Olfactory transduction and pathways

Chapters:

Watch video only

Introduction0:00–0:19

The sensation of smell, also called olfaction, is carried out by the olfactory nerve or cranial nerve I, and it comes from specialized sensory neurons located in the roof of the nasal cavity, within the nose.

Nasal cavity0:19–1:39

The nasal cavity is made up of three regions. The first, is the nasal vestibule which is the area just inside the nostrils.
The second is the respiratory region, which is just above the nasal vestibule and is separated by three shelf-like bony structures; the superior, middle and inferior nasal conchae.
Lining the respiratory region is a layer of epithelial cells that create mucus to moisten the air and trap pathogens. The third, is the olfactory region which is at the top of the nasal cavity and is involved in smelling.
Lining the olfactory region is a layer called of special epithelial cells which form the olfactory epithelium. The olfactory epithelium consists of olfactory receptor cells which are chemoreceptors that respond to molecules, called odorants.
The olfactory epithelium also contains columnar epithelial cells which support those olfactory receptor cells. Below the olfactory epithelium is a layer of connective tissue called the lamina propria.
The lamina propria contains olfactory glands or Bowman’s glands which produce nasal mucus that protects the surface of the olfactory epithelium.
Below the lamina propria is the roof of the olfactory region which is formed by the cribriform plate of the ethmoid bone.
Now, if we zoom in a bit, we can see that the olfactory receptor cells are bipolar neurons, meaning that they have two projections outside the olfactory epithelium.

Olfactory receptor cells1:39–3:38

One projection carries their dendrites to the bottom of the epithelium and gives off hair-like structures called the olfactory hairs, or cilia.
These cilia protrude beyond the nasal mucosa so that they can come into contact with odorants trapped by the mucus. The other projection is an axon that joins up with axons of other receptors to form tiny olfactory nerves - collectively called cranial nerve 1.
These tiny olfactory nerves pass through small openings of the cribriform plate of the ethmoid bone, called the olfactory foramina, to enter inside the olfactory bulb.
The olfactory bulb contains second order neurons in the olfactory pathway, and it sends information through the olfactory tract to the olfactory cortex in the temporal lobe.
So, when you inhale air in a smelly locker room, stinky odorants, say from an old sock, travel to the roof of the nasal cavity where they get trapped on the nasal mucosa and make contact with the cilia of the olfactory receptor cells.
Now, within the membrane of the cilia there are olfactory receptor proteins, called G-protein coupled receptors and the odorants bind to them.
Now, there are over a thousand different types of olfactory receptor proteins, and each reacts most strongly to a specific odorant but can also bind to multiple odorants.
And each receptor cell usually has just a single type of receptor on their cilium. For example this receptor cell has receptors that responds strongly to stinky sock odorant, but those receptors respond weakly to other smells, like bacon or perfume.
As a result, there’s an incredible combination of different receptor proteins that might get activated together - strongly and weakly - which is why we can smell over 10,000 distinct odors, using only 1000 types of odor receptors.
So, after an odorant binds to a receptor, signal transduction occurs - meaning that it activates a G protein inside the cell called the G-olfactory protein.

Transduction3:38–5:47

This triggers the G-protein coupled receptor pathway, which ultimately opens up calcium and sodium channels - allowing those ions to flow into the cell.
As calcium flows in, it causes Ca++-activated chloride channels to open up. Since there’s a higher concentration of Cl- ions inside the receptor cell relative to outside the cell, the Cl- ions flow out of the cell.
The combination of positive ions coming in and negative ions going out causes the receptor cell membrane to depolarize, or become less negative.
This causes the neuron to fire and transmit the sensory information up the olfactory nerve towards the olfactory bulb. Once the signal reaches the olfactory bulb, the receptor cells that make up the olfactory nerve synapse with second order neurons called mitral cells.
When activated, the receptor cells release neurotransmitters, like glutamate, which causes depolarization of the mitral cells.
This signal then travels through the mitral cell axons which form the olfactory tract. The olfactory tract splits into a lateral and medial tract.
The lateral tract runs to an area deep within the ipsilateral, or same side, temporal lobe, called the piriform cortex which is the primary center for olfaction.
From there some fibers can also go to the limbic system, which is composed of the thalamus, hippocampus and amygdala, which are the centers for emotion and memory.
And this is why certain odors can trigger emotions or memories from the past, like the smell of the ocean might remind you of family vacations to the beach.
Now, the medial tract crosses over to the contralateral, or opposite side, piriform cortex meaning that smell from one olfactory receptor can reach both sides of the brain.
So, if you have a cold and the right nasal cavity is completely clogged, some signals can still pass to the right side of the brain from the left nasal cavity.

Adaptation5:47–6:19

One interesting feature of smell is that although the olfactory receptors are very sensitive, after some time of stimulation by the same odorants they start sending fewer and fewer signals and finally lose their ability to respond to that particular smell.
This is called adaptation and with most odors complete adaptation can result within less than a minute. This explains why when you go to a gas station, the smell of gas hits you really strongly at first, but after a few seconds this sensation drops, and reaches a point where you can’t even smell it anymore.

Review6:19–6:56

Alright, as a quick recap. When smell molecules enter the nose they get trapped by mucus at the roof of the nasal cavity and bind to G-protein coupled receptors found on the cell membrane of specialized sensory neurons, called olfactory receptors.
This triggers a nerve signal, which travels through the axons of the olfactory receptors to reach the olfactory bulb that sits on the cribriform plate of the ethmoid bone.
From there the signal travels through the olfactory tract to reach the piriform cortex in both temporal lobes, which are the primary centers for olfaction, and the limbic system where smell is associated with memory and emotions.