STUDIO PREVIO
III. Carpinterías
• The mucous membrane of the mouth receives its sensory innervation mainly from branches of the trigeminal nerve.
■ Roof of mouth:
• Greater palatine nerve
• Nasopalatine nerve
• Floor of mouth:
• Lingual nerve
• Taste fibres in chorda tympani
■ Cheek:
• Buccal nerve
5.12.1 Permanent teeth
• 32 permanent teeth
• In each jaw there are:
■ 2 canines
■ 4 premolars
■ 6 molars
• Approximate ages of eruption are:
■ First molars: 6 years
■ Central incisors: 7 years
■ Lateral incisors: 8 years
■ First premolars: 9 years
■ Second premolars: 10 years
■ Canines: 11 years
■ Second molars: 12 years
■ Third molars (wisdom teeth): 17-30 years
5.12.2 Innervation
• The teeth of the upper jaw are innervated by the maxillary division of the trigeminal via the anterior superior alveolar nerve and posterior superior alveolar nerves.
• The teeth of the lower jaw are innervated by the mandibular division of the trigeminal via the inferior alveolar nerve.
• Dental anaesthesia:
■ Lower teeth are anaesthetized by a mandibular block - injection of local anaesthetic into the infratemporal fossa lateral to the pterygomandibular ligament anaesthetizes the inferior
alveolar and lingual nerves. This anaesthetizes the lower teeth, dorsum of anterior two-thirds of tongue, mucosa of the floor of the mouth, gingiva of the mandibular arch and incisors, and the mucosa of the lower lip. This is augmented by infiltration of local anaesthetic behind the lower 3rd molar, which in addition blocks the long buccal nerve.
■ Upper teeth - require infiltration of anaesthetic on both the inside and outside of the
maxillary process, just distal to the tooth to be anaesthetized. This local infiltration provides adequate anaesthesia as the maxilla has a thin layer of cortical bone, and the spongy bone within allows rapid diffusion of the anaesthetic.
5.12.3 Structure of the tooth
See Figure A.5.7.
Figure A.5.7 Structure of a tooth
5.12.4 Hard palate
• Thin, horizontal plate made from:
■ Palatine processes of the maxilla
■ Horizontal plates of palatine bones
• It is continuous with the soft palate behind and forms the floor of the nasal cavities.
• Its blood supply is from the greater palatine artery.
• It is innervated by the greater palatine nerve and nasopalatine nerve.
5.12.5 The tongue
• The tongue is divided into 2 parts by the sulcus terminalis (see Figure A.5.8):
Figure A.5.8 The structure of the tongue
■ Anterior two-thirds or oral part
■ Posterior third or pharyngeal part
• Blood supply of the tongue is mainly from the lingual artery. Secondary blood supply is from tonsillar branch of facial artery and ascending pharyngeal artery.
• Nerve supply of tongue:
■ Posterior third:
• Taste: Glossopharyngeal nerve
• Sensation: Glossopharyngeal nerve
■ Anterior two-thirds:
• Taste: Chorda tympani
• Sensation: Lingual nerve (from mandibular nerve)
■ Motor innervation to the whole tongue is from hypoglossal nerve (except palatoglossus, supplied by cranial nerve X)
• Lymphatic drainage:
■ Posterior third: Drains to deep cervical lymph nodes
■ Anterior two-thirds: Drains via submandibular nodes to the deep cervical nodes (tip drains via submental nodes)
• Because the tongue is a midline structure the lymph frequently drains to both sides of the neck
(eventually to the jugular lymphatics), meaning it is imperative to detect cancer early. Resection of the tongue is a radical and complicated procedure, and chemo and radiotherapy are frequently used in place of surgery.
5.12.6 Floor of the mouth
• Formed by the anterior two-thirds of the tongue and the mucous membrane from the sides of the tongue to the gum on the mandible.
• In the midline the frenulum connects the under-surface of the tongue to the floor of the mouth.
