Anatomy and Physiology of the Nose

External Anatomy

• The nasal bones form the upper bridge (nasion/rhinion)
• Laterally, the frontal processes of the maxilla provide support

• Upper lateral cartilages (ULC) lie below the nasal bones and form the middle third of the nose
• Lower lateral cartilages (LLC) — also called alar cartilages — form the lower third; each has a medial crus, intermediate crus, and lateral crus
• Fibroareolar tissue occupies the base of the nose

Nasal Tip Support Mechanisms

• Shape and strength of the lower lateral cartilages
• Attachment of medial crural footplates to the caudal septum
• Attachment of ULC (caudal border) to LLC (cephalic border)

• Interdomal ligament
• Cartilaginous septal dorsum
• Sesamoid complex extending lateral crura to piriform aperture
• Anterior nasal spine and membranous septum

Internal Anatomy

Nasal Septum

• Quadrangular cartilage (anterior)
• Perpendicular plate of the ethmoid (posterior-superior)
• Vomer (posterior-inferior)

Nasal Cavity

• Floor: hard palate
• Roof: cribriform plate of ethmoid and sphenoid bone
• Medial wall: nasal septum
• Lateral wall: turbinates (conchae)

Turbinates (Conchae)

• Inferior turbinate — largest; most important for airflow and humidification
• Middle turbinate — landmark for sinus surgery; the ostiomeatal unit lies laterally
• Superior turbinate — smallest; lies near the olfactory cleft

Nasal Valve

Embryology

Blood Supply

Innervation

• Olfactory nerve (CN I): special sensory; unmyelinated axons from olfactory receptor cells pass through the cribriform plate to the olfactory bulb
• Ophthalmic branch (V1): anterior ethmoidal nerve → external nasal nerve (skin of tip and dorsum)
• Maxillary branch (V2): posterior nasal nerves via sphenopalatine ganglion (mucosa of nasal cavity and septum)
• Autonomic: parasympathetic (via vidian nerve) → vasodilation, mucosal secretion; sympathetic → vasoconstriction

Physiology

Airflow and Resistance

1. Humidification
2. Warming of inspired air
3. Particle filtration
4. Olfactory contact

The Nasal Cycle

Autonomic Control of Mucosal Blood Flow

• Parasympathetic: vasodilates sinusoids → congestion and increased secretions
• Sympathetic: provides steady vasoconstrictor tone → decongestion
• Certain medications (e.g., oral contraceptives) alter neural input → mucosal hypertrophy

Mucociliary Clearance

Olfactory Epithelium

Olfactory Transduction (Signal Cascade)

1. Odorant molecules bind to G protein-coupled receptors on olfactory cilia (~1,000 receptor types)
2. Receptor activation → Golf protein → adenylyl cyclase activation
3. ATP → cAMP increase → opens cation channels (Na⁺, K⁺, Ca²⁺ permeable)
4. Depolarization → action potential propagated centrally via CN I to the olfactory bulb

• Septal deviation → turbulent airflow → perceived obstruction even without complete blockade
• Turbinate hypertrophy (mucosal or bony) → persistent obstruction; bilateral mucosal hypertrophy suggests allergic/vasomotor rhinitis
• 50% of inspired air travels along the nasal floor — turbinate and septal floor deviations together significantly affect total flow
• Anterior epistaxis (90%) occurs at Kiesselbach's plexus; posterior epistaxis is rarer but life-threatening and more common after age 40

Anatomy and Physiology of the Pharynx

Overview

Subdivisions and Boundaries

Nasopharynx

• Subunits: lateral walls (containing the fossa of Rosenmüller and eustachian tube orifice), vault/roof, and posterior wall
• Lined by pseudostratified ciliated columnar (respiratory) epithelium superiorly, transitioning to stratified squamous epithelium inferiorly
• Contains the pharyngeal tonsil (adenoid) on the posterior wall

Oropharynx

• Subunits: soft palate and uvula, base of tongue, palatine arches (tonsillar fossae, palatine tonsils, anterior and posterior pillars), lateral and posterior walls, valleculae, and pharyngoepiglottic/glossoepiglottic folds
• Lined by non-keratinized stratified squamous epithelium

