Summary

The mouth is the facial opening of the gastrointestinal tract. The oral cavity, which is bounded by the lips anteriorly, cheeks laterally, and the oropharynx posteriorly, encloses the tongue, palates, gums, and teeth. The mouth's primary function is the initiation of the digestion process, which involves ingestion, chewing to break down food (mastication), the release of digestive enzymes from the salivary glands into the oral cavity, and swallowing (deglutition). Secondary functions include taste (gustation), sound production and speech articulation, ventilation, facial expression, and touch. The oral cavity can be anatomically divided into the oral vestibule, the space between the teeth and the mucosa of the lips and cheeks, and the oral cavity proper, the space bounded anteriorly and laterally by the teeth and posteriorly by the oropharynx. The teeth are hard, calcified structures used for sound articulation as well as biting and masticating food. The oral cavity is separated from the nasal cavity by the palate, which is divided into the soft and the hard palate. The masticatory movements of the jaw are enabled by the muscles of mastication, and swallowing is facilitated by the palatine muscles. The tongue is the organ responsible for the gustatory sense, which occurs via stimulation of the taste buds situated in the lingual papillae. The oral cavity is, furthermore, a secondary respiratory channel, which communicates with the nasal cavity posteriorly.

Anatomy

Overview

The oral cavity is divided into two spaces: the oral vestibule and the oral cavity proper.

Anatomical boundaries

Oral vestibule
- Anterior/lateral: buccal mucosa (i.e., mucosa of the cheeks and the lips)
- Posterior/medial: anterior (facial) surface of the teeth and the gingiva (gums) facing the buccal mucosa
Oral cavity proper
- Anterior/lateral: posterior surface of the teeth and gums
- Superior (roof): soft and hard palate
- Inferior (floor): floor of the mouth
- Posterior: communicates with the oropharynx

Anatomical structures

Structures of the oral cavity
Lips	Form the oral fissure, which is the opening of the oral cavity. Can be divided anatomically and histologically into three regions: Facial skin (external) Keratinized stratified squamous epithelium, containing hair follicles and sebaceous glands Nasolabial folds: extend from the edge of the nose to the corners of the mouth Philtrum: the vertical groove between the middle of the upper lip and the nose Vermilion zone (external and internal) Zone of transition between facial skin and lip mucosa Keratinized stratified squamous epithelium penetrated by short dermal papillae that contains fewer sebaceous glands than the facial skin and no hair follicles. Can be divided in dry vermillion (external) and wet vermillion (internal) Vermilion border (external): the line of demarcation between the highly keratinized squamous epithelium of the facial skin and the less keratinized epithelium of the vermillion zone Labial commissures (external and internal): corners of the mouth Mucosa (internal): nonkeratinized stratified squamous epithelium of the buccal mucosa with no sebaceous glands or hair follicles
Oral (buccal) mucosa	Mucous membrane lining the sides of the oral vestibule and the floor of the mouth to the gums Nonkeratinized stratified squamous epithelium with no sebaceous glands or hair follicles Composed of the following: Lining mucosa, which covers the lips, the cheeks, the floor of mouth, and the ventral surface of the tongue Masticatory mucosa, which covers the palate and gums Specialized mucosa, which covers the dorsal surface and lateral borders of the tongue (carry nerve endings for sensory and taste perception)
Intermaxillary commissure	The space behind the last molars that allows communication between the oral vestibule and oral cavity proper even when the teeth are in occlusion
Teeth	Hard, calcified structures used to break down food Embedded in the alveolar processes of the mandible and maxilla (see “Teeth” below)
Gingiva	Mucosal tissue covering the surfaces of the maxilla and mandible in the oral cavity. It surrounds the alveolar surfaces of the maxilla and mandible and protects the roots of the teeth and the underlying bone (see “Gingiva” below)
Salivary glands	Exocrine glands that produce saliva, which reaches the oral cavity via a system of ducts Include three paired major glands (parotid, submandibular gland, and sublingual gland) and hundreds of minor salivary glands The ducts from parotid and some minor salivary glands open into the oral vestibule. Ducts from submandibular gland, sublingual gland, and other minor salivary glands open into the oral cavity proper.
Tongue	Muscle involved in gustation, mastication, sound production, and speech articulation Located on the floor of the oral cavity proper (see “Tongue” below)
Palate	Consists of an anterior, bony part (hard palate) and a posterior muscular part (soft palate) Separates the oral cavity from the nasal cavity (oral cavity roof) Muscles of the soft palate are involved in ventilation and deglutition Both hard and the soft palates are important for sound production and speech articulation (see “Palate” below)
Salivary gland

