Course of the olfactory nerve viewed from the left side of a parasagittal section.
Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial on the olfactory nerve. Before we begin, let me give you a quick overview of the topics we'll discuss in this tutorial. First, we'll cover the structure and function of the olfactory nerve then we'll move on to look at some important relations of this nerve. Once we're competent in these areas, we'll systematically work our way through the gross anatomy of the olfactory pathway followed by the histology of the olfactory mucosa. We'll also look at the neural pathways involved in the perception of smell. Finally, we'll conclude this tutorial with some clinical notes relating to the olfactory nerve and its pathway.
Now let's start with the first item on the list which is the structure and function of this nerve. So what exactly is the olfactory nerve? The olfactory nerve is the first of twelve cranial nerves. It can also be acronymed to CN I. It's a sensory nerve that transmits sensory information to the brain allowing us to have a sense of smell. In this illustration which is a sagittal cross section of the skull, we can see the olfactory nerve fibers arising in the roof of the nasal cavity. More specifically, these fibers arise in the olfactory mucosa and merge to form up to twenty nerve bundles. These bundles collectively known as the olfactory nerve course through the foramina in the cribriform plate of the ethmoid bone to enter the cranial activity.
Now that we're familiar with the olfactory nerve and its function, let's move on to discuss some bony and membranous structures related to this nerve. The cribriform plate as we mentioned previously is part of the ethmoid bone and it separates the nasal cavity from the brain. In the sagittal plane, we can see that it's a horizontal bony lamina and has a sieve-like structure that transmits the fibers of the olfactory nerve. This structure also supports the olfactory bulb which we will talk about later on in this tutorial.
Anterior to the cribriform plate, we can see this hollow region within the frontal bone – the frontal sinus. This is a paired structure lined by nasal mucosa and is one of four paranasal sinuses. Trauma to this area may cause damage to the olfactory nerves resulting in a loss of the sense of smell.
Finally, we'll talk about this structure here – the dura mater. The dura mater is the outermost layer of the meninges which are the membranous coverings of the brain and spinal cord. It's a tough thick membrane and we can see it lying between the cribriform plate and the olfactory bulb. Keep in mind that the olfactory nerve will also need to pass through two other meninges of the brain – the arachnoid mater and the pia mater – before it reaches the olfactory bulb.
Now, we'll move on to discuss the gross anatomy of the olfactory pathway. Before we'll look at each element individually, let me first show you this illustration here which is a basal view of the brain. The olfactory pathway is highlighted in green and we can see that this pathway is bilateral, that is, that there is one on the right and left side of the brain. Once the fibers of the olfactory nerve travel through the cribriform plate and into the cranial cavity, they enter the olfactory bulb. This is an elongated ovoid structure and the nerve fibers from the olfactory mucosa synapse here.
The olfactory bulb is continuous with the next structure we're going to talk about – the olfactory tract. The olfactory tract connects the olfactory bulb to the brain. Posteriorly, it divides into the medial olfactory stria which we can see here and the lateral olfactory stria which we can see here. We will discuss both of these structures in greater detail later on in this tutorial.
At the point of bifurcation of the olfactory tract into the medial and lateral olfactory striae, the olfactory tract also gives off a short posterior extension known as the intermediate stria. This stria forms a trigone called the olfactory trigone. The olfactory trigone forms the anterior boundaries of a region of gray matter called the anterior perforated substance. The intermediate stria extends slightly into the anterior perforated substance and its most posterior portion forms an elevated oval structure called the olfactory tubercle. We can see the olfactory tubercle in this image on the right.
Moving posteriorly, we can see a structure that we've mentioned previously – the anterior perforated substance. Numerous small blood vessels pierce this layer of gray matter to supply the brain giving it a perforated morphology. This structure is bordered medially by the medial olfactory stria and laterally by the lateral olfactory stria. We'll now move on to talk about the medial olfactory stria. As we mentioned previously, the medial stria is a division of the olfactory tract and forms the medial border of the anterior perforated substance. It forms connections with medial regions of the brain including the subcallosal area and the paraterminal gyrus. It also forms connections with the anterior commissure.
