OLFACTORY NERVE

The olfactory nerve is the first and the shortest nerve among the twelve pairs of cranial nerves. It is one of the only two nerves that do not project from brainstem. It is in charge of transferring sense of smell-related information which arises first in the epithelium sited in the upper part of nasal cavity (olfactory region) and through the olfactory nerve and olfactory pathway, the smell is transmitted further to the primary olfactory cortex in brain. Some neurological diseases such as Parkinson’s are associated with the quality of the smell.

The importance of olfactory nerve is linked to the quality of smell. We will ever appreciate the smell of morning cup of coffee or the odor an authentic brand perfume if we perform great function of olfactory nerve. The secretion of the salvia in the mouth will be increased if the smell of a great meal of chicken touches the cilia in olfactory region located in the top of nasal cavity to transmit that smell to the brain. Moreover, great memories are connected to the sense of smell. Old days will touch our memories and draw happiness in our faces if the olfactory nerve is able to detect and transform specific smell that is rich in old memories. Any damages to olfactory nerve will reduce the privilege of the smell and therefore will decrease the enjoyment of our lifestyle.

 At the top of nasal cavity, the olfactory epithelium (Fig. 1) is found where the olfactory receptors are embedded to detect chemicals that activate the sense of smell. The axons of those cells form fila olfactoria where they are collected into small bundles of true olfactory nerves. Through the puncture of small foramina in cribriform plate of the ethmoid bone, the axons pass into the cranial cavity. Immediately after reaching the cranial cavity, those fibres access the olfactory bulb found in the olfactory groove within the anterior cranial fossa. Three cell types are found within the epithelium: the olfactory receptor cells, basal (stem) cells and supporting cells (Fig. 2).

The olfactory receptor cells have two protrusions from their cell body. The first is the dendrites which travel and expands into the surface of the olfactory epithelium. The dendrite’s surface has cilia that are embedded into the fluid layer enveloping the epithelium in the nasal cavity (Fig. 3). There are receptors for odor molecules in the cilia and they run into the nasal cavity to be captured in the fluid encasing the olfactory epithelium. The second protrusion of olfactory receptor cells is  the unmyelinated axon. 

The epithelium includes other cell types such as basal stem cells (Fig. 4). The basal stem cells can substitute damaged receptor cells since the lifespan of the olfactory receptor cell ranges from 30-60 days. However not all receptors are substituted across a lifespan which mean that the sense of smell can decrease with age.

Olfactory Nerve

There is one axon in each receptor cell that extends from the basal surface located directly inferior to the cribriform plate of the ethmoid bone that act as the bony roof of the nasal cavity. The bundles of nerve fibers are composed through the combination of the axons which protrude from the cell body with other receptor cell axons, and the olfactory nerve is considered as the collection of these axonal bundles. The dura and arachnoid maters surround these bundles of nerve fibers and they travel in the cribriform plate of the ethmoid bone through the foramina to move superiorly toward the olfactory bulb (Fig. 5).

Olfactory Bulb

Mitral cells, the specialized neurons, are the components of the olfactory bulb, the ovoid-like structure. When the fibres of the olfactory nerve synapse with the mitral cells, collections known as synaptic glomeruli is formed. Inside the bulb, the second order nerves are now composed to move posteriorly within the olfactory tract. Through the olfactory nerve, some axons project from the olfactory receptor cells, and within the olfactory bulb, they terminate. 

The olfactory bulb is considered as the main relay station within the olfactory pathway. The location of each one (right and left) is inside the cranial cavity, superior to the cribriform plate of the ethmoid bone and lateral to the crista galli. Consequently, it sites on the underside of medial aspect of the frontal lobe. The olfactory bulb contains bundles of nerve fibers called glomeruli, and incoming receptor cell axons are observed connected with the dendrites of mitral relay neurons. (Fig. 6).

Olfactory Striae

The olfactory tract, on the posterior and anterior sides of the optic chiasm, is divided into lateral and medial olfactory striae. While the lateral stria continues on to structures associated with the olfactory cortex, the medial stria protrudes to the anterior commissure to contralateral olfactory structures (Fig. 7).

Olfactory Cortex

The combined area of the cerebral cortex refers to the olfactory cortex. The primary olfactory cortex is responsible of sending nerve fibres to different areas of the brain including the amygdala, the piriform cortex, and the secondary olfactory cortex (Fig. 8). These areas are involved in the memory and evaluation of olfactory sensations. It explains the intricate link between the sense of smell and memory. The function of the mucosal layer is not only to detect smell, but also to sense  advanced aspects of taste.