Teeth

Function

Food break-down into smaller particles, which facilitates digestion and absorption
Sound production and speech articulation

Types of teeth according to age

Primary teeth (also known as deciduous teeth or milk teeth)
- Develop during embryogenesis, erupt during infancy, and shed by the age of 6–12 years
- Five in each quadrant (right, left, upper, and lower) 20 in total
  - Two incisors (8 in total)
  - One canine (4 in total)
  - Two molars (8 in total)
Permanent teeth
- Usually completed by 13 years of age
- Eight in each quadrant (right, left, superior, and inferior), 32 in total
  - Two incisors (8 in total)
  - One canine (4 in total)
  - Two premolars (8 in total)
  - Three molars (12 in total)
- Wisdom teeth (vestigial third molars)
  - Usually erupt by the age of 17–38 years
  - One in each quadrant (right, left, superior, and inferior), 4 in total
  - Partial eruption may cause inflammation and infection in the surrounding gingiva (pericoronitis), which are often indications for extraction.
- Developmental alterations may affect the number, size, shape, and structure of permanent teeth.
  - Hypodontia: the congenital absence of one or more teeth
  - Hyperdontia: the condition of having one or more extra teeth (supernumerary teeth)

Types of teeth according to function

Type	Characteristics
Molars	Primarily used for grinding of food Contain 3–5 cusps Upper molars have 3 roots and attach to the maxilla. Lower molars have 2 roots and attach to the mandible.
Premolars	Primarily used for grinding of food Contain 2 cusps Contain a single root The first upper premolar can be bifid.
Canines	Primarily used for tearing of food Contain a single downward cone-shaped prominence Contain a single root
Incisors	Primarily used for biting and cutting food Contain a single root The free end is shaped like a chisel.

Innervation

Upper teeth: (maxillary) branches of the maxillary nerve (CN V₃), including the anterior, middle, and posterosuperior alveolar nerves
Lower teeth: (mandibular) branches of the mandibular nerve (CN V₃), including the inferior alveolar nerves

Structure

Part	Characteristics
Crown	Covered by enamel Projects above the gingiva (gums)
Neck	Located between the crown and the root
Root	Located in the alveolar surface of the mandible and/or maxilla Connected to the bone by the periodontal ligament (a type of modified periosteal layer) Covered with cementum Mandibular molars have 2 roots. Maxillary molars have 3 roots.

Composition

Structure	Characteristics
Enamel	The outer layer of the tooth that covers the crown Develops from the surface ectoderm Produced by ameloblasts Whitish, yellowish, or grayish in color Hardest part of the tooth Most highly mineralized substance in the human body Primarily composed of hydroxyapatite Does not contain collagen Can be damaged and degraded by acids in food and drinks Does not contain blood vessels or nerves
Dentin	Located beneath the enamel Secreted by odontoblasts Supports the crown of the tooth Nourished by dental tubules of odontoblasts located around the central pulp
Pulp	Located in the central cavity of the tooth Continuous with the root canal Contains blood vessels, nerves, and lymphatics, which enter the pulp via the apical foramen (at the apex of the root canal) Contains odontoblasts (i.e., dentin producing cells), located peripherally along the borders of the dental pulp Dental pulp and odontoblasts develop from the neural crest ectoderm. Contains immune cells (e.g., macrophages, T cells)
Cementum	Hard, bone-like structure that covers the root of the tooth Composed of hydroxyapatite and collagen Secreted by cementoblasts Site of attachment for periodontal ligaments Forms the cementoenamel junction

Gingiva

Structure

Definition: a mucosal tissue that covers the following:
- The alveolar surfaces of the maxilla and mandible
- The neck and root of teeth
Description: pinkish color (can be pigmented in dark-skinned individuals)
Types: can be divided into the following 2 types:
- Attached gingiva
  - Keratinized tissue that is firmly attached to the bone structure
  - Fills the interdental spaces beneath each tooth to form the interdental papillae
- Free gingiva: forms a collar around the tooth but is not directly attached to the tooth surface

