The Nasal Cavity

Sumamry Ever wondered how you can identify smells? This lesson will describe the nasal cavity, the physiology of smell and why these are relevant.

Introduction

The function of the nose and nasal cavity is to clean and warm the air that enters the respiratory tract.

It is lined by respiratory mucosa, ie. Pseudostratified ciliated epithelium with goblet cells.

The goblet cells secrete mucus to trap particles/pathogens, as well as warming it via the rich blood supply. The cilia aid this by encouraging movement of the mucus.

The conchae in the nasal cavity are present to increase the surface area in which these processes take place. A small part of the nasal cavity is covered by the olfactory mucosa.

The nasal cavity meets the nasopharynx at the posterior nasal apertures (1) between the medial pterygoid plates.

Its height extends from the cribriform plate of the ethmoid bone to the palate. It is divided by the nasal septum andReviseDental.com extends below the orbit. The Nose

The ‘external nose’ is made up of the nasal bones, upper & lower cartilages along with the septum cartilage.

The bridge of the nose is made up of the nasal bones and the frontal processes of the maxilla. The main part of the nose that we see is cartilage. The nostrils bulge due to fatty fibrous tissue deposits. This then opens inside the nostrils and makes the nasal vestibule. This contains hairs that filter the air we breathe in.

The Nasal Cavity

ReviseDental.com The Roof

This is made of the nasal bones, cribriform plate and the body of the sphenoid.

The Floor

This is the bony palate (palatine processes of the maxilla and the horizontal plates of the palatine bones).

Anteriorly, the palatine processes have the incisive canal which the nasopalatine nerves pass through (+ terminal branch of greater palatine artery). Clinically, these foramina sit in the incisive fossa of the palate.

The nasopalatine nerve supplies the mucosa of the anterior part of the hard palate.

Lateral Wall

The maxilla and labyrinth of the ethmoid make up most of the lateral wall. On these walls are the conchae, folds of bone that stick out into the nasal cavity. The maxilla makes up the part below the middle concha, and the ethmoid includes the middle concha and anything above it.

Conchae and Surrounding Anatomy

The nasal cavity is divided into 3 parts by the conchae. The superior meatus, the middle meatus and inferior meatus.ReviseDental.com These sit between their respective conchae.

Above the superior concha, is the sphenoethmoidal recess. The nasal cavity opens into the maxillary sinus via the maxillary hiatus. In the skull, this is covered by the articulating bones and conchae.

The ethmoidal air cells open into the nasal cavity into the superior and middle meatus.

The nasolacrimal groove lies behind the frontal process. The lacrimal bone sits in the lateral wall of the nasal cavity, between the maxilla and ethmoidal labyrinth. On its other side, it contributes to the medial wall of the orbit. The nasolacrimal canal starts at the lacrimal groove in the orbit and opens just below the inferiorReviseDental.com concha in the nasal cavity. Its function is to drain tears.

The maxillary sinus opens into the posterior part of the hiatus semilunaris (curved gap between a swelling below the middle concha). The frontal sinus opens into the anterior part of this hiatus via the frontonasal canal.

The Septum

This is a thin bone extending from the floor to the roof of the nasal cavity. This comes from the perpendicular plate of the ethmoid and the vomer.

It is innervated by the medial nasal branches of the maxillary nerve.

It receives blood from the sphenopalatine branch of the maxillary artery.

Both of these enter the nasal cavity via the sphenopalatine foramen.

Posteriorly, venous blood from the septum drains into the pterygoid plexus, whilst lymph drains into the retropharyngeal nodes. Anteriorly, it is drained by the facial vein, whilst lymph drains to submandibular nodes.

Mucosa

The nasal cavity is lined by the respiratory mucosa. However, the roof and down to just below the superior conchae, are lined by olfactory epithelium.

Olfactory epithelium contains olfactory receptor cells. These are like primary sensory neurons. These have unmyelinated axons which pass through the cribriform plate and synapse in the olfactory bulb, leading to olfactory nerves.

