First neuron and nerve formation
The olfactory pathway originates in a specific area of the nasal cavity: the olfactory region of the mucous membrane. This is where the first neurons are located: olfactory neurosensory cells (cellulae neurosensoriae olfactoriae).
Instead of merging into a single large nerve trunk, the axons of these sensory cells gather into 15–20 thin bundles. These bundles are called olfactory filaments (fila olfactoria). In essence, they constitute the olfactory nerves themselves.
Entry into the cranial cavity and second neuron
The formed olfactory filaments ascend and leave the nasal cavity. To do this, they:
- Pass through the small foramina of the cribriform plate of the ethmoid bone (lamina cribrosa ossis ethmoidalis).
- Enter directly into the cranial cavity.
- Terminate on the cells of the olfactory bulb (bulbus olfactorius).
The olfactory bulb is a crucial relay station in the transmission of odors, as it houses the cell bodies of the second neurons of the olfactory pathway.
Olfactory tract and further pathway
The axons of the second neuron originate from the cells of the olfactory bulb. They form the olfactory tract (tractus olfactorius), which projects toward brain structures.
The subsequent anatomy of this pathway is as follows:
- The tract reaches the olfactory trigone (trigonum olfactorium), whose structure is formed primarily by an accumulation of nerve cells.
- In the region of the trigone, the pathway divides into olfactory striae (striae olfactoriae).
- The olfactory striae carry impulses directly to the cortical olfactory centers.
Terminal nerve (cranial nerve 0)
An interesting feature of cranial nerve I is its close proximity to the so-called zero cranial nerve: the terminal nerve (n. terminalis).
Fibers of the terminal nerve run directly among the fibers of the olfactory nerve. They originate in the mucous membrane of the olfactory region located on the nasal septum. Having followed the same path through the foramina of the cribriform plate, the n. terminalis fibers continue into the medial olfactory stria.
Briefly: answers to topical questions
Where are the cell bodies of the third neurons of the olfactory pathway located?
The cell bodies of the third neurons of the olfactory pathway are located in several brain structures. They originate from the prepiriform area. Additionally, the cell bodies of the third neurons are localized in the anterior perforated substance and the septum pellucidum, where the striae from the olfactory trigone arrive. From these structures, the fibers project to the cortical representations.
Into which specific olfactory striae does the olfactory pathway divide in the region of the trigone?
In the region of the olfactory trigone, the fibers of the olfactory pathway diverge and divide into the following structures:
- Medial olfactory stria — its fibers decussate and cross to the opposite side;
- Lateral olfactory stria — the fibers bypass the limen insulae;
- Longitudinal stria.
These diverging pathways project to the cortical and subcortical centers.
Which brain structures are classified as subcortical olfactory centers?
According to the source, the cortical and subcortical olfactory centers include: the subcallosal area (Broca's olfactory area), the prepiriform area, the uncus, the amygdala, and area 28 (entorhinal area).
Where are the cortical olfactory centers located?
The cortical olfactory centers are located in the cortex of the temporal lobes of the brain. The primary and secondary cortical olfactory areas, where the subjective perception of odors arises, include:
- Uncinate gyrus (uncus, uncus);
- Parahippocampal gyrus (gyrus parahippocampalis), specifically its anterior part, which corresponds to Brodmann area 28.
The mnemonic of the first letters will help you remember the sequence of the olfactory pathway structures: "Cells — Filaments — Bulb — Tract — Trigone — Striae".
Class and exam questions
Does the olfactory nerve form a single nerve trunk?
No, the olfactory nerve does not have a single trunk. It consists of 15–20 fine olfactory filaments (fila olfactoria).
Where is the cell body of the second neuron of the olfactory pathway located?
The second neuron is located in the olfactory bulb (bulbus olfactorius), which lies in the cranial cavity directly above the cribriform plate.
Through which openings does the nerve enter the cranial cavity?
The olfactory filaments pass through the foramina of the cribriform plate of the ethmoid bone (lamina cribrosa ossis ethmoidalis).
What else to study on this topic
- Cytology and histology of olfactory neurosensory cells
- Topographic anatomy of the cribriform plate and ethmoid bone
- Cortical olfactory centers and their localization in the brain
- Functional significance of the terminal nerve (cranial nerve 0)
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