Function

Protection of the root of the teeth and the underlying bone from mechanical insults and infections
Modulation of inflammatory response and tissue repair

Innervation

Buccal (outer, anterior) surface
- Maxillary (superior) side: innervated by branches of the infraorbital nerve (anterosuperior, middle, and posterior branches)
- Mandibular (inferior) side: innervated by the mental and buccal nerves
Lingual (inner, posterior) surface
- Maxillary (superior) side: innervated by the nasopalatine and greater palatine nerves
- Mandibular (inferior) side: innervated by lingual nerves

Tongue

Structure

Location: on the floor of the oral cavity
Parts
- Body: anterior two-thirds
- Root: posterior one-third
- Apex: tip of the tongue
- Body and root of the tongues are divided by the V-shaped sulcus terminalis
- Left and right half are divided by the sulcus medianus
Attachments
- Hyoid bone
- Mandible
- Palate
- Styloid process of the temporal bone
- Pharynx

Function

Gustation
Speech and sound production
Mastication and deglutition
Touch

Histology

The tongue consist of the following layers:

Mucous membrane
- Dorsum has a velvety appearance due to the presence of numerous papillae (see below)
- Inferior surface appears smooth and thin
Epithelium
- Keratinized surface
- Nonkeratinized stratified squamous epithelium
Muscle layer (skeletal muscle)
Corium: connective tissue with mucous and serous glands
Adipose tissue

Lingual papillae

Lingual papillae represent small mucosal projections that cover the dorsal surface of the tongue.
Increase the surface area of the tongue
Located on the dorsum of the tongue, mostly on the anterior two-thirds
Some papillae contain taste buds, clusters of 50–150 taste receptor cells responsible for gustation ^[1]

Lingual papillae type	Characteristics	Innervation
Filiform papillae	Most numerous type of the lingual papillae Follow the V shape of the sulcus terminalis throughout the anterior two-thirds of the tongue, becoming more transverse at the tip of the tongue Slender, small, and conical in shape The only keratinized papillae Responsible for Sensation of touch Increasing friction between the tongue and the food to facilitate mastication and digestion The only papillae without taste buds	Lingual nerve of the mandibular branch of the trigeminal nerve (CN V₃) for sensation of touch
Fungiform papillae	Located on both sides of the body of the tongue and the apex Mushroom-shaped Deep red color is due to highly vascularized connective tissue core Contain very few taste buds	Chorda tympani of the facial nerve (CN VII) for taste sensation
Foliate papillae	Located in front of the palatoglossal arches and the lingual tonsils, on both sides of the tongue Rudimentary in humans Leaf-shaped Contain abundant taste buds
Circumvallate papillae	The largest of the papillae 8–12 in number Follow the V shape of the sulcus terminalis anteriorly Dome-shaped (narrower at the base and larger at the apex) Surrounded by a slight circular mucosal elevation (vallum or wall) which is separated from the papilla by a circular sulcus Covered by stratified squamous epithelium Contain abundant taste buds	Glossopharyngeal nerve (CN IX) for taste sensation The only part of the anterior two-thirds of the tongue that is not innervated by the CN V₃ or CN VII

Lingual tonsils

Characteristics
- Located at the base of the tongue bilaterally
- Two small aggregates of lymphoid tissue
- Covered by nonkeratinized stratified squamous epithelium
- Surrounded by a thin capsule
Function: immune protection (contain T cells and B cells, which produce IgA)
Blood supply
- Lingual artery (branch of the external carotid artery)
- Facial artery (tonsillar branch)
- Ascending pharyngeal branch (external carotid artery)
Innervation: tonsillar branches of glossopharyngeal nerve (CN XII)

Muscles of the tongue

Extrinsic muscles of the tongue

The extrinsic muscles of the tongue originate outside of the tongue.