Hypopharynx (Laryngopharynx)

• Subunits:
  • Pyriform sinuses (pyriform fossae): paired lateral recesses bordered medially by the aryepiglottic folds and anterolaterally by the thyroid cartilage; the apex is at the level of the cricoid
  • Posterior and lateral pharyngeal walls
  • Postcricoid region: extends from the arytenoids to the inferior margin of the cricoid, contiguous with the medial walls of the pyriform sinuses
• The narrowest point of the pharyngeal cavity is at the inferior constrictor's attachment to the cricoid

Layers of the Pharyngeal Wall

1. Mucosa — stratified squamous epithelium (respiratory in nasopharynx)
2. Pharyngobasilar fascia — inner fibrous layer, attaches to the base of skull superiorly
3. Muscular layer — constrictors (outer circular) and longitudinal muscles (inner)
4. Buccopharyngeal fascia — outer fascia; contains the pharyngeal plexus of nerves and vessels

Muscles

Pharyngeal Constrictor Muscles (Circular)

Clinically Important Gaps in the Pharyngeal Wall

• Killian's dehiscence: triangular gap between the inferior constrictor and cricopharyngeus → site where a Zenker's diverticulum herniates
• Killian–Jamieson space: lateral dehiscence inferior to the cricopharyngeus → passage of branches of the inferior thyroid artery
• Laimer–Hackermann space: between the posterior cricopharyngeus and esophageal musculature
• Oropharyngeal triangle: gap between superior/middle constrictors and posterior border of mylohyoid → allows passage of stylopharyngeus muscle, lingual artery and vein, lingual nerve, CN XII (hypoglossal), CN IX (glossopharyngeal), and lymphatics

Pharyngeal Longitudinal Muscles (Elevators)

Soft Palate

Function

• Elevation → closes the pharyngeal isthmus → seals off the nasopharynx (prevents reflux of food into the nasopharynx during swallowing)
• Depression → closes the oropharyngeal isthmus → seals the oral cavity from the oropharynx (allows simultaneous chewing and breathing)

Muscles of the Soft Palate

Waldeyer's Ring (Lymphoid Tissue)

• Pharyngeal tonsil (adenoid) — midline posterior nasopharynx
• Palatine tonsils — between palatoglossal and palatopharyngeal arches
• Lingual tonsils — posterior third of tongue
• Tubal (Gerlach) tonsils — at the lip of the fossa of Rosenmüller, posterior to the torus tubarius
• Lateral pharyngeal bands — lymphoid tissue behind the posterior pillars

Blood Supply

Innervation

Motor

• CN X (vagus) via the pharyngeal plexus → all pharyngeal muscles except stylopharyngeus
• CN IX (glossopharyngeal) → stylopharyngeus (only muscle)
• CN V3 (mandibular) → tensor veli palatini (only soft palate muscle)

1. Pharyngeal branch of CN X (from the inferior vagal ganglion — the dominant motor nerve)
2. Branches from the external laryngeal nerve (branch of the superior laryngeal nerve of CN X)
3. Pharyngeal branches of CN IX

Sensory

Lymphatic Drainage

Physiology

Swallowing (Deglutition)

Three Phases of Swallowing

• Oral preparatory: food taken into mouth, chewed, mixed with saliva; bolus formed by the tongue
• Oral propulsive: tongue propels bolus posteriorly to the pharyngeal inlet in a piston-like action → triggers the involuntary pharyngeal reflex

1. Soft palate elevates → closes nasopharynx, preventing nasal regurgitation
2. Larynx elevates against the epiglottis (epiglottis deflects posteriorly) → airway protection; glottis closes; respiration inhibited
3. UES (cricopharyngeus) relaxes — notably, the UES relaxes just before arrival of the bolus, as laryngeal elevation pulls it open via traction, creating suction that assists bolus entry
4. Sequential peristaltic contraction of constrictors (superior → middle → inferior) propels the bolus through the open UES into the esophagus
5. Breathing resumes with expiration (not inspiration), reducing aspiration risk

