The Nasal Cavity
Total Page:16
File Type:pdf, Size:1020Kb
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).