The human being, like the rest of the animals, captures the stimuli coming from the environment through the senses. Although there are modalities such as proprioception (or self-perception of the body) or nociception (perception of pain), as a general rule we understand as such sight, hearing, taste, touch and smell.
All of them offer us different types of information that allow our adaptation and survival, processing and integrating the information received in different brain nuclei. In the case of smell, this processing is carried out in the olfactory bulb , one of the oldest parts of the brain in our evolutionary line. Let’s see what its characteristics are.
The sense of smell
Although in human beings 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 smell by capturing volatile chemical substances that reach our organism 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 we approach or move away from it depending on the need. Thanks to this we can adjust our behaviour to different stimuli or agents. In addition, smell also has an important relationship with the perception of taste , allowing us to taste food.
To capture this information, a specialized system is needed to translate and transmit the information to the rest of the body. 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 very important part for the capture of odorous stimuli, the process by which the odor is captured does not start in it .
The odor molecules arrive and enter the nostrils, being trapped by the nasal mucosa. This 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 amielinic. Transduction is carried out in these areas , this being the step in which the information is passed from a specific type of signal (in this case chemical) 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 to capture and process information from the odoriferous receptors located in the nasal mucosa. In fact, we have two of these bulbs, with one in each hemisphere of the brain.
This small extension of the cerebral cortex is located under the closest area to the eyes in the frontal lobe and connects to the innermost part of the nostrils.
How does it work?
In terms of their involvement in odour capture and processing, odour molecules previously absorbed by the nasal mucosa and which have been captured and transformed into bioelectrical activity by neurons located there 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 patterns of activation depending on what has been captured and thanks to whose differentiated activity it is possible to distinguish different odours. 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 orbitofrontal cortex, 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 it is configured by a series of layers that differ from each other mainly by the type of cells that compose them.
Although up to seven layers can be found, as a 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 the glomeruli are located, the structures in which the synapse between the receptor and the mitral cell will be produced and in which the different reactions are observed according to the perceived stimulus that will end up allowing the distinction between odours. In fact, the glomeruli are grouped in such a way that similar odours will be detected by specific neuronal groups.
2. External plexiform layer
This layer contains the somas of plumed cells, which have a similar function to mitrals. In this layer are present several interneurons that make possible the process of lateral inhibition, while connecting several neurons to each other.
3. Layer of mitral cells
In this layer are located the somas of the mitral cells, which will transmit the olfactory information to the rest of the structures connected to the bulb. Thus, it is in this layer where the mitral cells receive the information from the receptors .
4. Internal plexiform layer
In the inner plexiform layer, the axons of the mitral and plume cells can basically be found. That is to say, it is a layer in which the information captured begins to be retransmitted to other structures .
5. Granular cell layer
This last layer, the deepest one, is made up of granular cells, thanks to which it is possible for different mitral cells to connect their dendrites to each other .
The olfactory bulb is considered to be the main nucleus of olfactory information processing, which is delivered to it from receptors located in the mucosa or nasal epithelium. This role implies that the bulb performs several important functions .
Allow the capture of olfactory information
Being the main core of olfactory information processing, the olfactory bulb allows the human being to perceive the information coming from the sense of smell. It has been proven that the presence of damage or the removal of any of the two bulbs produces anosmia or lack of olfactory perception.
Distinction between odors
The olfactory bulb plays a major role in the ability to distinguish between various types of odours. The differentiation is due in particular to the different patterns of activation of the neurons responsible for olfactory perception, which react differently depending on the odour in question .
Specifically, it is speculated that what produces this reaction is the form, structure and electrical charge of the particles that reach us in the olfactory system.
Lateral inhibition of olfactory information
Lateral inhibition is understood to be the process by which we are able to disregard certain stimulations in order to focus on a specific stimulation. An example of this would be being able to smell the perfume of the 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 has a role, as the interneurons of the bulb act to inhibit the effect that the capture of certain odours would normally have. This is why after a while in the presence of a certain odour, its perception decreases to a great extent.
Participates in emotional processing of information
The connection of the olfactory bulb with the amygdala, both directly and indirectly through the primary or piriform olfactory cortex, allows emotions to be linked to olfactory stimuli . For example, the sensation of disgust or repulsion before an odour that we consider negative.
On the other hand, the nerve circuit of the sense of smell, unlike those of sight and hearing, does not pass first through the thalamus, and therefore has a more direct connection with the limbic system. This, among other things, makes smells especially powerful in making us recall memories , even if they are of experiences that occurred many years ago and that we thought had been forgotten.
Allows odour recognition
In this case, due to its connection with the hippocampus, the olfactory bulb participates in the process of learning to identify previously perceived odours, which in turn allows them to be associated with specific situations or stimuli . This is why we can associate an aroma with a person or a specific stimulus.
Helps to capture the flavor
It is quite 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 taste from the one we normally attribute to a food. That is why there are food flavourings .
Since it allows olfactory information to be processed, the olfactory bulb is thus relevant to the perception of taste. In fact, people with anosmia tend to be unable to capture certain flavours.
Helps regulate sexual behavior
Although the existence of this in humans has been questioned in many studies, in a large number of animals there is a structure called the accessory olfactory bulb. This structure is specialized in the capture of a certain type of substance: pheromones.
Through them, beings of the same species are able to transmit certain types of information to each other, modifying the behavior of their fellow beings. One of the best known examples is the role of pheromones in the control of sexual behaviour , participating in aspects such as attraction. In humans, androestadienone and strategtraenol are two of the best known, both influencing human sexual response.
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- 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. Res. Tech.24 (2): 142-56.