Muscle	Origin	Insertion	Innervation	Function
Palatoglossus	Palatine aponeurosis (aponeurosis of the soft palate)	Posterolateral sides of the tongue	Vagus nerve (CN X), via the pharyngeal plexus	Elevation of the back of the tongue Depression of the soft palate
Genioglossus	Mandible (genial tubercle)	Body of the hyoid bone Inferior surface of the tongue	Hypoglossal nerve (CN XII)	Protrusion and depression of the tongue
Hyoglossus	Hyoid bone (greater horn and body)	Lateral and inferior surface of the tongue (lateral to the genioglossus and medial to the styloglossus)		Retraction and depression of the tongue
Styloglossus	Temporal bone (styloid process)	Lateral and inferior surface of the tongue		Elevation of the sides of the tongue to facilitate swallowing Retraction of the tongue

The genioglossus normally protrudes the tongue evenly in the midline, but if there is unilateral hypoglossal damage, the tongue will deviate towards the side of the damage.

The Genie propels out of the bottle (the genioglossus protrudes the tongue).

Intrinsic muscles of the tongue

The intrinsic muscles of the tongue originate and insert within the tongue.

Muscle	Origin	Insertion	Innervation	Function
Superior longitudinal muscle	Median fibrous septum, close to the epiglottis	Lateral edges of the tongue	Hypoglossal nerve (CN XII)	Retracts the tongue, making it short and thick
Inferior longitudinal muscle	Root of the tongue	Apex of the tongue		Retracts the tongue, making it short and thick
Vertical muscle	Dorsum of the tongue (submucosal fibrous layer)	Anterior borders of the tongue		Flattens and broadens the tongue
Transverse muscle of the tongue	Median fibrous septum	Lateral edges of the tongue		Narrows and elongates the tongue

Blood vessels and nerve supply of the tongue

Structure	Characteristics
Arterial supply	Lingual artery Branch of the external carotid artery (at the level of the hyoid bone in the carotid triangle) Branches Sublingual artery Dorsal lingual artery Suprahyoid artery Deep lingual artery (terminal branch of the lingual artery)
Venous drainage	Lingual veins Originate on the dorsum, sides, and inferior surfaces of the tongue Drain into the internal jugular vein Follow the course of the lingual artery Ranine vein Begins on the tip of the inferior tongue Follows the course of the hypoglossal nerve Other veins Sublingual vein Dorsal lingual vein Suprahyoid vein Deep lingual artery These veins are the route of absorption for sublingual nitroglycerin.
Innervation	Motor innervation Hypoglossal nerve (CN XII): hyoglossus, genioglossus, and styloglossus muscles Vagus nerve (CN X): palatoglossus muscle Sensory innervation Anterior two-thirds of the tongue (excluding papillae vallatae) Taste: chorda tympani from facial nerve (CN VII) General sensation: lingual nerve from the mandibular branch of the trigeminal nerve (CN V₃) Posterior one-third of the tongue (including papillae vallatae): taste and general sensation through glossopharyngeal nerve (CN IX) Epiglottic region of the tongue and epiglottis: taste and general sensation through internal laryngeal nerve from vagus nerve (CN X)

All of the tongue muscles (intrinsic and extrinsic) are innervated by the hypoglossal nerve (CN XII), except for the palatoglossus (CN X).

Embryology

Structure	Characteristics
Anterior two-thirds of the tongue	Develops from the 1^st pharyngeal arch A median swelling (median tongue bud or tuberculum impar) develops within the 1^st pharyngeal arch during the 4^th week of embryonic development. Lateral lingual swellings develop on the right and left sides of the 1^st pharyngeal arch during the 5^th week of embryonic development, expand, and cover the medial lingual swelling. The median sulcus represents the line of their fusion.
Posterior one-third of the tongue	Develops from the 2^nd, 3^rd, and 4^th pharyngeal arches A swelling (copula) develops in the midline of the 2^nd pharyngeal arch during the 4^th week of embryonic development. A subsequent swelling (hypopharyngeal eminence) of endoderm of the 3^rd and 4^th pharyngeal arches expands and covers the copula during the 5^th and 6^th weeks of embryonic development.
Foramen cecum	Located at the intersection between the sulcus terminalis and sulcus medianus Site of embryonic origin of the thyroglossal duct
Muscles of the tongue	Both the intrinsic and extrinsic muscles of the tongue are derived from myoblasts of occipital somites that migrate towards the tongue.

