Nasal Cavity Anatomy

The Nasal Cavity

• The nose serves as the primary sensory organ for smell
• It contributes to respiration and speech production
• The nasal cavity is located behind the two nostrils, forming the interior part of the nose
• The nasal cavity, along with the paranasal sinuses, oral cavity, pharynx, and larynx, constitute the respiratory system

Vestibule

• The vestibule area is located right behind the nostrils
• It is the most exposed part of the nasal cavity
• It is lined with stratified squamous epithelium
• This lining acts as a protective barrier
• Stiff hairs known as vibrissae are present, embedded in the cell lining and covered with thick mucus

Respiratory Region

• The respiratory region starts at the end of the vestibular region
• It includes the nasal cavity except for the olfactory region
• It features a ciliated pseudostratified epithelium lining containing goblet cells, secreting mucus to form the mucous membrane

Olfactory Region

• The olfactory region sits atop the nasal cavity near the nasal septum
• It contains olfactory cells and olfactory mucosa
• The mucosa contains millions of cilia with olfactory receptor proteins for the sense of smell

Medial Wall of Nasal Cavity

• The medial wall refers to the septum, dividing the nasal cavity into two equal sections
• The septum's bony upper and lower parts are formed by the perpendicular plate of the ethmoid bone and the vomer
• The lower front part is formed by a quadrangular hyaline cartilaginous structure known as the septum cartilage
• The lower part of the smooth medial wall is visible externally, divided by the nostrils

Superior Border (Roof)

• The roof consists of:
  • Two nasal bones
  • Lower part of the frontal bone
  • Cribiform plate (of the ethmoid bone)
  • Sphenoid bone

Anterior Border (Lateral Wall)

• The front part of the nasal cavity houses:
  • Lower parts of the nasal bones
  • Attached cartilage
  • Outer nose
  • Nostrils

Posterior Border (Back Wall)

• The upper back and sides are bordered by cranial bones:
  • Maxilla
  • Ethmoid bone
  • Palatine bone
  • Sphenoid bone
  • Lacrimal bone
  • Inferior concha

Floor

• The floor separates the nasal cavity from the oral cavity
• It contains the hard palate formed by the palatine process of the maxilla
• The palatine bones (horizontal plates) also contribute to its smooth surface

Nasopharynx

• The nasal and oral cavities connect through the nasopharynx
• It is located at the back of the throat
• The soft palate and uvula move to close off this connection when eating, preventing food from entering the nasal cavity

Paranasal Sinuses

• These are air-filled extensions of the nasal cavity
• There are four paranasal sinuses named after the bones they are located in:
  • Maxillary
  • Frontal
  • Sphenoid
  • Ethmoid
• Each sinus is lined with ciliated pseudostratified epithelium with mucus secreting goblet cells

Paranasal Sinuses Functions

• Debated, but the most logical functions include:
  • Lightening the weight of the head
  • Supporting the immune defence of the nasal cavity
  • Humidifying inspired air
  • Increasing resonance of the voice

Paranasal Sinuses Formation and Drainage

• They form during development as the nasal cavity erodes surrounding bones
• All sinuses drain back into the nasal cavity
• Openings can be found on the roof and lateral nasal walls

Frontal Sinuses

• Two frontal sinuses are located within the frontal bone of the skull
• They are the most superior
• They are triangular
• Drainage occurs via the frontonasal duct
• They open out at the hiatus semilunaris within the middle meatus of the nasal cavity
• Innervation: Supraorbital nerve (branch of the ophthalmic nerve)
• Blood supply: Anterior ethmoid artery (branch of the internal carotid artery)

Sphenoid Sinuses

• Situated within the sphenoid bone
• They open into the nasal cavity in an area superior/posterior to the superior concha, known as the spheno-ethmoidal recess
• Innervation: Posterior ethmoidal nerve (branch of the ophthalmic nerve) and branches of the maxillary nerve
• Blood supply: Via the pharyngeal branches of the maxillary arteries

Ethmoid Sinuses

• There are three ethmoid sinuses within the ethmoid bone
  • Anterior: Opens onto the hiatus semilunaris (middle meatus)
  • Middle: Opens onto the lateral wall of the middle meatus
  • Posterior: Opens onto the lateral wall of the superior meatus
• Innervation: Anterior and posterior ethmoid branches of the nasociliary and maxillary nerves
• Blood supply: Anterior and posterior ethmoid arteries

Maxillary Sinuses

• They are the largest sinuses, located laterally and slightly inferior to the nasal cavity
• Drainage occurs into the nasal cavity via the hiatus semilunaris underneath the frontal sinus opening
• This is a potential pathway for infection, allowing fluid drainage from the frontal sinus to enter the maxillary sinus
• Innervation: Branches of the alveolar nerve plus the infraorbital nerve
• Blood supply: Branches of alveolar arteries plus infraorbital and greater palatine arteries

Clinical Relevance of Paranasal Sinuses

• Upper respiratory tract infections can spread to the sinuses due to their continuity with the nasal cavity, causing inflammation and pain known as sinusitis
• Positioning and the use of aerosols should be considered carefully if the patient is having difficulty breathing
• Inflammation of the maxillary sinus can manifest as toothache due to the maxillary nerve supplying both the maxillary sinus and maxillary teeth