Olfactory bulb: definition, parts and functions

The human being, like other animals, captures stimuli from the environment through the senses. Although there are modalities such as proprioception (or self-perception of one's own body) or nociception (perception of pain), as a general rule we understand as such sight, hearing, taste, touch and smell.

They all offer us different types of information that allow our adaptation and survival, processing and integrating the information received in various brain nuclei. In the case of smell, such processing takes place in the olfactory bulb, one of the oldest parts of the brain in our evolutionary line. Let's see what its characteristics are.

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The sense of smell

Although in the human being it is a relatively undeveloped sense compared to sight and hearing, smell is a fundamental mechanism when it comes to capturing stimuli that come to us from the environment. It is the sense that allows us to process odor through the capture of volatile chemical substances that reach our body through the air we breathe.

The main function of this sense is mainly to detect elements that the body needs to subsist and those that can be harmful, so that we approach or move away from it depending on the need. Thanks to this we can adjust our behavior to different stimuli or agents. In addition, smell also has an important relationship with the perception of taste, allowing us to savor the food.

To capture this information, the presence of a specialized system capable of translating and transmitting the information to the rest of the organization is necessary. This is the olfactory system, within which the role played by the olfactory bulb stands out.

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Before reaching the bulb

Although the bulb is a very important part for capturing odorous stimuli, the process by which the odor is captured does not start in it.

The odorous molecules arrive and enter the nasal passages, being trapped by the nasal mucosa. It collects these molecules and absorbs them, acting according to the intensity with which they reach the system.

Within the mucosa we can find various areas in which there are numerous olfactory neurons of different types, although they tend to be bipolar and unmyelinated. In them the transduction is carried outThis being the step in which information is passed from a specific type of signal (in this case chemical) to a bioelectric signal that can circulate through the nervous system. Later, they go through the olfactory nerve until reaching the olfactory bulb.

The olfactory bulb

The olfactory bulb is a small vesicular structure whose main function is capture and process information from odorant receptors located on the nasal mucosa. In fact, we actually have two of these bulbs, having one in each hemisphere of the brain.

This small extension of the cerebral cortex is located below the area closest to the eyes of the frontal lobe and connects with the innermost part of the nostrils.

How does it work?

With regard to their participation in the capture and processing of odor, odorous molecules previously absorbed by the nasal mucosa and that have been captured and transformed into bioelectric activity by the neurons located in it send their axons to the bulb.

In the olfactory bulb these neurons make synapses with other neurons called mitral cells in structures called glomeruli which will have different activation patterns depending on what has been captured and thanks to whose differentiated activity it is possible to distinguish different odors. This differentiated activation will depend on the slowness or speed with which the substance has been transported through the mucosa and its chemical composition.

After being processed in the glomeruli of the bulb, the information will be transmitted through the mitral cells to different brain regions such as the primary olfactory cortex, the secondary olfactory cortex, the cortex orbitofrontal, the amygdala or the hippocampus.

Parts of the olfactory bulb

The olfactory bulb is not a uniform and homogeneous element in all its extension, but is configured by a series of layers that are distinguished from each other mainly by the type of cells that make them up.

Although up to seven layers can be found, as a general rule five of them are considered, which form the structure of the olfactory bulb.

1. Glomerular layer

This is the part of the bulb where are the glomeruli, the structures in which the synapse between receptor and mitral cell will occur and in which observe the different reactions according to the perceived stimulus that will end up allowing the distinction between smells. In fact, the glomeruli are grouped in such a way that similar odors will be detected by specific neuronal groups.

2. Outer plexiform layer

This layer contains the body of tufted cells, which have a similar function to mitral cells. Various interneurons are present in this layer that make the lateral inhibition process possible, while connecting various neurons to each other.

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3. Mitral cell layer

In this layer are located the bodies of the mitral cells, which will transmit the olfactory information to the rest of the structures connected to the bulb. So in this layer it is where mitral cells receive information from receptors.

4. Inner plexiform layer

In the inner plexiform layer, the axons of the mitral and tufted cells can basically be found. That is, it is a layer in which the information captured begins to be retransmitted to other structures.

5. Granule cell layer

This last layer, the deepest, is made up of granular cells, thanks to which it is possible that different mitral cells connect your dendrites to each other.

Principal functions

The olfactory bulb is considered the main nucleus of olfactory information processing, which comes from the receptors located in the mucosa or nasal epithelium. This role assumes that the bulb performs various functions of great importance.

Allow the capture of olfactory information

Being the main nucleus of olfactory information processing, the olfactory bulb allows the human being to perceive the information coming from the sense of smell. The presence of damage or removal of either bulb has been found to cause anosmia or lack of olfactory perception.

Distinction between smells

The olfactory bulb is largely involved in the ability to distinguish between various types of odors. The differentiation is due in particular to the different activation patterns of the neurons in charge of olfactory perception, which they react differently depending on the smell in question.

Specifically, it is speculated that what produces this reaction is the shape, structure and electrical charge of the particles that reach the olfactory system.

Lateral inhibition of olfactory information

Lateral inhibition is understood as the process by which we are capable of not paying attention to certain stimulations in order to focus on a specific stimulation. An example of this would be being able to smell the perfume of a loved one in the middle of a crowd.

Although part of this process is due to the brain areas that regulate attention, the olfactory bulb plays a role, as the interneurons of the bulb act to inhibit the effect that the uptake of certain smells would normally have. That is why after a while in the presence of a certain smell, its perception greatly diminishes.

Participates in the emotional processing of information

The connection of the olfactory bulb with the amygdala, both directly and indirectly through the primary or piriformis olfactory cortex, allows emotions to be linked to olfactory stimuli. For example, the feeling of disgust or revulsion at a smell that we consider negative.

On the other hand, the nervous circuit of the sense of smell, unlike those of sight and hearing, does not pass through the thalamus first, and therefore has a more direct connection with the limbic system. This, among other things, makes that smells are especially powerful when it comes to making us evoke memories, even if they are from experiences that occurred many years ago and that we thought were forgotten.

Allows odor recognition

In this case, due to its connection with the hippocampus, the olfactory bulb participates in the process of learning to identify previously perceived odors, which in turn is allows associating them to specific situations or stimuli. That is why we can associate an aroma with a person or a specific stimulus.

Helps to capture the flavor

It is well known that smell and taste are closely related and even connected. The fact that certain odors reach us can cause us to feel a potent or different flavor than the one we normally attribute to a meal. That is why there are food flavorings.

Since it allows the olfactory information to be processed, the olfactory bulb is therefore relevant in the perception of taste. In fact, people with anosmia tend to be unable to pick up on certain flavors.

Helps regulate sexual behavior

Although multiple studies have questioned the existence of this in humans, in a large number of animals there is a structure called the accessory olfactory bulb. This structure is specialized in capturing a certain type of substances: pheromones.

Through them, beings of the same species are capable of transmitting certain types of information to each other, modifying the behavior of their peers. One of the best known examples is the role of pheromones in controlling sexual behavior, participating in aspects such as attraction. In humans, androestadienone and estratetraenol are two of the best known, both influencing human sexual response.

Bibliographic references:

• Carlson, N.R. (1998). Physiology of behavior. Madrid: Pearson. pp: 262-267

• Goldstein, E.B. (2006). Sensation and Perception. 6th edition. Debate. Madrid.

• Scott, J.W.; Wellis, D.P.; Riggott, M.J. & Buonviso, N. (1993). Functional organization of the main olfactory bulb. Microsc. Beef. Tech. 24 (2): 142-56.