Olfactory Bulb

You must have wondered why the sense of smell is so strong. How is it possible that you can sense a  smell in less than a second? You have also wondered how a particular smell is so deeply related to emotions. Well, you will get all your answers in this article on olfactory bulb.

The olfactory bulb is a nervous structure related to
the sense of smell. It is an extension of the nervous tissue of the brain that
extends to the roof of the nasal cavity. It is not only associated with the
sense of smell but also with the emotions.

In this article, we will discuss the anatomical features such as location, relations, structure, histology, connections and blood supply of olfactory bulb. We will also discuss various roles performed by the olfactory bulb and its clinical significance. In the end, the article will be concluded as a brief summary along with references.

Location

The olfactory bulb is a part of the forebrain that is located just above the nasal cavity. It is actually a part of the limbic system. It is present just above the cribriform plate of the skull in the anterior cranial fossa.

The olfactory bulb is located at the base of the forebrain in olfactory sulcus. It is continuous posteriorly with the olfactory tract. It surrounded medially by the gyrus rectus and laterally by the orbital gyri.

Structure

Olfactory bulb is a bulb-shaped nervous tissue made up
of multiple layers of cells. Histologically, it is divided into five layers.
Here, we will discuss these layers individually.

Glomerular Layer and Glomeruli

This layer consists of spherical structures called glomeruli (singular glomerulus). It is the outer most layer of olfactory bulb and is the site of synapse formation.

The glomeruli present in the glomerular layer of the olfactory bulb are actually spherical or globular structures made up of globular tangles of axons of olfactory neurons and the dendrites of other cells such as mitral cells, tufted cells.

These globules are surrounded by heterogeneous collection cells, collectively known as juxtaglomerular cells. These include periglomerular, short axon and external tufted cells. Like other nervous structures, they are also surrounded by glial cells. 

The glomeruli act as the sites where synapses are
formed between the axons of the dendritic cells, carrying the olfactory
signals, and the dendrites of cells present in other layers of the olfactory
bulb.  These include mitral cells,
periglomerular cells, and the tufted cells. 

This layer is the first site where synaptic processing
of olfactory sensation coming from the nose takes place.

External Plexiform Layer

It is the second histological layer of the olfactory bulb. It comprises of the nerve fibers passing to and from the glomerular layer. It mainly contains the axons and dendrites of the cells located in the upper and lower layers. In addition, this layer also contains the tufted cells.

The tufted cells act as interneurons or projection neurons. They receive input from the olfactory neurons and process it by filtering od sharpening it. They play an important role in lateral inhibition.

Mitral Cell Layer

It is the middle layer of the olfactory bulb. This
layer consists of special neurons called mitral cells. Mitral cells act as the
projection neurons. They receive information from the olfactory neurons and
send this information to other areas of the brain.

Each cell in this later has a single long axonal dendrite and a few small lateral dendrites. The large axonal dendrite is sent to the glomerular layer while the lateral dendrites project to the external plexiform layer.

The dendrite in the glomerular layer synapses with the axons of olfactory neurons present in this layer. The lateral dendrites in the external plexiform layer synapse with the tufted cells and granular cells.

These cells receive excitatory signals from the olfactory
neurons and the tufted cells while the inhibitory signals are received from the
granular cells. These inhibitory signals are as a result of lateral inhibition.

Internal Plexiform Layer

It the fourth histological layer in the structure of the olfactory bulb. Unlike the external plexiform layer that contains tufted cells and interneurons, the inner plexiform layer is devoid of neurons and solely consists of nerve fibers.

These fibers include the axons of the mitral cells and the dendrites of the granular cells. Although some exceptions are still there. The exceptions are seen in the form of granule cells present in the inner plexiform layer.

The inner plexiform layer is very thin, and its sole
purpose is to provide passage to the nerve fibers.

Granule Cell Layer

This is the innermost layer of the olfactory bulb. It comprises of granule cells. These cells have a very small cell body and are mostly present in the granule cell layer. Some cell bodies are also located in the inner plexiform layer.

The granule cells have a characteristic apical dendrite
that extends to the external plexiform layer. Here they synapse with the mitral
cells. The granule cells are mainly the inhibitory neurons and cause lateral
inhibition of the mitral cells.

Other cells present in the granule layer are the
short-axon cells. These cells have cell bodies larger than the granule cells.
They have dendrites that extend to the external plexiform layer. The axons of
these cells extend to all other layers of the olfactory bulb. These cells are
also sometimes referred to as deep short-axon cells or dSA.

Physiology

In this section, we will discuss the functions of the olfactory bulb and its different layers.

Discrimination Among the Odors

The olfactory bulb through its neuronal circuits helps
discriminate among different odors. It is believed that this discrimination
occurs at the level of the olfactory bulb and higher centers are not involved.

Enhancing the Selective Odors

The olfactory bulb has odor selectivity. It has a special feature to selectively enhance special odors. The selectivity of odors depends upon the association of odor with the emotions of a person.

Filtering out the Odors

The olfactory bulb not only enhances the selective odors associated with emotions but also filters out unnecessary odors. These include the background odors present in the environment.

Modification of Odor Detection and Discrimination

The olfactory bulb has connections with higher centers
of the brain via olfactory tracts. Through these connections, the olfactory
bulb allows the higher centers of the brain to modify the detection and
discrimination of different odors. These include those smells that are
associated with the arousal and attention of a person.

Clinical Significance

The clinical condition associated with the pathology
of the olfactory bulb is amnesia.

Anosmia

This condition is characterized by loss of ability to perceive any smell. A person with anosmia will not be able to differentiate the odor of a fragrance from the smell of garbage. Anosmia may be total or partial, ipsilateral or contralateral.

The destruction of the olfactory bulb is the major
cause of anosmia. If the olfactory bulb is destroyed on one side, it results in
ipsilateral and partial anosmia, whereas the destruction on both sides results
in complete and bilateral anosmia.

It should be kept in mind that anosmia from the destruction of olfactory epithelium is reversible. It can be repaired. On the other hand, anosmia resulting from the damage to the olfactory bulb is permanent and irreversible.

This is because the olfactory bulb is a nervous structure. It can not be regenerated. Any damage to olfactory bulb results in scar formation.

Conclusion/Summary

Olfactory bulb is a nervous structure, a part of
forebrain, associated with the perception and processing of olfaction and
smell.

It is present in the anterior cranial fossa and communicates with the olfactory neurons via the cribriform plate.

Its structure is made up of five layers.

The glomerular layer is the outermost layer,
containing spherical glomeruli formed by axons of olfactory neurons and the
dendrites of mitral cells. It also contains juxtaglomerular cells.

The next layer is the external plexiform layer that
contains dendrites of mitral cells as well as tufted cells.

The third one is the mitral cell layer. This layer contains mitral cells, the second type of projection neurons in the olfactory bulb.

The next one is the internal plexiform layer. This
layer contains nerve fibers. It also contains cell bodies of some granular
cells.

The most internal layer is the granular layer. it
contains the inhibitory neurons present in the olfactory bulb, the granular
cells.

The functions of the olfactory bulb are all associated
with the process of olfaction. These include:

• Discrimination of odors
• Filtering out background odors
• Enhancing some selective odors
• Modification and detection of odors

Damage to the olfactory bulb due to any reason results in an inability to perceive any smell, called anosmia. The anosmia may be complete or partial, ipsilateral or bilateral.

References

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