From the olfactory cortex and via the dorsal medial nucleus of the thalamus, information about smell is sent to the orbitofrontal cortex. As a part of the prefrontal cortex, the orbitofrontal cortex is located on the underside of the frontal lobe and above the eye orbit (Fig. 8) and (Fig. 9), and the failure to differentiate various odors can occur as a consequence of lesions of this cortical region. Also portions of the hypothalamus and brainstem receive odor information and they trigger autonomic responses involved in salivation, appetite, and gastric contraction.

The Olfactory Epithelium

The roof of the nasal cavity is the site of the olfactory epithelium which is made up of pseudostratified columnar epithelium with a number of cells. The basal cells that are composed of new stem cells act to develop the new olfactory cells. Sustentacular cells which are similar to the glial cells in the central nervous system, and they act as structural support. Olfactory receptor cells which are bipolar neurons with two processes, dendritic process and central process. Besides the epithelium, Bowman’s glands are also found in the mucosa of the respiratory epithelium for the purpose of mucus secretion (Fig. 4).

During the fourth week of embryonic development, the face is developed from five bulges derived from the first and second pharyngeal arches. These bulges are called facial prominences, and they include the frontonasal, and the two paired of mandibular and maxillary prominences (Fig. 10). A thickened area of ectoderm originates on each side of the frontonasal prominence called the olfactory placodes, which continue to increase in size until the sixth week when the center of each placode invaginates to form the nasal pits (Fig. 11). The nasal pits eventually give rise to the olfactory epithelium, from which the olfactory nerves arise, and divide into a medial and lateral nasal process.

The nasal cavity, including the olfactory epithelium located on the top of the cavity, receives blood supply from several branches. The carotid (external and internal) arteries, the sphenopalatine artery and the ethmoidal (anterior and posterior) arteries are among those branches. The external carotid artery gives off the maxillary artery (Fig. 12), which it then gives off the sphenopalatine artery. The sphenopalatine artery within pterygopalatine fossa reaches the upper nasal cavity posterosuperior to the middle nasal conchae to form extensive network of major branches include the posterior lateral nasal and nasal septal arteries.

The ophthalmic artery, which is a branch of the internal carotid artery gives off two branches, the anterior and posterior ethmoidal arteries (Fig. 13). The posterior ethmoidal artery originates from a proximal portion of the ophthalmic artery and passes through the posterior ethmoidal foramen located next the optic canal. 

The anterior ethmoidal artery, however, originates from distal portion of the ophthalmic artery and moves toward the medial wall of the orbit to enter the anterior ethmoidal foramen located behind the orbital margin. Both the anterior and posterior ethmoidal arteries, along with the sphenopalatine artery, form extensive anastomotic networks to provide blood supply the olfactory mucosa.

Within the cranium, the olfactory nerve receives the blood supply from olfactory artery, which is a branch of the anterior cerebral artery (Fig. 14). The olfactory artery originates from the lateral aspect of the anterior cerebral artery. The olfactory artery runs above the optic nerve, where it gives three terminal branches to supply both the olfactory tract and olfactory bulb.

Anosmia

It is defined as an alteration in the intensity of smell perception, or the full lack of smell perception. Numerous neurological degenerative diseases are usually associated with this condition, such as Schizophrenia, Parkinson’s disease, Alzheimer’s disease, Pick’s disease, Huntington’s disease, Diabetes and multiple sclerosis. Other forms of brain injury can also lead to anosmia. Examples of those injuries can fall into the head trauma with an anterior cranial fossa or the superior orbit fracture and the brain tumors or aneurysms that compress a segment of the olfactory pathway.

Dysosmia

This disease can be defined as a distortion in the quality of the perception of a smell (known as parosmia) or as the perception of a smell when no smell is actually present (known as phantosmia). As a result of upper respiratory tract infections, damage to olfactory nerve fibers can occur. When the number of nerves fibers decreases as a consequence of these infections, there will not be enough different fibers to precisely distinguish smells, thus, it eventually leads to parosmia. Phantosmia is associated commonly with temporal lobe epilepsy, and it may mark a partial seizure which can disseminate later to temporal lobe structures.

Hyposmia

It is the reduction of the ability to smell and to detect odors. This condition may be caused by several factors including allergies, viral infections or head trauma. Older people are most subjected to have hyposmia since the quality of smell is associated with aging. Hyposmia could be an early sign of Parkinson's disease.

Hyperosmia

It is a hypersensitivity to smell due to some reasons including genetics, hormone changes and migraines. The sense of taste could also be affected since there is a strong association between the two senses (smell and taste). Hyperosmia can be temporary and remain minor, or it can last for a long time and lead to more complicated conditions. Common causes of this condition include pregnancy, autoimmune diseases and neurological disease. 

Testing the sense of smell can easily be conducted through using well and strong odors, such as menthol, coffee or peppermint. This test can be done in the alterations of odor perception or if there is damage to any sinus or nasal cavity issues (Fig. 15).