes rarely appear as bare recall. They appear inside clinical stems: why a block failed, which nerve is at risk, where a cancer will metastasise, why a specimen was reported as having an involved margin.
Innervation of the Teeth and Oral Mucosa
All sensory supply to the teeth is from the trigeminal nerve (CN V).
| Region | Nerve | Division |
|---|---|---|
| Upper incisors and canine | Anterior superior alveolar | V2 |
| Upper premolars (and often MB root of first molar) | Middle superior alveolar (inconstant) | V2 |
| Upper molars | Posterior superior alveolar | V2 |
| Palate, canine to canine | Nasopalatine (via incisive foramen) | V2 |
| Posterior hard palate | Greater palatine | V2 |
| Soft palate | Lesser palatine | V2 |
| All lower teeth | Inferior alveolar | V3 |
| Lower labial gingivae and lip, canine to midline | Mental (terminal IAN branch) | V3 |
| Buccal gingivae of lower molars | Long buccal | V3 |
| Lingual gingivae, floor of mouth, anterior two-thirds of tongue (general sensation) | Lingual | V3 |
| Taste, anterior two-thirds of tongue | Chorda tympani (travelling with the lingual nerve) | CN VII |
The clinically important consequence is that a failed inferior alveolar nerve block still leaves buccal soft tissue sensate unless a long buccal infiltration is added, and that a lower molar extraction needs inferior alveolar, lingual and long buccal anaesthesia.
Motor supply and the facial nerve
The four muscles of mastication — masseter, temporalis, medial pterygoid and lateral pterygoid — are all supplied by V3. The lateral pterygoid protrudes the mandible and, acting with the suprahyoids, depresses it; unilateral action deviates the mandible to the opposite side.
The facial nerve (CN VII) is motor to the muscles of facial expression and divides into five terminal branches: temporal, zygomatic, buccal, marginal mandibular and cervical. The marginal mandibular branch runs close to the lower border of the mandible and is the branch most often injured during submandibular gland surgery or a submandibular incision for drainage; the result is weakness of the depressors of the lower lip and an asymmetrical smile.
Blood Supply and Lymphatic Drainage
The maxillary artery (a terminal branch of the external carotid) supplies the teeth through the inferior alveolar, posterior superior alveolar, greater palatine and sphenopalatine branches. The facial artery supplies the lips and, as the submental branch, the floor of mouth.
Cervical lymph nodes are described in levels I–VI:
| Level | Group | Typical drainage |
|---|---|---|
| Ia | Submental | Tip of tongue, lower lip centre, floor of mouth anteriorly — often bilateral |
| Ib | Submandibular | Most of the oral cavity, upper lip, lateral lower lip |
| II–IV | Upper, mid and lower deep cervical (jugular chain) | Tongue, oropharynx, larynx |
| V | Posterior triangle | Nasopharynx, scalp |
| VI | Central compartment | Thyroid, subglottis |
Bilateral drainage of midline structures explains why a tongue cancer approaching the midline is staged and treated with the contralateral neck in mind.
Dental Hard Tissues
| Tissue | Mineral content (by weight) | Key properties |
|---|---|---|
| Enamel | ~96% | Acellular, avascular, non-vital; cannot repair itself; prism structure permits acid etch retention |
| Dentine | ~70% (≈20% organic, ≈10% water) | Tubular and vital via odontoblast processes; capable of tertiary/reparative dentine formation |
| Cementum | ~65% | Anchors Sharpey's fibres of the periodontal ligament; thin cervically, thicker apically |
| Pulp | Soft tissue | Vascular and innervated; A-delta fibres give sharp stimulus-evoked pain, C fibres dull throbbing pain |
Because enamel cannot repair, prevention and remineralisation operate at the enamel surface, while pulpal responses (tertiary dentine, sclerosis) are a dentine phenomenon. The A-delta versus C fibre distinction is the physiological basis of the reversible/irreversible pulpitis split: sharp evoked pain is A-delta; dull, spontaneous, poorly localised pain implies C-fibre involvement and advanced inflammation.
