The Anatomy of the Olfactory Nerve

Your sense of smell and what can damage it

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Actually a pair of cranial nerves, the olfactory nerve transmits information to the brain from smell receptors in the nose. The olfactory nerve is sometimes referred to as the first cranial nerve, or CN1. Cranial means "of the skull." Of the 12 cranial nerves, the olfactory nerve and the optic nerve, which relays visual information to the brain, are the only ones that don't connect to the brainstem.

Anatomy

The olfactory nerve is the shortest nerve in the human head. It originates in the olfactory mucosa (mucous membrane) along the roof of your nasal cavity (nostril). This nerve is made of many small nerve fibers called fascicles that are bound together by thin strips of connective tissue.

The bundle extends from the nasal cavity through the ethmoid bone behind your nose. From there, the fascicles go inside a structure called the olfactory bulb. You have a bulb for each nostril, and they send the information along what's called the olfactory tract and into the brain.

These impulses go to several regions of your brain, including the:

  • Uncus
  • Hippocampal gyrus
  • Amygdala
  • Entorhinal cortex

Function

Unlike many other nerves, the olfactory nerve has one job—making you able to smell things.

When particles in the air enter your nasal cavity, they interact with the receptors on the olfactory nerve and a type of tissue called the olfactory epithelium, which is in several areas of the nasal cavity and contains millions of receptors.

All those receptors then send that information they've gathered to the central nervous system. Your brain then interprets that information as scent.

Associated Conditions

A number of conditions can interfere with your sense of smell, as can an injury. These can cause a reduced or completely lost sense of smell, phantom smells, or even a heightened sense of smell.

The following terms are used to describe certain symptoms associated with disordered odor perception:

  • Dysosmia: Dysosmia can cause a distortion in how you perceive an actual odor, or it can cause phantom smells—odors that aren't actually there. This is called an olfactory hallucination. In most cases, this is due to degeneration of the olfactory epithelium.
  • Anosmia: Anosmia is the total loss of the sense of smell. It can be caused by infection, blockage, or head injury.
  • Hyposmia: Hyposmia is a reduced ability to detect odors. It can be caused by allergies, nasal polyps, viral infections, and head trauma.
  • Hyperosmia: Hyperosmia is a relatively rare condition involving a heightened sense of smell. It sometimes happens on its own but can also occur as part of another condition, including autoimmune disease, Lyme disease, and pregnancy. Certain odors may cause severe discomfort and may lead to headache, migraine, nausea, and vomiting. The smells most likely to trigger symptoms include chemical smells, perfumes, cleaning products, and scented candles.

Changes to your sense of smell have a big impact on your sense of taste, as well.

Post-Viral Olfactory Loss

The most common condition to impact the olfactory nerve is the common cold, but other viral illnesses can have the same effect.

You probably know that when nasal congestion fills your sinuses, it can result in a lowered ability to smell that comes back after the congestion clears up.

Sometimes, though, it takes a while to come back all the way. This is called post-viral olfactory loss (PVOL), and everyone probably experiences it at some point. Researchers don't understand exactly why this happens, but they suspect it's because certain viruses—including the common cold and influenza—somehow damage the mucous membrane and the olfactory epithelium.

Some people will have a sudden and noticeable drop in smell sensitivity. In others, it's a gradual loss over the course of several acute illnesses which most people get a few times a year.

Post-Traumatic Olfactory Loss

Anosmia or hyposmia can result from a head injury, which is called post-traumatic olfactory loss (PTOL). The loss is connected to both the severity of the injury as well as the part of the head that's damaged. Injuries to the back of the head are the most likely ones to cause loss of smell.

That might seem odd since the olfactory nerves are in the front of the brain. When there's an impact on the back of the head, the brain can come forward and collide with the inside front of the skull—right where the olfactory nerve is. Then, as the brain bounces back, it yanks on the delicate nerve fibers, which can snag on the rough edges of the tiny holes in the skull that they extrude through.

The olfactory nerves can become severed in this way, but often the smell loss is due to bruising of the olfactory bulb.

PTOL also can be caused by damage to the face, such as a blow to the nose, as well.

Other Causes of Olfactory Loss

A decreased sense of smell can also occur due to tumors, such as meningiomas of the olfactory groove as well as be an early feature of some neurological diseases such as Parkinson's disease, Alzheimer's disease, and Lewy body dementia.

Treatment

If you've lost your sense of smell to a known cause that's treatable—such as by surgically removing nasal polyps, straightening the septum, or clearing out the sinuses—it's possible that your sense of smell will improve over time.

That's what happens in many cases of post-viral olfactory loss, although the sense may never be fully restored. A study published in 2014 found that more than 80 percent of participants reported improvement in olfactory ability one year after they were diagnosed with loss.

If your decreased sense of smell is due to a Parkinsonian syndrome or Alzheimer's disease, the treatment is usually directed towards the condition itself, often with no significant impact on the diminished ability to smell.

Several studies suggest that smell training may be beneficial to some people. Researchers believe that could be due to the brain stimulation that results from repeatedly detecting or even imagining particular scents.

Most studies have used essential oils that people are familiar with.

The prognosis is generally worse for people with post-traumatic olfactory loss. Some will never regain their sense of smell, but others may see some improvement over time. Usually, the degree of recovery depends on the severity of the damage.

While olfactory nerve cells can grow new nerve fibers, scar tissue may prevent them from ever being able to re-connect to the olfactory bulb. The new fibers also may be unable to find their way through the tiny holes in the bone behind the nose.

Smell training may be able to help some people with PTOL and Parkinson's disease.

If you're interested in smell training, talk to your doctor. You may also be able to research this treatment and try it at home, but be sure to involve your doctor in your decisions and in the process to make sure you're not doing anything to harm yourself.

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