The olfactory epithelium also contains serous glands.

Nerve SupplyReviseDental.com

The following diagrams demonstrate the nerves that surround and pass through the nasal cavity. ReviseDental.com

You can split the lateral wall of the nasal into 4. Each part is innervated and supplied by different nerves/vessels. You can find the details in the diagram below: Nerve supply and arterial supply of the quadrants of the lateral nasal wall

The mucous glands in the nasal cavity receive parasympathetic secretomotor innervation from branches of the facial nerve, which have synapsed in the pterygopalatine ganglion.

Blood Supply

The venous drainage of the nasal cavity is divided anteriorly and posteriorly. Posteriorly, the nasal cavity is drained into the pharyngeal and pterygoid plexuses. Anteriorly, the nasal cavity is drained by the facial vein.

ReviseDental.com ReviseDental.com Lymph

Again, this can be split into anterior and posterior parts. Anteriorly, the lateral wall drains to the submandibular nodes. Posteriorly, this drains to the retropharyngeal nodes.

Physiology of Olfaction (2)

Smell is a chemical sense, involving olfactory receptors.

In order to be sensed, the molecule stimulants need to be dissolved.

Fun Fact: The impulses involved in the sense of smell can terminate in the limbic system and hence cause emotional responses/links (2).

Fun Fact 2: Humans can distinguish about 10,000 types of odours (2). Hence, the nose contains between 10 million-100ReviseDental.com million receptors for olfaction (smell).

These receptors sit within the olfactory epithelium at the top of the nasal cavity (covers superior nasal concha and the inferior part of the cribriform plate).

The olfactory epithelium is made up of olfactory receptor cells, supporting cells and basal cells.

Olfactory receptor cells: These are first-order neurons in the olfactory pathway. At one end, there is an exposed dendrite, and at the other, an axon extending into the olfactory bulb. This axon passes through the cribriform plate.

There are non-mobile cilia which sit on the surface of the dendrite. This is where olfactory transduction occurs. This is because, the membranes of these cilia contain olfactory receptors that bind to odorants. Upon binding, an action potential is created.

The supporting cells are columnar epithelium from the lining of the nose. These support, nourish, insulate and detoxify the olfactory receptor cells.

Basal cells are ReviseDental.comstem cells that sit between the supporting cells. Their function is to differentiate into new olfactory receptor cells and neurons every month.

Underlying this epithelium is CT, which contains Bowmans glands. These produce mucus which wets the epithelium and dissolves the odorants to allow binding and transduction to occur.

The supporting cells and Bowmans glands are innervated by the parasympathetic supply of the facial nerve. This is why stimulation can sometimes then stimulate the lacrimal glands and mucous glands in the nasal cavity (hence you cry/sniffle at certain pungent scents). Depolarisation of the olfactory receptor membranes occurs and this generates an action potential.

1. An odorant binds to an olfactory receptor protein in a cilia. This activates G-proteins inside the cell. 2. This G-protein then activates the enzyme adenylate cyclase to produce cAMP. 3. The cAMP opens a sodium ion channel, which allows Na+ ions to enter the cell, hence causing depolarisation of the membrane. 4. This generates an action potential down the axon of the olfactory receptor cell, which reaches the olfactory bulb.

Adaptation:

Olfaction has a low threshold. Therefore, only a few small molecules can be present in order to be detected and stimulate a response.

We can stop noticing certain smells very quickly. This is called adaptation/decreasing sensitivity. The smell is still there, but our body has learned to ignore it.

Pathway:

The axons of the olfactory receptors pass through many small foramina in the cribriform plate (of the ethmoid bone) and extend and collect into the olfactory nerves (2).

These olfactory nerves terminate in a mass of grey matter termed the olfactory bulbs. These sit below the frontal lobes of the cerebrum and lateral to the crista galli of the ethmoid bone (2).

