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Olfactory bulb: definition, parts and functions

Olfactory bulb: definition, parts and functions

April 24, 2024

The human being, like the rest of animals, captures the stimuli coming 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), we generally understand sight, hearing, taste, touch and smell as such.

All offer 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, said processing takes place in the olfactory bulb , one of the oldest parts of the brain in our evolutionary line. Let's see what their characteristics are.

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The sense of smell

Although in humans it is a relatively undeveloped sense compared to sight and hearing, Smell is a fundamental mechanism when it comes to capturing stimuli that come from the environment. It is the sense that allows us to process the 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 survive 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, the smell also It has an important relationship with the perception of taste , allowing us to savor the food.

In order to capture this information, the presence of a specialized system capable of translating and transmitting the information to the rest of the body is necessary. This is the olfactory system , which highlights the role played by the olfactory bulb.

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Before reaching the bulb

Although the bulb is a part of great importance for the capture of the odorous stimuli, the process by which the odor is captured does not start in it .

The odorous molecules arrive and enter the nostrils, 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 different areas in which there are many olfactory neurons of different types, although they tend to be bipolar and unmyelinated. In them transduction is performed , this being the step in which the information of a specific type of signal (in this chemical case) is passed on to a bioelectrical signal that can circulate through the nervous system. Later, 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 the information coming from the odor receptors located in the nasal mucosa. In fact, we actually have two of these bulbs, 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 nasal passages.

How does it work?

In regard to their participation in the collection and processing of odor, the 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 said neurons synapse with other neurons called mitral cells in structures called glomeruli They will have different activation patterns depending on what has been captured and thanks to whose differentiated activity it is possible to distinguish different scents. This differentiated activation will depend on the slowness or speed with which the substance has been transported by 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 is configured by 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, they form the structure of the olfactory bulb .

1. Glomerular layer

It is about the part of the bulb where are the glomeruli , the structures in which the synapse between the receptor and the mitral cell will occur and in which the different reactions are observed according to 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.

2. External plexiform layer

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

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3. Mitral cell layer

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

4. Internal plexiform layer

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

5. Granular cell layer

This last layer, the deepest, is formed by granular cells, thanks to which it is possible that different mitral cells connect their dendrites to each other .

Principal functions

The olfactory bulb is considered the main nucleus of olfactory information processing, which comes from receptors located in the mucosa or nasal epithelium. This paper 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. 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 smells

The olfactory bulb participates to a large extent in the ability to distinguish between different types of odors. The differentiation is due in particular to the different activation patterns of the neurons responsible for olfactory perception, which they react differently according to 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 able to not pay 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 the loved one in the middle of a crowd.

Although part of this process is due to the cerebral areas that regulate the attention, the olfactory bulb has participation, when the interneurons of the bulb act to inhibit the effect that the uptake of certain smells would normally. That is why after a while in the presence of a certain odor, your perception diminishes greatly.

Participate 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 sensation of disgust or repulsion towards an odor 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 the smells are especially powerful at the time of making us evoke memories , although they are experiences that happened many years ago and that we thought were forgotten.

Allows the recognition of odors

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 . That is why we can associate an aroma to a person or a specific stimulus.

It helps to capture the flavor

The fact that smell and taste are closely related and even connected is well known. The fact that we get certain odors can cause us to feel an enhanced flavor or different to what we normally attribute to a meal. That is why there are food flavorings .

Since it allows 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 capture certain flavors.

Helps regulate sexual behavior

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

Through them, beings of the same species are able to transmit certain types of information to each other, modifying the behavior of their congeners. One of the best-known examples is the role of pheromones in the control of sexual behavior , participating in aspects such as attraction. In the human being, androestadienona and estratetraenol are two of the best known, influencing both in the 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. Res. Tech.24 (2): 142-56.

Olfactory Pathway - Nerve and Tracts (April 2024).

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