• The floor contains:
■ Sublingual gland
■ Lingual nerve
■ Submandibular gland
■ Submandibular duct
■ Hypoglossal nerve
5.12.7 The pharynx
• The pharynx is situated posterior to the nasal cavities, mouth, and larynx.
• It is funnel-shaped, with the wider end under the skull and the narrow end becoming continuous with the oesophagus at the level of C6.
• Divided into 3 layers - from outside in:
■ Mucous
■ Fibrous
■ Muscular
• Muscular layer arranged as 3 sheets of constrictor muscles, superior, middle, and inferior, whose fibres run in a circular direction.
• Lymph drainage of the pharynx:
■ Direct drainage into deep cervical nodes
■ Indirect drainage into deep cervical nodes via retropharyngeal or paratracheal nodes
• Innervation of pharynx:
■ Main nerve supply is from the pharyngeal plexus formed from branches of:
• Glossopharyngeal nerve
• Vagus nerve
• Sympathetic nerves
■ Motor nerve supply from pharyngeal plexus
■ Sensory nerve supply:
• Nasopharynx - Maxillary nerve
• Oropharynx - Glossopharyngeal nerve
• Entrance to larynx - Internal laryngeal branch of vagus nerve
5.12.8 Interior of pharynx
• The pharynx is arranged into 3 continuous sections:
■ Nasal part
■ Oral part
■ Laryngeal part
• Nasal part of the pharynx
-■ The adenoids or pharyngeal tonsils are located in the nasal part of the pharynx, high in the submucosa of the posterior wall
■ The opening of the auditory tube is on the lateral wall of the pharynx, at the level of the floor of the nose. This tube equalizes pressure between the pharynx and the middle ear
• Oral part of the pharynx
-■ Contains the palatine tonsils:
• Lie in the tonsillar fossa on each side of the pharynx
• Blood supply is via tonsillar branch of facial artery
• Tonsillar bed is very vascular and there is a risk of postoperative haemorrhage following tonsillectomy
■ Veins drain to the pharyngeal venous plexus and into the facial, external palatine, or pharyngeal vein
■ Lymph drainage is to nodes that drain into the deep cervical nodes
■ The jugulodigastric lymph node, below and behind the angle of the mandible, can become particularly swollen and tender in tonsillitis
■ The valleculae are situated in the oropharynx, just behind the root of the tongue, on each side of the median epiglottic fold, and are an important landmark in intubation
• Laryngeal part of the pharynx
-■ The laryngopharynx lies behind the larynx and the opening into the larynx
■ Its anterior wall comprises the inlet of the larynx, with the piriform fossa on either side.
■ This fossa is a common site for foreign bodies to become lodged and can be viewed with a laryngoscope as seen in Figure A.5.9
Figure A.5.9 View of the larynx via a laryngoscope
5.12.9 Soft palate
• The soft palate is a fold of tissue attached to the posterior aspect of the hard palate.
• It is comprised of a mucous membrane and a series of paired muscles, which are able to tighten and move the soft palate to aid with speech and closure of the nasopharynx.
• The muscles of the soft palate are innervated by the pharyngeal plexus.
• Sensation of the soft palate is innervated via the maxillary division of the trigeminal nerve (via the palatine branches).
• Secretomotor fibres to the soft palate are from the pterygopalatine ganglion.
• The uvula is a small flap of tissue that hangs from the posterior edge of the soft palate.
• The uvula contains fibrous tissue and muscle and is involved in closing off the nasopharynx from the mouth during speech and swallowing, and in the gag reflex.
5.12.10 The gag reflex
• This is a reflex contraction of the back of the throat, elicited by touching the soft palate or uvula.
• It plays an important role in preventing material from entering the throat and thus prevents choking.
• The afferent pathway controlling this reflex is from the glossopharyngeal nerve (cranial nerve IX).
• The efferent pathway is via the vagus nerve (cranial nerve X).
• The gag reflex is absent if there is damage to cranial nerve IX and X and in brainstem death.
• The reflex may also be triggered deliberately to induce vomiting.