• UES closes once bolus passes
• Primary peristaltic wave (coordinated by the swallowing reflex) travels the length of the esophagus
• If residue remains, secondary peristaltic wave (initiated by enteric nervous system at the site of distension) clears it
• Lower esophageal sphincter (LES) relaxes; bolus enters the stomach

Speech and Resonance

Airway Functions

• The pharynx maintains a patent airway at rest via tonic activation of pharyngeal dilator muscles (especially tensor veli palatini and genioglossus)
• Loss of this tonic activity during sleep underlies obstructive sleep apnea
• The gag reflex (afferent: CN IX; efferent: CN X) protects against foreign body aspiration

• Zenker's diverticulum herniates through Killian's dehiscence (between inferior constrictor and cricopharyngeus) — typically in older adults; presents with dysphagia, regurgitation of undigested food, and halitosis
• Cricopharyngeal dysfunction (failure of UES to relax) → dysphagia; can be treated with dilation or cricopharyngeal myotomy
• Peritonsillar abscess forms in the space between the palatine tonsil and the superior constrictor
• Nasopharyngeal carcinoma classically presents in the fossa of Rosenmüller and causes ipsilateral serous otitis media (from Eustachian tube obstruction), epistaxis, and cervical lymphadenopathy

Anatomy and Physiology of the Ear

The Temporal Bone

• Squamous part — forms the lateral wall of the middle cranial fossa; contains the zygomatic process
• Petrous part — pyramid-shaped; houses the inner ear; contains the internal auditory canal (IAC) and carotid canal
• Mastoid part — contains air cells communicating with the middle ear via the antrum
• Tympanic part — forms the walls of the bony external auditory canal (EAC) and anterior/floor of the middle ear

1. External Ear

Auricle (Pinna)

External Auditory Canal (EAC)

• Lateral third — cartilaginous (membranous); thick mobile skin; contains ceruminous (apocrine) glands, sebaceous glands, and hair follicles
• Medial two-thirds — bony; thin immobile skin without glands or hair; continuous with the TM epithelium

• Auriculotemporal nerve (CN V3) — anterosuperior wall
• Arnold's nerve (auricular branch of CN X) — posterior wall → explains cough reflex from ear stimulation
• Great auricular nerve (C2, C3)

2. Middle Ear

Tympanic Membrane (TM)

1. Lateral — stratified squamous epithelium (continuous with EAC skin)
2. Middle (fibrous layer) — radial and circular collagen fibers
3. Medial — simple squamous/cuboidal epithelium (continuous with middle ear mucosa)

• Pars tensa — the larger, taut inferior portion, supported by the fibrous annulus (Gerlach ring)
• Pars flaccida (Shrapnell's membrane) — the smaller, lax superior portion above the lateral process of the malleus; lacks a fibrous middle layer; retraction here leads to cholesteatoma formation

Middle Ear Spaces (Tympanic Cavity)

• Epitympanum (attic) — above the TM; contains the head of malleus and body/short process of incus
• Mesotympanum — at the level of the TM; main middle ear space
• Hypotympanum — below the TM; overlies the jugular bulb
• Mastoid antrum — posteriorly, communicates via the aditus ad antrum

Ossicular Chain

• Tensor tympani (innervated by CN V3): pulls malleus handle medially → tenses TM
• Stapedius (innervated by CN VII): pulls stapes posteriorly → reduces ossicular mobility

Important Middle Ear Landmarks

• Promontory: bony bulge of the basal turn of the cochlea on the medial wall
• Oval window: niche for stapes footplate, leads to scala vestibuli
• Round window: covered by the secondary tympanic membrane, provides pressure relief for the cochlea (opens into scala tympani)
• Facial nerve canal (fallopian canal): courses along the medial wall just above the oval window — critically vulnerable during middle ear surgery
• Cochleariform process: bony pulley for the tensor tympani tendon; landmark for the tympanic segment of CN VII

Eustachian Tube

• Equalizes middle ear pressure with atmospheric pressure
• Drains middle ear secretions
• Protects middle ear from nasopharyngeal pathogens and sound

