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Your olfactory nerve is responsible for your sense of smell and plays a big part in your sense of taste, too. Learn about how it works, disorders that affect it, and how it can regenerate.

Can You Smell That?

Close your eyes and imagine that you are walking through a garden of roses. How do you know there are roses around you if your eyes are closed? You can smell them, right? The smell of a rose is easily identifiable to most people. In fact, you have the ability to detect millions of different odors, from the pleasant smell of a rose garden to the harsh smell of a burning building. Without a properly functioning olfactory nerve, you wouldn’t be able to smell any of these things.

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Your olfactory nerve is also very important in giving you a sense of taste, so if it is not functioning, then you wouldn’t be able to taste much either!

Function of the Olfactory Nerve

The olfactory nerve is responsible for your sense of smell and partially responsible for your sense of taste. It is also known as cranial nerve 1 because it is the shortest of the cranial nerves and one of only two nerves (the other is the optic nerve) that bypass the brain stem and connect directly to your brain.The job of your olfactory nerve is to detect chemicals floating around in the air and transmit information about them to your brain, where you interpret this as a certain smell.

Let’s look at exactly what happens when you smell something, like a rose.First, small droplets of chemicals that are released by the rose enter your mouth and nose when you breathe. These chemicals dissolve into the mucous layer that lines your nasal cavity. High up inside your nose, about 7 cm above and behind your nostrils, is a region of the nasal cavity known as the olfactory epithelium.

The cells there, called olfactory epithelial cells, are very specialized. On the side facing your nasal cavity (and so exposed to the smell-inducing chemicals), they are covered with tiny hairs called cilia, and on the other side, they are connected to an axon, which is the main body of a nerve cell.The cilia react to specific chemicals and send electrical signals through the axons of the olfactory nerve to your brain.

Each axon passes through small holes in your skull in an area known as the cribriform plate. Just below your brain, they all pass into the olfactory bulb, where the signals from each axon come together and are sent on to the brain via the olfactory tract.Once these electrical signals get to your brain, neurons there interpret the specific pattern of impulses as a specific smell.

Some of the parts of your brain responsible for smell are also responsible for memory and emotions, which is why a certain smell can trigger a specific memory or emotion every time you encounter it.

Disorders of the Olfactory Nerve

Damage to the olfactory nerve can result in three different disorders. The first, anosmia is an inability to smell at all. Some people with olfactory nerve damage develop anosmia, but some simply have a reduced ability to smell.

This is called hyposmia. Finally, olfactory nerve disorders can cause people to develop dysosmia, where smells are mixed up so that a pleasant aroma like that of a rose might smell bad, like rotting garbage.Most of the time, anosmia and hyposmia are temporary and are caused by inflammation in the nasal cavity which prevents chemicals from reaching the olfactory epithelium. This is what happens when you have a cold or sinus infection. Sometimes, however, these conditions can become permanent. This can happen if certain viruses or toxins damage the epithelial cells of the olfactory system, or if they are damaged by trauma. This often happens when the cribriform plate is broken, severing the axons of the olfactory nerve.

Even if the cribriform plate is not broken, a blow to the head or face can damage these delicate cells and result in a partial or total loss of smell.Anosmia can also be a symptom of several degenerative brain diseases, like Parkinson’s disease and Huntington’s disease. In these diseases, neurons throughout the brain are affected, but the olfactory nerve is often one of the earliest nerves to develop noticeable dysfunction.In addition, most people gradually lose some of their sense of smell as they get older. We don’t really know exactly why this happens, but it is thought that after many years of damage from viral infections and small traumas, the epithelial cells lose their sensitivity. This is not typically a total loss of smell, but damage to the olfactory nerve means that elderly people typically cannot detect odors as well as young people.

Regeneration of the Olfactory Nerve

Although most nerve cells do not regenerate at all, the olfactory nerve is unusual because it does have the capacity to regenerate in some situations. If damage occurs to only the epithelial cells, they have the ability to regrow in both children and adults. If the damage is more extensive and there is damage to the olfactory bulb, scar tissue may make it impossible for the nerve to regenerate.One interesting side effect of olfactory nerve regeneration is one of the conditions mentioned earlier, dysosmia. Sometimes, when the nerves are repairing themselves following an infection or injury, they get a little bit mixed up, and odors which once were identified correctly in the brain now are associated with a different pattern of signals. This makes you unable to accurately identify certain smells.

Pleasant smells may smell very bad to you, while unpleasant smells may smell good.

Lesson Summary

To review, the olfactory nerve is responsible for your sense of smell. It does this by detecting chemicals floating around in the air and transmitting information about them to your brain, where you interpret the signal as a certain smell.The olfactory nerve begins in the region of the nasal cavity known as the olfactory epithelium.

The epithelial cells there are covered with tiny hairs called cilia on one side, and they are connected to an axon, which is the main body of a nerve cell, on the other side. These axons pass through a part of the skull with many tiny holes in it, called the cribriform plate and come together in the olfactory bulb. All the signals are collected there and sent on to the brain through the olfactory tract.Disorders of the olfactory system include anosmia (an inability to smell), hyposmia (a reduced ability to smell), and dysosmia (a distortion of your perception of a smell).

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