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).
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).
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).
The olfactory system permit smell via two basic methods:
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:
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).
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).
The following are the clinical relevances of the olfactory nerve:
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.
Medical conditions and other factors that can affect the olfactory nerve are:
⦁ Nasal polyps
⦁ Concussions from the head injury
⦁ Poor oral hygiene
⦁ Chemical poisoning
⦁ Drugs such as antibiotics
⦁ Ear, nose, and throat infections
⦁ Cancers of head and neck
⦁ Alzheimer's disease
⦁ Parkinson disease
⦁ Seizure disorder.
More than fifty percent of patients with covid-19 develop a loss of sense of smell. Studies are currently ongoing as to why, but there is a likelihood of the SARS-COV-2 viral infection having a direct impact on the olfactory nerve causing damage to the nerve or the receptors. Although some of the patients regain their ability to smell after a while it is not always immediately, as it can take several months.
The following are preventive measures to help keep your olfactory nerve healthy.
Practice good dental hygiene
Get covid-19 vaccine
Avoid blunt trauma to the head
Wear protective head gear for sport activities (1).