3. Inner Ear

Cochlea (Auditory Portion)

Organ of Corti

• ~3,500 inner hair cells (IHC) — arranged in a single row; receive 90–95% of afferent cochlear nerve fibers; primary sensory detectors
• ~12,000 outer hair cells (OHC) — arranged in 3–4 rows; fewer afferent fibers but many efferent (olivocochlear) fibers; function as active amplifiers ("tuning" the system via electromotility — prestin motor protein)
• Stereocilia project upward from hair cells and are embedded in (OHC) or contact (IHC) the overlying tectorial membrane
• Rods of Corti (pillar cells): triangular support structures; with the reticular lamina and basilar membrane, form a rigid unit

Physiology of Hearing (Auditory Transduction)

Step 1 — Sound Collection and Transmission

Step 2 — Impedance Matching

• Area ratio: TM surface area (~55 mm²) vs. stapes footplate (~3.2 mm²) → ~17-fold force concentration
• Lever action of ossicles: ~1.3-fold mechanical advantage
• Combined effect: ~22-fold increase in force delivered to cochlear fluid
• Without the ossicular system, hearing sensitivity drops 15–20 dB — Guyton and Hall Textbook of Medical Physiology

Step 3 — Basilar Membrane Tonotopy

• Base (near oval window): narrow, stiff → responds to high frequencies (up to 20,000 Hz)
• Apex (helicotrema): wide, flaccid → responds to low frequencies (~20 Hz)
• High frequencies: maximum displacement near base (e.g., 8,000 Hz)
• Low frequencies: maximum displacement near apex (e.g., <200 Hz)
• This place coding (tonotopy) is the principal mechanism of frequency discrimination — Guyton and Hall

Step 4 — Hair Cell Transduction

1. Basilar membrane vibration causes the reticular lamina to rock; stereocilia shear against the tectorial membrane
2. Deflection toward the kinocilium/longer stereocilia → tip links pull open mechanically gated K⁺ channels → K⁺ flows in from high-K⁺ endolymph → depolarization
3. Depolarization opens voltage-gated Ca²⁺ channels → Ca²⁺ influx augments depolarization → release of glutamate from hair cell basal synapses → activates afferent cochlear nerve fibers
4. Deflection in the opposite direction → hyperpolarization → decreased firing

• Guyton and Hall Textbook of Medical Physiology

Step 5 — Acoustic Reflex

Step 6 — Central Auditory Pathway

Vestibular System (Inner Ear, Balance)

Anatomy

• Horizontal (lateral) canal — detects yaw (left–right rotation)
• Superior (anterior) canal — detects pitch (nodding)
• Posterior canal — detects roll (tilting side to side)
• All three are perpendicular to each other → together cover all three axes of rotation

• Utricle — macula oriented horizontally; detects forward/backward and lateral tilts
• Saccule — macula oriented vertically; detects up/down, pitch, and roll movements
• Both contain a macula: hair cells covered by an otolith membrane embedded with calcium carbonate crystals (otoconia/otoliths)

Vestibular Hair Cells

• Bending toward kinocilium → depolarization → increased afferent firing
• Bending away from kinocilium → hyperpolarization → decreased afferent firing

1. Head rotates left → canals and ampullae rotate left
2. Endolymph initially lags behind (inertia) → cupula deflected → hair cells on left canal depolarize, right canal hyperpolarizes
3. As rotation continues, endolymph catches up → deflection returns to baseline
4. When rotation stops → endolymph continues briefly → reverse deflection → perception of deceleration

Vestibular Pathways

• Cerebellum (via vestibulo-cerebellar tract) — coordination of gaze and posture
• Spinal cord (via lateral and medial vestibulospinal tracts) — postural reflexes
• Extraocular motor nuclei (via MLF) — vestibulo-ocular reflex (VOR) — stabilizes gaze during head movement
• Thalamus → cortex — conscious perception of motion and spatial orientation

