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The Nose

We have two nostrils in our nose, which serve as the entranceway into the nasal cavities. The nostrils are separated by a wall of cartilage called the nasal septum. Damage to the nose, such as from a punch or a hard fall, can cause a deviated septum, in which the wall of cartilage is no longer straight. A deviated septum can interfere with both proper breathing and the sense of smell. The nasal septum may also be punctured or perforated. For example, chronic cocaine use can cause holes to form in the septum, which can also interfere with breathing and olfaction. In human beings, the two nasal cavities are extremely close to each other, so they are essentially sampling the same air. Thus, there is no real analogy to the binocular vision our two eyes can achieve or the ability of our auditory system to integrate across ears, such as in sound localization.

Inside the nasal cavity, turbinates serve to disperse air. Turbinates are bony knots of tissue that ensure that some air will be passed upward through a space called the olfactory cleft and land on an area of tissue called the olfactory epithelium. Olfactory receptor neurons are located inside the olfactory epithelium, and these neurons serve as the transducers of the olfactory system. The olfactory receptor neurons are pretty far into the nasal cavity; indeed, they are just a couple of centimeters behind each eye. The air passes through the cleft and then rejoins the air being sent through the pharynx toward the lungs. Odorants from food find their way to the olfactory epithelium through a passage in the oral cavity in the back of the mouth.

The olfactory epithelium serves as the organ of transduction, taking chemical stimulation and transforming it into a neural signal. There is an olfactory epithelium at the top of each nostril, measuring about 1 to 2 cm2. In addition to the olfactory receptor neurons, the olfactory epithelium contains supporting cells and basal cells. Supporting cells provide metabolic supplies to the olfactory receptor neurons. Basal cells create olfactory receptor neurons. Interestingly, olfactory receptor neurons die out after about a month, so basal cells are continually resupplying the olfactory epithelium with olfactory receptor neurons. It is estimated that olfactory receptor neurons are completely regenerated every 28 days.

The olfactory receptor neurons have cilia extending into the mucus covering of the olfactory epithelium. These cilia contain the transducing elements of the cell on their tips. As a particular molecule of a particular odorant comes into contact with the tip of the cilium of the olfactory receptor neuron, a chain of chemical events is initiated, ending with an action potential leaving the olfactory receptor neuron along an axon heading toward the olfactory bulb. That is, the chemical triggers the cilia, causing a neural signal to begin. As few as seven molecules of the same odorant can trigger an action potential in an olfactory receptor neuron.

Answer the following questions:

Describe the anatomy of the nose How many receptors and types of receptors do we have? What makes us microsmatic as compared to macrosmatic?