The Olfactory Nerve

The olfactory nerve otherwise called cranial nerve 1, is the first, and shortest cranial nerve in the body. It helps to send a signal of smell to the brain via its special afferent nerve. The olfactory placode is the origin of the olfactory nerve, and it provides supportive cells called the glial cells to provide support for the nerve embryologically (2).

The nasal epithelium in the upper section of the nasopharynx is the site of the genesis of the sensitive olfactory nerve. The olfactory nerve finds its way to the brain by passing through the ethmoid bone, journeying from the olfactory mucosa. The tiny nerve fascicles synapse together in the olfactory bulb from where they send information or smell signals to the brain with the help of the olfactory tract (4). One of the special attributes of the olfactory nerve is the absence of a cortical link to the thalamus of the brain (3).

Structure Of The Olfactory Nerve

Structurally, the nose is constructed to make it easier for inspired air to travel forward toward the olfactory system. The septum, the upper section of the superior turbinate, and the topography and composition of the posterosuperior regions of both nasal canals are all covered by the specialized epithelial tissue known as the olfactory epithelium.

The nerve signals of bipolar olfactory neurons, where the olfactory nerve fibers begin and terminate apically, are found in this specialized olfactory epithelium. Averagely, about six to ten million of the sensory olfactory neurons possessed by man are well distributed along a surface area of close to 2.5 cm^2. The neuronal cells took their origin from the epithelial base and possess a half-life of thirty to forty days. The apex of the neuronal cells of the olfactory nerve has dendrites that made it easier for them to penetrate the epithelial surface, where they have interactions with the odoriferous particle through receptors called the “G protein-coupled receptors”

The olfactory neuron basal projections are later grouped into bundles of small nerve fiber pieces called the fila olfactory. This file olfactory goes up and finds its way through the cribriform plate of the ethmoidal bone. Each of the small file olfactory nerve bundles come together to form the olfactory nerve, and they are numbered between fifteen to twenty nerve bundles situated in each nasal cavity.

The landmark junction where the nerve passes through the cribriform plate is a point of clinical relevance in that it is prone to infection, trauma, and damage. The foramen in the cribriform plate of ethmoid bone serves as a source of entry for pathogenic microorganisms and agents and also facilitates olfactory nerve damage and its shearing in the time of trauma.

The fila olfactory finds its way into the olfactory bulb ventrally after passing through the subarachnoid space and cribriform plate of the ethmoidal bone. The olfactory
epithelium’s basement membrane emerges via the foramina while the dura mater covering the cerebral portion of the cribriform plate extends continuously.
The olfactory bulb is made up of the ventral surface, and the posterior surface.

The ventral or anterior surface is anatomically located on the posterior third of the cribriform plate of the ethmoid bone, and the posterior or dorsal layer is located at the lower side of the frontal lobes, especially “the orbital and the rectus gyri”.
The olfactory bulb then serves as the point of relay for the transmission of information between the brain’s primary olfactory cortex and the epithelium of the olfactory nerve.

The axons extensions of mitral and tufted cells create bundles that pass through to the olfactory bulb and travel dorsally, coming together to form the olfactory tract, after olfactory information is passed from the neurons assigned to the olfactory receptors to these cells in the glomeruli. Each olfactory tract extends dorsally from the olfactory bulb and terminates in the sensory olfactory trigone.

The olfactory trigone is a triangular enlargement of the terminating olfactory tract that is positioned precisely rostral towards the anterior perforated substance and superior to the front clinoid process. The fibers of the tract split into two major bundles, “the lateral and medial olfactory stria”, at this distinctive anatomical point (5).

The lateral olfactory striae, that is nerves are situated at the top of the nasal concha, while the medial olfactory nerves are located around the nasal septum (3). The medial olfactory nerve handles autonomic functions, and responses examples of which is stimulation of excess salivation in response to the aroma of food, and the medial olfactory nerves send information to the olfactory bulb with the help of the “ipsilateral anterior olfactory nucleus” and the anterior part of the contralateral olfactory bulb.

The excessive salivation that results from the sight and smell of food is brought about by the medullary stria pathway. This occurs from the stimulation of the upper and lower salivatory nuclei through the habenular nuclei and the tegmentum. In reaction to the scent of food, the posterior vagal nucleus in the medulla communicates with the olfactory-hypothalamic-tegmental network, increasing peristalsis and stomach production (5).

Mostly in the olfactory tract, the lateral olfactory stria has the most fibers and is where the greatest operational olfactory transmission occurs. The main olfactory cortex is found in the temporal lobe next to the uncus, and it is reached by the efferent terminals of the olfactory bulb that are carried via the lateral olfactory stria more towards the limen of the insula (5).

The processing of olfactory information that occurs in the brain mainly occurs in the primary olfactory cortex. This does not occur in isolation, it does this by communicating with various cortical and limbic structures, certain structures have been identified to receive the axons coming from the olfactory bulb and these structures are the amygdala, the piriform cortex, and the parahippocampal gyrus. Other identified structures are the olfactory tubercle and the anterior olfactory nucleus. These structures offer a wide range of capabilities that enable the olfactory sensory information to be integrated to encode, identify, and interpret events (5).

