OLFACTORY PATHWAYS
Description
The olfactory nerve is the first cranial nerve (CN I). It is a
sensory nerve that functions for the sense of smell. Olfaction is
phylogenetically referred to as the oldest of the senses. It is carried out
through special visceral afferent nerve. It is a cranial nerve with certain
unique features such as lacking a precortical connection to the
thalamus.[1] [2]
Root
The olfactory nerve terminates at the olfactory bulb, located just above the
ethmoid bone and below the frontal lobe. The olfactory bulb acts as a relay
center for the transmission of the impulses from the olfactory nerve to the
olfactory tract and then to the cerebral cortex (olfactory cortex).[3]
Branches
The olfactory nerve divides into two branches. These are:
1. Lateral olfactory nerves- located in the superior nasal concha [4]
2. Medial olfactory nerves- located along the nasal septum
Pathway
The olfactory epithelium is found in the nasal cavity. This contains
different types of cells,including: olfactory sensory neurons, microvillar‐
capped sustentacular cells, microvillar cells, the Bowman's duct cells, and
the basal cells or stem cells.
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The olfactory sensory neurons are bipolar cells that receive the stimuli
at its dendrite from the apical pole and conducts them to the axon from
basal pole. Groups of 15 to 40 axons are ensheathed with glial cells at
the level of the basement membrane, making up a small fascicle.
These small fascicles converge with other small fascicles giving rise to
larger fascicles, known as fila olfactoria.
These collection of axons crosses the cribriform plate of the ethmoid
bone, through its perforations, and reach the brain where it innervates
the olfactory bulb.
The olfactory bulb is a relay station for the transmission of the impulse
coming from the olfactory nerve. From the olfactory bulb, impulses then
travel through the olfactory tract. The olfactory tract divides into the
medial and lateral olfactory striae, then connects to the olfactory cortex.
The olfactory cortex are the parts of the brain that receive and process
the sensory input.[3]
Function
The olfactory nerve is purely a sensory nerve that functions for the sense of
smell.
Clinical Relevance
The olfactory nerve can be damaged through trauma eg TBI; Blunt trauma
to the head can lead to laceration of the olfactory nerve as it crosses the
ethmoid bone; Infections can also cause damage to the olfactory nerve.
The anatomical position of the olfactory nerve makes it an obstacle to
exploring the anterior cranial fossa [2]. Therefore, this leads to a potential
risk of injury especially during surgeries. During surgical procedures, there
are three mechanisms that can lead to olfactory nerve injury [5]:
1. Detachment of the cribriform plate and section of the fila olfactoria
during frontal lobe retraction.
2. Partial or total section of the olfactory tract during the process of
dissection
3. Ischemic injuries
Lesions to the Olfactory Nerve and/or to the Olfactory Pathway can lead to
the following symptoms:
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Anosmia- loss of sense of smell
Hyposmia- decrease ability to detect smell
Hyperosmia- increased sensitivity to the sense of smell
Dysosmia- distorted sense of smell or presence of unpleasant smell in
the absence of any actual odor (olfactory hallucinations)
Assessment
Assessment of the olfactory nerve is readily done either at bedside or in the
clinic. Testing of the integrity of the olfactory nerve involves either
pinching or blocking of one nostril while the patient is blindfolded or with
the eyes closed, then have the patient smell aromatic substances such as
coffee, vanilla, cinnamon, etc. Avoid using substances with strong or
noxious smell such as alcohol or ammonia. Have the patient identify the
substance based on its aroma. Then repeat the same procedure on the
other nostril. This is done to compare the sense of smell on both sides.
Treatment
Conservative management for olfactory nerve damage secondary to
different etiologies is olfactory training.[6][7]