Embryology of the Inner Ear

Key Clinical Correlates

Maxillary Sinus

Overview

Dimensions

• Volume: approximately 15 mL (range 9.5–20 mL)
• Height: ~33 mm; Width: ~23 mm; Depth (AP): ~34 mm
• The largest of all the paranasal sinuses (maxillary > frontal > sphenoid > ethmoid)

Embryology and Development

• Develops as an evagination from the lateral nasal wall (infundibulum/middle meatus region)
• In early childhood: the floor of the sinus lies above the nasal floor due to the presence of unerupted dentition, and the roof slopes downward medial to lateral because the orbit is disproportionately large relative to the midface
• As dentition erupts and the midface grows, the floor descends to and then below the level of the nasal floor
• Reaches near-adult size by puberty; continues slight growth into adult life
• Only the ethmoid and maxillary sinuses are present at birth — K.J. Lee's Essential Otolaryngology

Walls (Boundaries)

• Roof of maxillary sinus ≈ height of sphenoid sinus floor
• Posterior wall of maxillary sinus ≈ depth of sphenoid rostrum
• Medial maxillary wall normally lies in line with a vertical line drawn tangential to the lamina papyracea

The Maxillary Ostium and Ostiomeatal Complex

Natural Ostium

• Located in the posterior one-third of the ethmoid infundibulum
• Opens high on the medial wall, near the roof of the sinus (not at the floor — an inherently disadvantageous position for gravity drainage)
• Opens at a 45° angle into the floor of the infundibulum
• Diameter: 2.4–4.8 mm (may be enlarged by accessory ostia)

Ostiomeatal Complex (OMC)

• Laterally: lamina papyracea (medial orbital wall)
• Medially: middle turbinate

Mucosal Lining and Histology

• Cilia beat in a coordinated spiral pattern directed toward the natural ostium regardless of where material deposits in the sinus (including the floor)
• Mucociliary transport rate in the sinus: ~6 mm/min
• Mucous blanket: two layers — a sol (periciliary) layer and a gel (mucous) layer
• Mucociliary clearance is the primary defense mechanism; impairment (e.g., from viral URI, ciliary dyskinesia, cystic fibrosis) predisposes to sinusitis

Blood Supply

• Infraorbital artery (from maxillary artery, 2nd part) → roof and anterior wall
• Posterior superior alveolar artery (from maxillary artery) → posterior wall and floor
• Greater palatine artery (from pterygopalatine part of maxillary artery) → floor
• Anterior superior alveolar branch of the infraorbital artery → anterior wall/floor

• Into the pterygoid plexus (via veins accompanying the arteries) → internal jugular vein
• Small communications with the facial vein anteriorly

Innervation

Lymphatic Drainage

• Anterior: facial lymphatics → submandibular nodes
• Posterior: retropharyngeal and deep cervical nodes

Important Anatomic Relations and Clinical Significance

Relation to Teeth

1. First molar (most commonly projects into the sinus)
2. Second molar
3. Second premolar
4. Third molar

• Odontogenic sinusitis (dental infection seeding the sinus directly)
• Oroantral fistula after tooth extraction
• Referred dental pain in maxillary sinusitis

Relation to the Orbit

• Orbital complications of maxillary sinusitis (orbital cellulitis, subperiosteal abscess)
• Orbital floor fractures ("blowout fractures") → herniation of orbital contents into the sinus
• The infraorbital nerve runs in a canal in the roof and may be dehiscent (14%) → facial numbness in surgery or trauma

Relation to the Pterygopalatine Fossa

• Pterygopalatine ganglion
• Internal maxillary artery
• CN V2 (as it enters from foramen rotundum)
• Vidian nerve

Anatomic Variants Relevant to Maxillary Sinus

Physiology and Functions

1. Humidification and warming of inspired air — contribution is minimal given the small ostium
2. Lightening of the skull — reduction of cranial weight without sacrificing structural strength
3. Resonance of the voice (though this effect is minimal)
4. Immune defense — the mucous blanket traps and the mucociliary escalator clears bacteria and particles toward the ostium and into the nasal cavity
5. Thermal insulation of orbital contents and brain from facial temperature changes
6. Crumple zones — absorb mechanical shock from trauma to the facial skeleton

Key Clinical Correlates