In these bulbs, ReviseDental.comthe axons of the olfactory receptor cells synapse with the olfactory bulb neurone cell bodies. These neurones then extend backwards and form the olfactory tract. Some of the neurones of the olfactory tract can terminate to the primary olfactory area in the cerebral cortex.

Hence, the olfactory nerves are actually classified as outgrowths from the brain, rather than nerves in themselves.

Fun Fact: the olfactory impulses are the only ones to reach the cerebral cortex without first synapsing in the thalamus. (2) Some pathways will lead to the frontal lobe. It is here, in the orbitofrontal area, that we can identify and distinguish different odours. Damage to this area will result in difficulty in this.

Paranasal Air Sinuses

Introduction

There are 4 bones that contain sinuses that communicate with the nasal cavity. These bones are the maxilla, frontal, sphenoid and ethmoidal labyrinth. There is no uniform shape for these sinuses.

The sinuses grow the most during adolescence and will expand throughout life.

Again, they are lined by respiratory mucosa. Inflammation of this lining is common in head colds. The cilia move the mucus towards the opening in order to assist drainage. However, during sinusitis, the number of cilia deplete and hence decreases movement so recurrence of infection is more likely (chronic sinusitis). ReviseDental.com We are unsure of the function of the paranasal sinuses.

We do know they are involved in voice moderation and reduce the weight of the head due to the air they hold (1). (Note: wearing glasses would outweigh the effect of the sinuses (1)). The more accepted opinion is that they are banks of mucus to add to that of the nasal cavity for protection and reinforcement if the nasal cavity is dry. Maxillary Sinus

Also termed the ‘Maxillary Antrum’.

These are large sinuses that sit in the maxilla. They are pyramid shaped, with their tip in the zygomatic process.

The thin walls are made up of the nasal, orbital, anterior and infratemporal surfaces of the maxilla.

The floor sits above the mouth and forms the tops of the sockets of the maxillary premolars and molars. In someReviseDental.com cases, it may be absent with only a mucosal lining separating the sinus and the sockets.

The infra-orbital nerve in the base of the orbit (also the roof of the maxillary sinus) causes a ridge which can be seen in the roof of the sinus.

On the anterior wall, there is also a ridge containing the anterior superior alveolar nerves. The posterior and middle superior alveolar nerves pass along the posterior wall of the sinus to their respective teeth. Remember, the MB root of an Upper 6 is innervated by the middle superior alveolar nerve and hence may require 2 infiltrations.

These nerves supply the teeth and sensation to the mucosa lining the sinus. The relevant arteries travel with these nerves.

The venous drainage of the sinus goes into the pterygoid plexus and the lymph drainage is to the submandibular nodes.

Relation to nasalReviseDental.com cavity:

The sinus opens into the posterior part of the hiatus semilunaris in the middle meatus of the nose. The drainage of this sinus is reliant on the movement of cilia, as gravity is not functional to do this. Frontal Sinus

The frontal bone has two hollow sinuses that sit just above the orbits. These vary in size and can extend backwards over the orbit. The two sinuses are separated by a sheet of bone called the septum.

The supra-orbital nerve supplies the lining of the frontal sinuses.

It receives its blood supply via the supraorbital and anterior ethmoidal arteries, whilst being drained by the supra-orbitalReviseDental.com and superior ophthalmic veins.

Lymph drainage is to the submandibular nodes.

Relation to Nasal Cavity:

The frontal sinuses open into the nasal cavity via the frontonasal canal, into the anterior part of the hiatus semilunaris of the middle meatus. Occasionally, the canal may open directly into the anterior part of the middle meatus. As they drain very closely to one another, an infection of the frontal sinus can commonly spread to the maxillary sinus.

Frontal sinusitis can present as a dull ache in the region of the sinus, with some tenderness.

Sphenoidal Sinus

The body of the sphenoid is hollowed out into 2 spaces, which are separated by a septum of bone. These are the sphenoidal sinuses. These drain into the sphenoethmoidal recess of their respective sides.

They receive their nerves and blood supply via the posterior ethmoidal nerves and vessels.