Clinical significance

Macroglossia

Definition: severe enlargement of the tongue which may result in cosmetic and/or functional impairment
Etiology
- Congenital disorders
  - Vascular malformations (e.g., lymphangioma or hemangioma)
  - Muscular hypertrophy (e.g., Beckwith-Wiedemann syndrome or hemihyperplasia)
  - Down syndrome
  - Multiple endocrine neoplasia type 2B
  - Glycogen storage disease type 2
  - Neurofibromatosis
- Acquired conditions
  - Amyloidosis
  - Myxedema and hypothyroid disorders
  - Tumors (e.g., squamous cell carcinoma of the tongue)
Clinical features
- Dyspnea and obstructive sleep apnea
- Dysphagia
- Dysphonia
- Sialorrhea
Treatment: indicated for symptomatic cases
- Surgery (reduction glossectomy)
- Speech therapy
- Treatment of the underlying acquired cause (e.g., radiotherapy for tumors)

Oral hairy leukoplakia

Definition: a benign condition characterized by painless, white plaques that cannot be scraped off.
Etiology
- EBV
- Immunosuppression (e.g., patients with HIV or a history of organ transplantation)
Clinical features
- Painless, irregularly shaped white plaques with a feathery or hairy appearance
- Most commonly located on the lateral tongue, but can also occur on the floor of the mouth, palate, or buccal mucosa
- Patches cannot be scraped off (in contrast to oral thrush)
Treatment: treat the underlying condition (in contrast to leukoplakia)

Palate

Overview

The palate divides the oral cavity from the nasal cavity.
Constitutes the roof of the mouth and contributes to the floor of the nose

Embryology

4^th week of development
- Downward proliferation of the mesoderm covering the forebrain → development of the frontonasal prominence
- Local thickening of the surface ectoderm on both sides of the frontonasal prominence → formation of the nasal placodes
5^th week of development: invagination of the nasal placodes → formation of medial and lateral nasal prominences
6^th–7^th week of development
- Medial growth of the maxillary prominences and subsequent fusion with medial nasal prominences → formation of the intermaxillary segment, composed of:
  - Labial component → development of philtrum
  - Upper jaw component → development of the four incisor teeth
  - Palatal component → development of the triangular primary palate
- Development of palatal shelves (outgrowths slanted obliquely downward from the maxillary prominences) and subsequent fusion of the two palatal shelves in the midline → formation of the secondary palate
7^th–8^th week of development: fusion of the primary and secondary palates at the level of the incisive foramen → formation of the permanent palate

Structure	Fused structures	Developmental defects
Primary palate	Medial nasal prominences in the midline with palatine processes posterior to the triangular primary palate	Cleft lip: failed fusion of the palatine processes and medial nasal prominences
Secondary palate	Lateral palatine processes (palatal shelves) which derive from the maxillary prominences	Cleft palate: Failed fusion either of the palatine shelves or of the palatine shelves with the nasal septum and/or primary palate
Permanent palate	Primary and secondary palates with the nasal septum at the level of the incisive foramen	N/A

Anatomy of the definitive palate

Structure

Characteristics

Hard palate

A bony framework covered by mucosa
Formed by the horizontal plate of the palatine bone (posterior) and the palatal shelves of the maxilla (anterior)
Constitutes the anterior, greater part of the palate
Contains following foramina (see also skull):
Receives blood supply from the greater palatine arteries (branches of the descending palatine artery, which originates from the maxillary artery)
Innervated by the greater palatine and nasopalatine nerves (branches of the maxillary nerve)

Soft palate

A fibromuscular fold that forms the posterior part of the palate
Continuous with the palatoglossal folds and the palatopharyngeal folds
Supplied by the following branches of the external carotid artery:
- Ascending palatine artery, which arises from the facial artery
- Palatine branches of the ascending pharyngeal artery
- Lesser palatine arteries, branches of the descending palatine artery, which originates from the maxillary artery
Innervated by the lesser palatine nerve (secretomotor, sensory, and taste) and the vagus nerve (motor)

