Human beings, like other animals, capture stimuli from the environment through their 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 sight, hearing, taste, touch and smell as such.
They all offer us different types of information that allow our adaptation and survival, the information received being processed and integrated in various brain nuclei. In the case of smell, This 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.
Although in humans it is a relatively underdeveloped 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 the smell 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 survive and those that can be harmful, so we move closer or further away from them depending on the need. Thanks to this we can adjust our behavior to different stimuli or agents. Furthermore, the sense of smell 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 organism is necessary. This is the olfactory system within which the role carried out by the olfactory bulb stands out.
Before reaching the bulb
Although the bulb is a part of great importance for the capture of odorous stimuli, the process by which the smell is captured does not begin there
The odor molecules reach 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 out, this being the step in which the information from a specific type of signal (in this case chemical) is passed to a bioelectric signal that can circulate through the nervous system. Subsequently, they pass through the olfactory nerve until they reach the olfactory bulb.
The olfactory bulb
The olfactory bulb is a small vesicular structure whose main function is capture and process information from odor receptors located in the nasal mucosa. In fact, we actually have two of these bulbs, with 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 to the innermost part of the nasal passages.
How does it work?
Regarding their participation in the capture and processing of odor, the odor molecules previously absorbed by the nasal mucosa and that have been captured and transformed into bioelectric activity by the neurons located there send their axons to the bulb.
In the olfactory bulb, these neurons synapse 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 medulla, the information will be transmitted through the mitral cells to different brain regions such as the primary olfactory cortex, the secondary olfactory cortex, the orbitofrontal cortex, the amygdala or the hippocampus.
Parts of the olfactory bulb
The olfactory bulb is not a uniform and homogeneous element throughout its entire length, but is made up of a series of layers that are distinguished from each other mainly by the type of cells that compose them.
Although up to seven layers can be found, as a general rule five of them are considered, which they form the structure of the olfactory bulb
1. Glomerular layer
This is the bulb part where the glomeruli are located, the structures in which the synapse between receptor and mitral cell will occur and in which the different reactions are observed depending on the perceived stimulus that will end up allowing the distinction between odors. In fact, the glomeruli are grouped in such a way that similar odors will be detected by specific neuronal groups.
This layer contains the somas of tuft cells, which have a function similar to the mitral cells. Various interneurons are present in this layer that make the lateral inhibition process possible, while connecting various neurons with each other.
3. Mitral cell layer
The somas of the mitral cells are located in this layer, which will transmit 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 tuft 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 formed by granule cells, thanks to which it is possible for different mitral cells connect their dendrites to each other
Principal functions
The olfactory bulb is considered the main nucleus for processing olfactory information, which comes to it from the receptors located in the nasal mucosa or epithelium. This role assumes that The bulb performs various functions of great importance
Allow the capture of olfactory information
Being the main nucleus for processing olfactory information, the olfactory bulb allows humans to perceive information coming from the sense of smell. It has been proven that the presence of damage or removal of either bulb produces anosmia or lack of olfactory perception.
Distinction between odors
The olfactory bulb is largely involved in the ability to distinguish between various types of odors. The differentiation is due specifically to the different activation patterns of the neurons responsible for 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 through which we are able to ignore certain stimulations in order to focus on a specific stimulation. An example of this would be being able to smell the perfume of the person you love 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 bulb’s interneurons act to inhibit the effect that the uptake of certain smells it would normally have. That is why after a while in the presence of a certain odor, its perception greatly decreases.
Participates in the emotional processing of information
The connection of the olfactory bulb with the amygdala, both direct and indirect through the primary or piriform olfactory cortex, allows emotions to be linked to olfactory stimuli For example, the feeling of disgust or repulsion when faced with 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 first pass through the thalamus, 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 allows you to associate them with specific situations or stimuli This is why we can associate an aroma with a person or a specific stimulus.
Helps capture flavor
It is well known that smell and taste are closely related and even connected. The fact that certain smells reach us can cause us to feel an enhanced or different flavor than what we normally attribute to a food. That is why there are food flavorings
Since it allows the processing of olfactory information, the olfactory bulb is therefore relevant in the perception of flavor. In fact, people with anosmia tend to be unable to capture certain flavors.
Helps regulate sexual behavior
Although multiple studies have questioned its existence in humans, in a large number of animals there is a structure called the accessory olfactory bulb. This structure is specialized in capturing a specific type of substance: pheromones.
Through them, beings of the same species are capable of transmitting certain types of information to each other, modifying the behavior of their fellow humans. One of the best known examples is the role of pheromones in the control of sexual behavior, participating in aspects such as attraction. In humans, androstadienone and stratetraenol are two of the best known, both influencing the human sexual response.
Scott, J.W. Wellis, D.P.; Riggott, M.J. & Buonviso, N. (1993). Functional organization of the main olfactory bulb. Microsc. Tech Res.24 (2): 142-56.
