OLFACTORY BULB: STRUCTURE, ANATOMY AND FUNCTIONS - NEUROPSYCHOLOGY - 2024

The olfactory bulb is a fundamental brain structure for detecting odors. It is part of the olfactory system, and in humans it is found in the back of the nasal cavities.

There is an olfactory bulb for each cerebral hemisphere, and they are considered an evagination of the cortex. They consist of a pair of bumps that are located on the olfactory epithelium and below the frontal lobes of the brain. They participate in the transmission of olfactory information from the nose to the brain.

Olfactory bulb (yellow)

There are cells inside the nasal cavity that capture those chemical particles from the air that form odors. This information reaches the olfactory bulb.

This is thought to be responsible for detecting important odors, differentiating some odors from others, and amplifying sensitivity to them. In addition to sending this data to other areas of the brain for further processing.

The olfactory bulb appears to be different in humans and in animals. For example, in animals there is also the accessory olfactory bulb that allows them to capture sex hormones and defensive or aggressive behaviors.

On the other hand, the olfactory bulb stands out for being an area where there is adult neurogenesis. That is, new neurons continue to be born throughout life. The function of this neuronal regeneration is still being studied. In animals it seems to be related to sexual behavior and care of the young.

Location of the olfactory bulb

Olfactory bulb location (yellow)

In many animals, the olfactory bulb is located in the front part of the brain (rostral part), although in humans it is located in the brain, specifically in the lower lateral part of the brain, between the eyes. The frontal lobe is located on the olfactory bulb.

There is an olfactory bulb in each brain hemisphere, and they can be connected to each other through mitral cells.

How does the olfactory bulb work?

Sensory receptors in the human olfactory system. 1: olfactory bulb 2: mitral cells 3: bone 4: nasal epithelium 5: glomerulus 6: olfactory sensory receptor neurons

First of all, for a better understanding of the characteristics and functions of the olfactory bulb, it is necessary to explain the functioning of the olfactory system.

Olfactory system

Smell is a chemical sense whose most basic function is to recognize food and check whether it is in good condition or not. Although it is also useful to fully capture the flavors, or detect dangers or avoid poisoning.

It is essential for many species to detect predators. In addition to identifying family members, friends, enemies or potential partners.

Although we can discriminate between thousands of different smells, our vocabulary does not allow us to describe them exactly. It is usually easy to explain something we see or hear, but it is difficult to describe a smell. For this reason, it is said that the olfactory system aims to identify something, rather than analyze its characteristics.

smells

Smells, also called olfactory stimuli, are volatile substances that have a molecular weight between 15 and 300. They are usually of organic origin, and consist mainly of soluble lipids.

It is known that we have 6 million olfactory receptor cells located in a structure called the olfactory epithelium or mucous membrane. This is in the upper part of the nasal cavity.

Apparently, less than 10% of the air that reaches the nostrils enters the olfactory epithelium. For this reason, sometimes, to catch an odor, it is necessary to snort more intensely to reach the olfactory receptors.

Cribriform plate

Ethmoid bone seen from above. Source: Henry Vandyke Carter / Public domain

Just above the olfactory epithelium is located the lamina cribrosa. The cribriform plate is a portion of the ethmoid bone that lies between the olfactory epithelium and the olfactory bulb.

Said bone supports and protects the olfactory bulb, and has small perforations through which the receptor cells pass. Thus, they can transmit information from the olfactory epithelium to the olfactory bulb.

Particles get through the nose to the mucosa

We catch an odor when odorous molecules dissolve in the mucosa. The mucosa consists of secretions from the olfactory glands that keep the inside of the nose moist.

Once dissolved, these molecules stimulate receptors on olfactory receptor cells. These cells have the characteristic of continuously regenerating.

The olfactory bulb is located at the base of the brain, at the end of the olfactory tracts. Each receptor cell sends a single axon (nerve extension) to the olfactory bulb. Each axon branches out connecting with the dendrites of cells called mitral cells.