Muscles of the palate

Muscle	Origin	Insertion	Innervation	Function
Tensor veli palatini	Medial pterygoid plate of the sphenoid bone (scaphoid fossa) Auditory tube (cartilaginous part)	Palatine aponeurosis (aponeurosis of the soft palate)	Mandibular branch of the trigeminal nerve (CN V₃)	Tension of the soft palate Opening of the auditory tube inlet during deglutition
Levator veli palatini	Temporal bone (petrous region) Auditory tube (cartilaginous part)	Palatine aponeurosis (aponeurosis of the soft palate)	Vagus nerve (branches of the pharyngeal plexus)	Elevation of the soft palate
Musculus uvulae	Palatine bone (posterior nasal spine) Posterior border of the hard palate	Palatine aponeurosis (aponeurosis of the soft palate) Mucosa of the uvula		Closure of the nasopharynx Uvular movement
Palatoglossus	Palatine aponeurosis (aponeurosis of the soft palate)	Posterolateral side of the tongue		Elevation of the tongue
Palatopharyngeus	Palatine aponeurosis (aponeurosis of the soft palate)	Thyroid cartilage Lateral sides of the pharynx	Branches of the pharyngeal plexus from: Vagus nerve Glossopharyngeal nerve	Closure of the nasopharynx Elevation of the pharynx

In vagus nerve (CN X) lesions, the uvula deviates away from the side of the lesion.

Salivary glands

In addition to the three larger paired glands (parotid, submandibular gland, and sublingual glands), there are several hundred small salivary glands in the oral cavity and throat. As secretory glands, they secrete up to 1.4 L of saliva per day. The primary functions of saliva include:

Digestion
Protection of the mucosa and teeth
Immunological defense
Transport of soluble flavors to the taste buds

Parotid gland

Located on the surface of the masseter muscle, dorsal to the mandibular ramus and ventrocaudal to the external auditory canal in the retromandibular fossa.
Subdivided by the branches of the facial nerve which runs through the parotid gland
Its salivary duct (Stensen duct) runs forward along the masseter muscle and opens adjacent to the 2^nd molar in the vestibule of the mouth (the space between the cheeks and the teeth).
Produces mainly serous fluid and ∼ 40% of the total amount of saliva.

Submandibular gland

Located medial and caudal to the inner surface of the mandible above the mylohyoid and digastric muscles
The salivary duct (Wharton duct) ends in the sublingual caruncle
Produces seromucous fluid and the greater part (∼ 50%) of the secreted saliva

Sublingual gland

Located medial to the mandible above the mylohyoid muscle and below the sublingual fold
Its main salivary duct also ends in the Wharton duct
Produces mainly mucous fluid

Mastication

Muscles of mastication

Muscle	Origin	Insertion	Innervation	Function	Embryology
Masseter	Zygomatic arch (lower and medial surface)	Mandible (ramus, angle, and the lateral surface of the coronoid process)	Mandibular branch of the trigeminal nerve (CN V₃)	Deep part: retraction of the mandible Superficial part: elevation of the mandible (most powerful muscle)	1^st pharyngeal arch
Temporalis	Temporal fossa	Mandible (ramus and coronoid process)		Elevation and retraction of the mandible
Lateral pterygoid	Superior head: sphenoid bone (infratemporal fossa) Inferior head: sphenoid bone (lateral surface of the lateral pterygoid plate)	Mandible (neck and temporomandibular joint)		Bilateral contraction Superior head: elevation of the mandible Inferior head: depression and protraction of the mandible (major protractor) Unilateral contraction: laterotrusion (sideways movement) of the mandible to the contralateral side
Medial pterygoid	Superficial head Maxillary tuberosity Palatine bone (pyramidal process) Deep head Sphenoid bone (medial surface of the lateral pterygoid plate) Palatine bone (pyramidal process)	Mandible (medial surface, angle, and ramus)		Elevation and protraction of the mandible

Function

Mastication muscles facilitate the masticatory process via the following:
- Depression of the mandible (opening the mouth), which involves the following:
  - Lateral pterygoid
  - Certain suprahyoid muscles (anterior digastric, geniohyoid, and mylohyoid muscles)
- Elevation of the mandible (closing the mouth), which involves the following:
  - Masseter (superficial part)
  - Temporalis
  - Medial pterygoid
- Protraction, which involves the following:
  - Lateral pterygoid (major contributor)
  - Medial pterygoid
- Retraction, which involves the following:
  - Masseter (deep part)
  - Temporalis
  - Some suprahyoid muscles (digastric and geniohyoid muscles)

To remember the muscles of mastication that close the mouth: “Resist the TEMPtation to eat yuMMy food!” (TEMP = Temporalis; M = Masseter; M = Medial pterigoyd)