Salivary Physiology
Three paired major glands produce most saliva:
| Gland | Secretion | Duct | Parasympathetic supply |
|---|---|---|---|
| Parotid | Serous | Stensen's, opening opposite the upper second molar | CN IX via the otic ganglion |
| Submandibular | Mixed, mostly serous | Wharton's, opening at the sublingual caruncle | CN VII via chorda tympani and the submandibular ganglion |
| Sublingual | Mostly mucous | Ducts of Rivinus / Bartholin's | CN VII, as above |
Total daily output is roughly 0.5–1.5 litres. At rest the submandibular glands provide most of the flow; on stimulation the parotid dominates. Hyposalivation is defined objectively: unstimulated whole salivary flow below 0.1 mL/min, or stimulated whole salivary flow below 0.7 mL/min. Saliva buffers acid (bicarbonate and phosphate), supplies calcium and phosphate for remineralisation, and provides antimicrobial proteins (lysozyme, lactoferrin, secretory IgA) — which is why xerostomia drives rampant caries and candidiasis.
Terminology for Macroscopy and Specimen Orientation
Outcome C7.1 explicitly names specimen terminology. When tissue is sent for histopathology:
- Describe the specimen by site, size in three dimensions, colour and surface texture.
- Orientate the specimen for the pathologist — a marking suture at a named margin (for example "long suture = posterior"), or a diagram on the request form. Without orientation, an involved margin cannot be localised for further excision.
- Record which margin matters clinically and the clinical differential, because reporting is context-dependent.
- Fix immediately in 10% neutral buffered formalin at roughly ten times the specimen volume; an unfixed or under-fixed specimen autolyses.
- Never divide a small lesion between pots — it destroys the architecture the pathologist needs.
Normal Embryonic and Dental Development
Outcome C2.1 asks for knowledge of normal embryonic development, which underpins both orthodontic diagnosis and the developmental anomalies covered later.
The face forms in the fourth to tenth weeks of intrauterine life from the frontonasal process and the paired maxillary and mandibular processes of the first pharyngeal arch. Failure of fusion of the maxillary process with the medial nasal process produces cleft lip, and failure of fusion of the palatal shelves produces cleft palate; the two arise at different times and can occur separately, which is why cleft lip may occur with an intact palate.
Odontogenesis proceeds through recognisable stages from the dental lamina:
| Stage | What happens |
|---|---|
| Initiation (dental lamina) | Thickening of oral epithelium at about the sixth week; failure here gives hypodontia, over-activity gives supernumerary teeth |
| Bud | Epithelial bud invades the ectomesenchyme |
| Cap | Enamel organ, dental papilla and dental follicle become distinct |
| Bell | Cyto-differentiation into ameloblasts and odontoblasts; crown shape is determined — disturbance here produces defects of shape and structure |
| Apposition and mineralisation | Dentine is laid down first, then enamel; systemic insult at this stage produces enamel hypoplasia or hypomineralisation banded to the time of the insult |
| Root formation | Hertwig's epithelial root sheath maps the root; its remnants (rests of Malassez) later give rise to the radicular cyst |
| Eruption | Reduced enamel epithelium fuses with the oral epithelium; its persistence around an unerupted crown gives the dentigerous cyst |
The clinical value of the sequence is diagnostic timing: a symmetrical enamel defect banding the incisors and first molars points to a systemic insult at the age those crowns were mineralising, and the tissue affected tells you which cell line was disturbed.
A patient has weakness of the depressors of the lower lip and an asymmetrical smile after drainage of a submandibular abscess through an extra-oral incision. Which nerve has most likely been injured?
A dentist achieves profound pulpal anaesthesia of a lower first molar with an inferior alveolar nerve block but the patient still reports sharp discomfort when the buccal gingiva is engaged with forceps. What is the most likely explanation?
A squamous cell carcinoma arises at the tip of the tongue. Which nodal group is at greatest risk first, and what additional consideration applies?
Why can enamel not repair a carious lesion in the way dentine can lay down tertiary dentine?