Signals from the olfactory bulb to other areas of the brain

Olfactory neurons. Source: Henry Vandyke Carter / Public domain

Mitral cells are neurons in the olfactory bulb that send olfactory information to the rest of the brain to be processed.

They mainly send information to the amygdala, the piriform cortex, and the entorhinal cortex. Indirectly, the information also reaches the hippocampus, hypothalamus, and orbitofrontal cortex.

The orbitofrontal cortex also receives taste information. That is why it is believed that it could be related to the mixture of smell and taste that occurs in flavors.

On the other hand, different nerve fibers from different parts of the brain enter the olfactory bulb. These are usually acetylcholinergic, noradrenergic, dopaminergic, and serotonergic.

Noradrenergic inputs appear to be related to olfactory memories, and appear to be associated with reproduction.

Structure

Olfactory tract and bulb (red). Source: User Ancheta Wis on en.wikipedia / Public domain

The olfactory bulb is made up of 6 different layers. All of them carry out specific tasks that help the neuronal processing of odors. Ordered from bottom to top, these layers would be:

Nerve fiber layer

It is located just above the lamina cribrosa. In this layer are the axons of the olfactory neurons that come from the olfactory epithelium.

Glomerular layer

In this layer, the axons of the olfactory neurons synapse (that is, they connect) and the mitral cells dendritic arborizations. These connections form the so-called olfactory glomeruli, since they have an appearance of spherical structures.

Each glomerulus receives information from a unique type of receptor cell. There are different classes of these cells depending on the types of odors that their receptors capture. In humans, between 500 and 1000 different receptors have been identified, each sensitive to a different odor.

Thus, there are as many types of glomeruli as there are different receptor molecules.

The glomeruli also connect with the outer plexiform layer, and with the cells of the olfactory bulb of the other cerebral hemisphere.

Outer plexiform layer

It is the one that contains the bodies of the tufted cells. These, like mitral cells, connect with olfactory receptor neurons. They then send the olfactory information to the anterior olfactory nucleus, primary olfactory areas, and the anterior perforated substance.

It also has astrocytes and interneurons. Interneurons act as bridges linking different neurons.

Mitral cell layer

It is the part where the mitral cell bodies are located.

Inner plexiform layer and granule cells

This layer has axons of mitral cells and tufted cells. In addition to some granular cells.

Nerve fiber layer of the olfactory tract

In this layer are the axons that send and receive information to other areas of the brain. One of them is the olfactory cortex.

Features

The olfactory bulb is considered the main place where olfactory information is processed. It appears to function as a filter, however it also receives information from other areas of the brain involved in smell. For example, the amygdala, the orbitofrontal cortex, the hippocampus or substantia nigra.

The functions of the olfactory bulb are:

Distinguish some smells from others

To do this, it seems that a specific glomerulus receives information from specific olfactory receptors, and they send this data to specific parts of the olfactory cortex.

However, the question would be: how do we use a relatively small number of receptors to detect so many different smells? This is because a particular odor binds to more than one receptor. Thus, each odor would produce a different pattern of activity in the glomeruli to be recognized.

For example, a certain aroma might have a strong bond with one type of receptor, moderately strong with another, and weaker with the next. Then it would be recognized by that particular pattern in the olfactory bulb.

This was demonstrated in a study by Rubin and Katz (1999). They exposed the olfactory bulb to three different scents: pentanal, butanal, and propanal. While they observed their activity through computerized optical analysis.

They found that the three scents produced different patterns of activity in the glomeruli of the olfactory bulb.

Focus on detecting a specific odor

For example, even if we are in a bar where several different smells appear at the same time, thanks to the olfactory bulb we are able to identify some of them separately without the others interfering.

It seems that this process is achieved thanks to the so-called "lateral inhibition". That is, there are groups of interneurons whose function is to produce some inhibition in mitral cells. This helps discriminate specific odors, ignoring "background" odors.

Expand sensitivity to capture odors

This function is associated with lateral inhibition as well, since when we want to focus on detecting an odor, the receptor cells for that aroma increase their activity. While the rest of the receptor cells are inhibited, preventing other smells from "mixing".

