The nose is commonly known as the organ that produces the sense of smell, yet it is the structures deep to the nose in the nasal cavity that produce this important sense. Using the complex olfactory system of the nasal cavity, humans can differentiate at least 5,000 to 10,000 different scents. Flanking the nasal cavity are the infamous sinuses, which produce pain and discomfort when inflamed. Yet, in spite of their infamy, the sinuses provide several important functions to the head.
The nose is a structure of bone, cartilage, skin, and muscle that extends from the face and forms the anterior wall of the nasal cavity. Deep to the nose is the hollow nasal cavity, which contains many folds of epithelium-covered bone. These convolutions of the nasal cavity increase its surface area and direct the flow of air through the cavity on its way to and from the lungs.
On the roof of the nasal cavity is a special type of tissue known as olfactory epithelium that is specialized for producing the sense of smell. Like all epithelium, olfactory epithelium is a sheet of cells that cover the underlying tissues. Olfactory epithelium is unique in that it contains many sensory cells that protrude from its surface and are embedded in mucus secreted by goblet cells in the epithelium. Deep to the olfactory epithelium are neurons, which conduct sensory signals to the brain.
Surrounding the nasal cavity are the paranasal sinuses, a group of hollow structures within the bones of the face. Each sinus is lined with epithelium and closely resembles the nasal cavity in its structure.
- The maxillary sinuses are a pair of cavities within the maxillae of the face and are the largest of the paranasal sinuses. They are found inferior to the orbits, flanking the nasal cavity to the left and right. Tiny passages connect the maxillary sinuses to the nasal canal, allowing air to pass between them.
- The frontal sinuses are a pair of cavities found in the frontal bone superior to the orbits and superior to the nasal cavity. These sinuses are separated from the nasal cavity by a thin layer of bone and do not connect to the nasal canal.
- The ethmoidal sinuses are a group of small cavities within the ethmoid bone. These cavities are also known as air cells and vary in number and size between individual people. They are often arranged symmetrically along the body’s midline.
- The sphenoidal sinuses are a pair of cavities within the sphenoid bone. They are rounded and uneven in size, with a thin bony wall separating them.
The nose plays many important roles in the conduction of air into the lungs. Air entering the nose from the body’s exterior is often quite different in temperature and humidity from the air within the body. The convoluted inner structure of the nose increases the surface area of the respiratory tract and forces air to contact the mucous membranes lining the nasal cavity. As air passes over the mucous membranes, it is warmed and humidified so that it does not shock the delicate lungs. Hairs and mucus on the interior of the nose also catch any solid debris before it can enter the lungs. As air is exhaled from the lungs, it passes back through the nose where its moisture and heat is trapped by the membranes and used to warm and humidify the next breath of air.
Some of the air passing through the nose passes over the olfactory epithelium, whose thin layer of mucus traps some of the molecules. These molecules next come in contact with olfactory hairs extending from olfactory receptor neurons in the epithelium. Odorant molecules stimulate the olfactory receptor neurons to produce an action potential, which travels along their axons through holes in the ethmoid bone to the olfactory bulb. Neurons in the olfactory bulb receive the action potentials and carry the nerve signals into the olfactory area of the brain where the information is decoded and processed to produce the sense of smell.
The role of the sinuses is less clear than the roles of the nose and olfactory epithelium. One known function of the sinuses is to reduce the mass of bone in the skull and decrease the weight of the head. Another function of the sinuses is the production of vocal resonance, which affects the tone of the voice while singing and speaking. The air trapped within the sinuses may also produce some additional functions, such as balancing pressure within the head.