Superior to the medial olfactory stria, we can see the subcallosal area. It's also known as the parolfactory area. Despite having olfactory in its name, it's uncertain whether this area plays a major role in olfaction, however, it is involved in the production of speech.
Posterior to the subcallosal area, we can see the paraterminal gyrus. It's not a true gyrus but rather a narrow lamina on the medial surface of the cerebral hemisphere. In the next illustration, we can see this structure here – the anterior commissure which is a bundle of nerve fibers that connects the two temporal lobes of the brain. This structure facilitates the communication between the right and left olfactory pathways as it contains crossing fibers from each medial olfactory stria.
Now let's look at the lateral olfactory stria which is also a division of the olfactory tract and forms the lateral border of the anterior perforated substance. The fibers of these neural bundles synapse in nearby structures such as the parahippocampal gyrus, the ambient gyrus, the uncus, and the amygdaloid body. All of these structures are part of the limbic system – the area of the brain that deals with emotion and memory. We'll now talk about these structures in more detail.
The parahippocampal gyrus which we can see here highlighted in green is involved in the encoding and recognition of places. Olfactory input to this region has been observed in some mammals but it's unclear whether olfaction is linked to this area in humans. The next region we're going to talk about is known as the ambient gyrus. It refers to the rostral or front portion of the parahippocampal gyrus. The exact function of this gyrus and its involvement in olfaction is unknown, however, it's known to be involved in a form of dementia known as argyrophilic grain disease.
The parahippocampal gyrus hook sharply backwards to form this structure here, the uncus, which, funnily enough, means hook-like. The uncus plays a role in olfaction and seizures originating in this region are often preceded by hallucinations of disagreeable odors. Finally, the circular region we can see in this illustration is called the amygdaloid body. Its name is derived from Greek word for "almond" and it's sometimes referred to as the amygdala. The amygdaloid body is part of the olfactory system as well as the limbic system and plays a role in both the sense of smell and emotional behavior.
Now that we've looked at the anatomy of the olfactory pathway on a large scale, let's look at it on a microscopic level focusing specifically on the histology of the olfactory mucosa. It's located in the roof of the nasal cavity. Unlike all mucosae, the olfactory mucosa is comprised of an epithelium and a lamina propria. Before we begin, I'd like to point out that this image here is a zoomed-in version of the rectangle we can see here allowing us to see histological structures such as the olfactory epithelium.
The olfactory epithelium is classified as pseudostratified columnar epithelium and consists of three main cell types including the olfactory receptor cells, supporting cells and basal cells. This epithelial layer can be damaged by the inhalation of toxic fumes, physical injury to the interior of the nose or by the use of some nasal sprays. Damage to the olfactory epithelium is usually temporary due to its regenerative capacity but in extreme cases injury can be permanent leading to the loss of smell.
The olfactory receptor cells or the olfactory neurons are bipolar neurons spanning the thickness of the olfactory epithelium. We can see them here wedged between the supporting cells. The bodies of these neurons have two processes – a dendritic process and an axonal process. The axonal process projects from the body towards the olfactory bulb collecting into bundles that pass through the cribriform plate. The dendritic process on the other hand projects through the apical surface of the olfactory epithelium. It protrudes above the epithelial surface as a knob-like structure called the olfactory vesicle. Ten to twenty cilia arise from the olfactory vesicle ready to receive odiferous molecules.
In the next illustration, we can see the olfactory cilia which project into the olfactory mucus layer. These hair-like structures react to odors and stimulate the olfactory neurons. Surrounding the olfactory receptor cells, we can see the supporting cells which are also known as the sustentacular cells. These pseudostratified columnar epithelial cells play an important role in maintaining an optimal environment for the olfactory receptor cells by providing both mechanical and metabolic support. They also synthesize and secrete olfactory binding proteins which guide olfactory molecules to receptors on the olfactory cilia.
Notice now the cells resting on the lamina propria of the olfactory epithelium. These cells are known as basal cells. Their role is to differentiate into either supporting cells or olfactory nerve cells, therefore, basal cells can be considered the stem cells of the olfactory epithelium. They divide constantly ensuring that the olfactory epithelium can be thoroughly replaced within two to four weeks. Deep to the olfactory epithelium, we can see the lamina propria which is a layer of loose connective tissue. The lamina propria provides support to the basal cells and accommodates olfactory glands.