Functions Of The Cranial Nerve

The cranial nerve I, the Olfactory nerve, helps to detect different smells, aromas, and odors. The substance from which the smell is coming ejects small particles which are inhaled by the nose, and the olfactory cells receptors in the nasal mucosa, detect and take up the smell. These receptors transmit the information to the brain via the olfactory nerve, the interpretation makes it possible for us to perceive smell (1).

What Is The Mechanism Of Smell?

The olfactory system permit smell via two basic methods:

1. The nostrils: The tiny molecular particles given off by the substance from which the smell is coming triggers the olfactory receptors to action, and the receptors help to serve as the means of signaling the brain for smell interpretation and ability to differentiate a different kind of smell.
2. Behind the throat: Another route to detect smell is from the back of the throat. After chewing or drinking, tiny molecules have released that help to smell, the molecules are taken via the throat to the olfactory receptors located behind the nose.

The olfactory mucosa is inevitably important in the pathway of smell or in one’s ability to smell. The mucus membrane is located at the superior part of the nasal cavity, and it possesses different kinds of cells, which are:

• Olfactory receptor cells: These cells support two kinds of processes, namely the dendritic, and central processes. The dendritic processes project through the little hairs’ presence in the olfactory mucosa, through which they stimulate the olfactory cells. The central processes direct the cells in the opposite direction.
• Osustentacular cells: These groups of cells are supportive cells, they give support to the environmental tissues.
• Basal cells: The osustentacular cells and the olfactory receptor cells grow and develop in this area (1).

Embryology Of The Olfactory Nerve

According to embryology, 5 growths known as facial prominences that originate from the first and second pharyngeal bridges grow into the face during the 4th week of gestation. The anterior prominence and the coupled mandibular and maxillary angles are examples of these.

The olfactory placodes, a region of thickened ectoderm that appears on either side of the frontonasal eminence about the 4th week of early embryogenesis. Up until the sixth week, the nasal placodes continue to grow in size, but at that point, the nasal pits begin to develop in each placode’s core.

The squamous epithelium, from which the olfactory nerves emerge, eventually develops from the nasal pits, which then split into a medial and lateral nasal process. The nose, philtrum, and major palate then come into being (5).

Blood Supply To The Olfactory Nerve

The internal and external carotid arteries, the sphenopalatine artery, and the ethmoidal arteries, both anterior and posterior are the blood supply to the olfactory mucosa and neurons situated in the posterosuperior part of the nasal cavity.

The sphenopalatine artery is a branch of the pterygopalatine branch of the maxillary artery, which takes it branch from the external carotid artery. The sphenopalatine artery passes through the pterygopalatine fossa, before crossing to the sphenopalatine foramen from where it gets to the top of the nasal cavity, above the middle nasal conchae. It is at this junction that the big branches named the posterior lateral nasal and nasal septal group of arteries joined together and form an anastomosis (5).

Clinical Significance Of The Olfactory Nerve

The following are the clinical relevances of the olfactory nerve:

1. There is a possibility of the olfactory nerve being damaged from trauma, especially blunt trauma which often causes laceration to the nerve along the ethmoid bone. Such trauma can also predispose the nerve to infection (3)
2. Lesion or injury affecting the olfactory nerve or along the anatomical course of the nerve can lead to any of the following disorders
   1. Anosmia: This simply means the inability to smell
   2. Dysosmia: This occurs when an individual is perceiving a smell when there is nothing to smell at the time. This is also referred to as olfactory hallucination
   3. Hyposmia: This is a reduction in the ability to smell (3).

Anosmia

Anosmia is a medical condition that means the inability to smell or a total absence of the sense of smell. Anosmia can be temporary, it can be permanent, and at times, it can be congenital or progressive anosmia.

• Temporary anosmia: Temporary anosmia is usually a result of infection, for example, meningitis, or a local illness of the nose like the common cold.
• Permanent anosmia: This usually occurs as a result of trauma, such as head injury, or from the presence of a tumor in the olfactory groove, such as a meningioma.
• Neurodegenerative conditions: Anosmia also occurs due to the presence of neurological degenerative diseases such as Parkinson’s disease or Alzheimer’s disease. In such disorders, the anosmia supersedes the motor disorder in the patient but the patient might not pay attention to it.
• Genetic: Anosmia often also occurs from genetic diseases such as Kallmann syndrome, a syndrome characterized by the failure of the individual to start or finish the pubertal phase appropriately. It is also seen in primary ciliary dyskinesia, a disease that causes immobility of the cilia (2).

Frequently Asked Questions

References

1. https://my.clevelandclinic.org/health/body/23081-olfactory-nerve#:~:text=the%20olfactory%20nerve%3F-,Your%20olfactory%20nerve%20is%20the%20first%20cranial%20nerve%20(CN%20I,inside%20part%20of%20your%20nose.
2. https://teachmeanatomy.info/head/cranial-nerves/olfactory-cni/
3. https://www.physio-pedia.com/Olfactory_Nerve
4. https://en.wikipedia.org/wiki/Olfactory_nerve
5. https://www.ncbi.nlm.nih.gov/books/NBK556051/

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