Identification of stimuli by superior areas

Allow higher areas of the central nervous system to modify the identification or discrimination of olfactory stimuli.

However, it is not yet known with certainty if all these tasks are performed exclusively by the olfactory bulb, or if it really only participates in them together with other structures.

What has been shown is that injury to the olfactory bulb results in anosmia (lack of smell) on the affected side.

Connections with brain areas

Once the olfactory information passes through the olfactory bulb, it is then sent to other brain structures that will process it. Mainly these are the amygdala, the hippocampus and the orbitofrontal cortex. These areas are related to emotions, memory and learning.

Amygdala

The olfactory bulb establishes direct and indirect connections with the amygdala. Thus, it can be reached through the piriform cortex, a region of the primary olfactory cortex. Or, connect directly to certain specific areas of the amygdala.

The amygdala is a structure that is part of the limbic system. One of its functions is to learn associations between smells and behaviors. In fact, certain aromas can be pleasant and reinforcing stimuli while others can be aversive.

For example, through experience we learn that we like to go to a place that smells good, or that we reject the smell of a food that has made us sick in the past.

In other words, smells that are linked to positive aspects function as a "reward" for our behavior. While the opposite occurs when other odors are presented together with negative events.

In short, smells end up being associated with positive or negative emotions thanks to the amygdala. In addition, it has been shown that it is activated when unpleasant odors are detected.

Hippocampus

The olfactory bulb and amygdala also send information to the hippocampus. This region also has functions very similar to those of the amygdala, relating odors with other positive or negative stimuli.

On the other hand, it has an important role in the formation of autobiographical memory. It is one that allows us to remember important events or events in our life.

When we perceive a certain aroma that is stored in our memory in a different context, it is possible that memories come to mind. For example, smelling the perfume of our partner will surely evoke the memory of that person. Apparently, the structure involved in this event is the hippocampus.

In addition, both the amygdala and the hippocampus can modulate our olfactory perception. In this way, when we are in a physiological state such as hunger, the smell of food can seem very pleasant. This is produced by the learned association between the smell of food and the reinforcing act of eating.

Orbitofrontal cortex

Orbitofrontal cortex (Green). Source: PaulWicks / Public domain

The orbitofrontal cortex establishes connections with the olfactory bulb directly and through the primary olfactory cortex.

This area has many functions, and also participates in the smell-rewards association. One of its characteristic functions is to establish an assessment of the reward, that is, to weigh its benefits and costs.

The orbitofrontal cortex receives taste information and combines it with smell to form flavors. This area seems to be closely related to appetite and the reinforcing sensation of eating.

References

1. Carlson, NR (2006). Physiology of behavior 8th Ed. Madrid: Pearson. pp: 262-267.
2. Cheprasov, A. (nd). The Sense of Smell: Olfactory Bulb and the Nose. Retrieved on January 15, 2017, from Study.com: study.com.
3. Kadohisa, M. (2013). Effects of odor on emotion, with implications. Frontiers in Systems Neuroscience, 7, 66.
4. Olfactory bulb. (sf). Retrieved on January 15, 2017, from Wikipedia: en.wikipedia.org.
5. Purves D., Augustine GJ, Fitzpatrick D., et al., Editors. (2001). The Olfactory Bulb. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; Available from: ncbi.nlm.nih.gov.
6. Rubin, BC & Katz LC (1999). Optical imaging of odorant representations in the mammalian olfactory bulb. Neuron; 23 (3): 499-511.
7. What are the functions of the olfactory lobe? (sf). Retrieved on January 15, 2017, from Reference: reference.com.
8. What is the function of Olfactory Bulb? (sf). Retrieved on January 15, 2017, from Innovateus: innovateus.net.
9. Wilson Pauwels, L., Akesson, EJ, Stewart, PA, Spacey SD (2013). Olfactory nerve. In: Cranial Nerves. In health and illness. 3rd Ed. Editorial Médica Panameriana.