Now we'll take a look at these olfactory glands which we can see originating in the lamina propria and spanning the length of the olfactory epithelium. They are also known as Bowman's glands and secrete mucus onto the surface of the olfactory mucosa. This enables the dissolution of odiferous substances allowing them to be perceived by olfactory neurons. A constant flow of serous fluids from these glands is required to wash away old odors. Overlying the olfactory epithelium, we can see the olfactory mucus layer which is secreted by the olfactory glands. As I mentioned previously, the main function of this layer is to dissolve odiferous molecules and mediate their connection with specific receptors found in the cilia.
So let's bring our knowledge of the gross anatomy and histology of the system together by talking about the neural pathways. Once the nerve fibers of the olfactory receptor cells have entered the olfactory bulb, they synapse with mitral and tufted cells. This results in the formation of the structure we can see here which is the olfactory glomerulus. It's interesting to note that each odor activates a different pattern of glomeruli such that simply by analyzing the different sets of activated glomeruli, one could, in theory, decode the identity of an odor.
In the next illustration, we can see the neural axons of the mitral cells which can also be referred to as the afferent fibers of the olfactory bulb. The processing of the olfactory signals starts here. These nerve fibers then travel posteriorly and may synapse directly in regions of the olfactory cortex that we discussed previously such as the uncus. They may also synapse with cells in the anterior olfactory nucleus first before continuing to the olfactory cortical regions. In this illustration, we can see that the anterior olfactory nucleus is situated in the olfactory tract.
Finally, let's discuss the efferent fibers of the olfactory bulb. The primary efferent nerve fibers stem from the anterior commissure and pass through the olfactory tract. They synapse in the contralateral olfactory bulb on an inner nuclear cell. These cells are excited by mitral cells and inhibited by tufted cells upon which the inner nuclear cells fibers attach. The mitral and tufted cells fibers then continue on to synapse with the olfactory nerve fibers in the olfactory glomerulus. The olfactory nerve fibers converge into bundles and pass through the cribriform plate of the ethmoid bone. They terminate in the olfactory mucosa as olfactory cells in the roof of the nasal cavity. There is also a secondary efferent ganglion that stems from the olfactory trigone and continues along the same path as the primary efferent ganglion.
To conclude this tutorial, we'll go over some clinical notes relating to the olfactory pathway. Anosmia is the loss of the sense of smell and can be caused by injury or even a common cold. It's also associated with neurodegenerative and genetic disorders. Anosmia may be congenital meaning that the patient is born with the condition. In cases of congenital anosmia, the olfactory bulb is poorly developed which affects the normal functioning of the olfactory pathway.
There are three types of anosmia that can be recognized in clinical practice. The first of which is specific anosmia. This means that a patient will be unable to detect a specific odor while maintaining their ability to sense other odors. Specific anosmia may be caused by the absence of specific odorant receptors on the olfactory sensory cells. Secondly, we have hyposmia which refers to a diminished sense of smell. This is usually due to a respiratory infection and is most often temporary. Finally, there is general anosmia which is a complete loss of smell. This is usually due to injury of the olfactory pathway. When examining the olfactory nerves, each nostril should be tested independently as the anosmia may be unilateral.
Dysosmia is an impairment of the sense of smell and there are two types – troposmia and phantosmia. Troposmia is the distortion of an olfactory stimulation. This is caused by a decreased number of functioning olfactory sensory cells resulting in an incomplete characterization of an odorant. Phantosmia, on the other hand, involves sensing an odor when no odor is present. Many things including a head injury or respiratory infection, temporal lobe seizures, inflamed sinuses or even tumors can cause this condition.
So that brings us to the end of our complicated but hopefully interesting tutorial on the olfactory nerve. Neuroanatomy can be difficult but the main thing to take away from today is that the olfactory pathway consists of the olfactory mucosa, nerves, bulbs and tracts as well as the associated cerebral cortex. It's also useful to keep in mind how these aspects of the pathway are connected and how they are involved in the perception of smell. I hope you enjoyed the tutorial and thank you for listening.
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Now, good luck everyone, and I will see you next time.