The lymph drainage is to the retropharyngeal nodes.

Ethmoidal Sinus

The ethmoidal labyrinths contain air cells with thin walls, which interconnect. They can be divided into the anterior, middle and posterior groups.

The anterior group opens via many orifices into the frontonasal canal in the middle meatus.

The middle group sits in the bulla and opens by many orifices into the middle meatus (above the bulla). ReviseDental.com

The posterior group opens into the superior meatus via one orifice.

All ethmoidal sinuses are innervated by the anterior and posterior ethmoidal nerves. Their blood supply is via the corresponding arteries and veins to these nerves.

Lymph drainage passes through the submandibular nodes to the retropharyngeal nodes. The mucous glands in their linings are supplied by parasympathetic innervation of the facial nerve via the pterygopalatine ganglion.

Summary of Drainage into Nasal Cavity (1)

Opening? What drains via this? Spheno-ethmoidal recess Sphenoidal air sinuses Superior meatus Posterior ethmoidal air cells Middle meatus Frontal sinus, Maxillary sinus, anterior + middle ethmoidal air cells Inferior meatus Nasolacrimal canal

ReviseDental.com Clinical Relevance

Sinusitis – accompanied by fever and general illness. It can feel tender over the sinuses themselves also. Patients usually complain of pain when moving their head. It is commonly mistaken for maxillary toothache.

Referred Pain often occurs in the maxillary sinus, due to its proximity to the roots of the maxillary teeth.

Ie. Patients complaining of toothache with no clear cause.

To investigate, it is possible to press over the maxillary sinus and test for tenderness, as well as taking an OPG. If in the OPG, the floor is intact and the sinus is radiolucent, then it is normal. If the sinus appears radio-opaque, this could suggest infection.

The opposite can also occur- where a patient believes they have sinusitis when they actually have dental problems.

Maxillary sinusitis is usually treated with analgesic and routine advice (eg. Steaming), and if persistent, ephedrine nasal drops (0.5%) or 500mg amoxicillin three times daily for 7 days can be prescribed. (3)

Maxillary Sinus can be involved in extraction of maxillary teeth & some surgical procedures.

Spread of infection can occur or appear to occur into the maxillary sinus. Prior to extraction of maxillary premolars and molars, a thorough radiological examination is to be carried out to assess the risk of an oro-antralReviseDental.com communication.

Once the extraction has been completed, the roots should be assessed for any bone fragments from the floor of the sinus, and the socket should be checked for air bubbles or a hole. These are all indications of an OAC. If this OAC is left or undiagnosed, it may become lined by epithelium and form a oro-antral fistula.

To test for an OAC/OAF, patients can be asked to hold their nose and blow out which may cause bubbles in the socket (caution: this may cause the sinus floor to break if thin or compromised), or to close their mouth and blow out their cheeks (1). Patients may also complain of nasal regurgitation.

Upper respiratory tract infections can be related to inflammation of the lining in these sinuses.

Conclusion You should now be aware of the anatomy of the nasal cavity and paranasal sinuses, including their nerve, blood and lymph supply/drainage. You should also now understand the physiology of olfaction.

Third Party Links It is recommended to look at the references and third party reading for more text and diagrams that may aid your understanding.

Bell GW, Joshi BB, Macleod RI. Maxillary sinus disease: diagnosis and treatment. British dental journal. 2011 Feb;210(3):113-8. Ali K, Prabhakar E, editors. Essential Physiology for Dental Students. Newark: John Wiley & Sons.

References

1. Atkinson ME. Anatomy for dental students. Oxford University Press; 2013 Mar 14. 2. Tortora G. J., and Bryan Derrickson. Principles of Anatomy and Physiology. Hoboken. 3. Drug Prescribing for Dentistry. Third Edition. 2016 Jan. 4. Norton NS. Netter’s head and neck anatomy for dentistry e-book. Elsevier Health Sciences; 2016 Sep 13

Please note: The images in this lesson are original drawings and have been used by collating and averaging images from the sources above.

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