Deglutition

Deglutition (swallowing) is the transfer of a bolus of food from the mouth down the pharynx and esophagus to the stomach.
Encompasses the oral, pharyngeal, and esophageal phases

Phase	Characteristics
Oral phase	Voluntary process The bolus of food is masticated, moistened in saliva, and propelled towards the oropharynx by the tongue (primarily via the palatoglossus and styloglossus muscles).
Pharyngeal phase	Involuntary process Once the bolus of food is propelled backwards, the levator veli palatini and tensor veli palatini muscles elevate the soft palate, preventing the passage of food into the nasopharynx. Walls of the pharynx are elevated by the contraction of the following longitudinal pharyngeal muscles: Salpingopharyngeus Palatopharyngeus Stylopharyngeus The suprahyoid muscle contracts and elevates the hyoid bone and the larynx. Epiglottis closes and thereby: Protects the opening of the larynx Prevents passage of food into the trachea and respiratory tract (prevents aspiration) Can be affected by stroke of the brainstem
Esophageal phase	Involuntary process Contraction of the pharyngeal constrictor muscles (superior, middle, and inferior) → propulsion of the bolus of food downward to the esophagus Esophageal peristalsis → propulsion of the bolus of food towards the stomach Affected by achalasia

Taste (gustation)

Overview

Definition: a chemosensory process triggered by ions and molecules in solution stimulating receptors located in the taste buds
Location: Taste buds are primarily located in the tongue, palate, and epiglottis.
Tastants: any molecule capable of eliciting taste

Anatomy of taste

Gustatory pathway

The nerves responsible for taste transmit information to the solitary nucleus in the medulla, which in turn projects to three different brain areas:

Ventral posteromedial nucleus (VPM) of the thalamus, which projects to the basal portion of the postcentral gyrus (conscious gustatory perception)
Hypothalamus (emotional component of gustatory perception)
Amygdala (emotional component of gustatory perception)

Taste papillae and taste buds

Taste papillae
- Connective tissue projections covered by squamous cells
- Four types
  - Foliate
  - Filiform
  - Fungiform
  - Circumvallate
- Contain different types of taste buds
Taste buds
- Contain microvilli that connect to a central opening (pore)
- Tastants are sensed by taste receptors in the pore.
- Different mechanisms of taste signaling involve:
  - Transmembrane ionic channels
    - Salty (e.g., sodium chloride)
    - Sour (e.g., hydrogen ions)
  - G protein-coupled receptors
    - Sweet (e.g., carbohydrates)
    - Bitter (e.g., quinine, nicotine)
    - Umami (e.g., glutamate, aspartate)

Physiology of taste

Types of taste	Tastant	Mechanism	Function
Salty	Ions (e.g., Na⁺, NH⁴⁺, K⁺, SO₄^2-, Cl⁻)	Ions (e.g., sodium chloride) enter the cell via ionic channels → cell depolarization → generation of action potentials in nerve fibers → brain perception of salty taste	Regulation of electrolyte intake
Sour	Hydrogen ions (H⁺)	Hydrogen ions enter the cell via transmembrane ionic channels → acidic intracellular environment → generation of action potentials in nerve fibers → brain perception of sour taste	Warning signal for toxins
Sweet	Carbohydrates and some proteins	Binding of G protein-coupled receptors (gustducin) → increased levels of second messengers (inositol trisphosphate, IP₃; diacylglycerol, DAG) → release of intracellular calcium stores into the cytoplasm of the cell → sodium influx into the cell → release of ATP → generation of action potentials in nerve fibers → brain perception of sweet, bitter, or umami taste	Increase appetite for calorie-dense foods
Bitter	Highly variable (e.g., quinine, nicotine)		Warning signal for toxins
Umami (savory)	Various amino acids (e.g., glutamate, aspartate)		Increase appetite for protein-rich foods

Clinical significance

Neurologic disorders

Inflammatory and infectious diseases

Immunologic disorders

Congenital diseases and dysmorphisms

Neoplastic disorders

Oral cavity cancer
Throat cancer (squamous cell carcinoma of the tonsil)

Other disorders

Obstructive sleep apnea (can be exacerbated by hypertrophy of the lingual tonsils)
Bruxism