How does the sense of smell relate to your eyes? Why can we smell something in the first place? These questions and many more will be answered as we look into the olfactory bulb, chemoreceptors, cranial nerve I, and the olfactory nerve.
The Smell of Good Food
The olfactory bulbs, which allow us to sense smells, are located right between the eyes.
Ah, the smell of something delicious, especially when we're hungry. It brings a smile to one's face. We can even smell mom's great home cooking all the way up on the second floor. We begin to salivate and think of all the great tastes we'll soon experience. Then we run downstairs to savor the smell, taste, and look of some tasty food. The ability to sense these delicious smells depends on special receptors and nerves, some of which are right between your eyes!
When your mom is cooking your favorite meal, the food releases certain chemicals that fly up into the air. Eventually these chemicals disperse all around the kitchen and then throughout the entire house. As you sit upstairs, watching the Study.com videos on your screen, some of these molecules begin to reach your nose and travel way up into it.
These molecules are able to be recognized by special sensory nerves known as chemoreceptors. Chemoreceptors are receptors that are able to sense external chemical stimuli such as odor molecules or molecules that produce the sensation of taste.
The Olfactory Epithelium
Odor molecules enter through the nose and travel to a patch of chemoreceptors near the eyes.
As the odor molecules enter your nose they travel up and back to a little patch of chemoreceptors located underneath and in the middle of the place where your eyes sit in your skull. This little patch is called the olfactory epithelium, and it has a bunch of chemoreceptors in it.
Just in case you were wondering, the word 'olfactory' refers to our sense of smell, technically called olfaction.
During the process of olfaction, odor molecules entering your nose will latch onto the chemoreceptors in the olfactory epithelium. However, there's a small catch.
The olfactory epithelium involved in the first step of the sense of smell has a mucous layer covering the chemoreceptors. This mucous layer implies that only water-soluble odor molecules will be able to pass through it easily and activate the chemoreceptors involved in the sensation of smell. However, the mucous layer also has proteins, appropriately called odor-binding proteins, that help to transport non-water-soluble molecules through the mucous and to the chemoreceptors so they can activate the chemoreceptors as well.
Odor-binding proteins allow non-water-soluble molecules to pass through the mucous layer.
The Olfactory Nerve
The entire collection of chemoreceptors, in our specific case called olfactory receptor neurons, as well as their projections called axons are known as the olfactory nerve. The olfactory nerve, one on either side of your body, is also known as cranial nerve I. The olfactory nerve is the most important nerve involved in the sensation of smell.
The Olfactory Bulb
Cranial nerve I transmits information collected at the olfactory epithelium via axons traveling through your skull to a place directly above the olfactory epithelium called the olfactory bulb. The olfactory bulbs are a pair of swellings above the olfactory epithelium and underneath the frontal lobes of the brain that transmit odor information from the nose to the brain.
The olfactory bulb receives signals from the olfactory nerve and then transmits them to the brain.
Essentially, the olfactory nerve connects to other nerves in the olfactory bulbs, and these other nerves will then transmit electrical signals farther up into your brain where these signals will be processed and interpreted as a particular kind of smell. Think of the olfactory bulb as nothing more than a relay center for the information of smell.
Let's review how it is that you are able to smell odors. Odor molecules enter up your nose. The water-soluble molecules dissolve across the mucous lining, a structure known as the olfactory epithelium. Non-water-soluble molecules will be transported to the olfactory epithelium using odor-binding proteins.
Once the odor molecules reach the olfactory epithelium, they activate receptors that are able to sense external chemical stimuli, which are known collectively as chemoreceptors.
These chemoreceptors, in our specific case called olfactory receptor neurons, send projections through the skull and into the olfactory bulb. The entire collection of olfactory receptor neurons and their projections are known as cranial nerve I , synonymously referred to as the olfactory nerve. The olfactory nerve, remember, is the most important nerve involved in the sensation of smell.
As I just mentioned, the projections of the olfactory nerve enter structures known as olfactory bulbs. The olfactory bulbs are a pair of swellings above the olfactory epithelium and underneath the frontal lobes that transmit odor information from the nose to the brain. Upon reaching more central locations in your brain, this information is processed and interpreted into a specific smell.
Completing this lesson should prepare you to:
- Describe the roles of chemoreceptors, the olfactory epithelium, the olfactory nerve, and olfactory bulbs in the process of olfaction
- Explain how water-soluble and non-water-soluble molecules reach the olfactory epithelium