Olfactory region of the mucous membrane
The specialized zone in the nasal cavity is named the olfactory region of the nasal mucosa (regio olfactoria tunicae mucosae nasi). It occupies the superior nasal meatus and the upper parts of the nasal septum.
This region has several striking differences from the rest of the nasal (respiratory) mucosa:
- Thickness: the olfactory mucosa is significantly thicker.
- Color: has a characteristic yellowish-brown coloration.
- Epithelium: the typical ciliated epithelium of the respiratory tract is completely absent.
- Glands: contains specific olfactory glands (glandulae olfactoriae, or Bowman's glands). Their main function is the secretion of a serous fluid necessary to dissolve odorants (molecules can interact with receptors only in a dissolved state).
Structure of the olfactory epithelium
The surface of the olfactory region is lined by a specialized pseudostratified olfactory epithelium (epithelium olfactorium). It comprises three key cell types, each performing a unique task:
- Olfactory neurosensory cells (cellulae neurosensoriales olfactoriae). These are bipolar neurons that act as true receptors. Their apical (upper) pole forms olfactory vesicles with cilia that capture molecules. The basal (lower) pole gives rise to processes that form nerve fibers.
- Sustentacular cells (cellulae sustentaculares). Perform supporting and trophic (nutritional) functions. In addition, they play a crucial role in detoxifying odorants, protecting receptors from damage.
- Basal cells (cellulae basales). Located at the base of the epithelium. They serve as a cambial reserve—a source of regeneration for the olfactory epithelium, ensuring the continuous renewal of neurosensory cells.
Olfactory nerves and pathway
The central (basal) processes of olfactory neurosensory cells do not run individually. They bundle together to form the olfactory nerves (nn. olfactorii).
The further path of the impulse is as follows:
- The bundles of nerve fibers enter the cranial cavity through the openings of the cribriform plate of the ethmoid bone (lamina cribrosa ossis ethmoidalis).
- The fibers terminate in the olfactory bulb (bulbus olfactorius), where the first synaptic transmission occurs.
- From the bulb, the impulse travels via the olfactory tract (tractus olfactorius) directly to the primary olfactory centers in the cerebral cortex.
A crucial feature of the olfactory analyzer: the nerve impulse reaches the cerebral cortex bypassing any relay in the thalamus (unlike most other sensory systems).
Briefly: answers to topical questions
Which structures of the cerebral cortex serve as the primary olfactory centers?
The primary olfactory centers of the cerebral cortex are structures of the temporal lobes. These include:
- Uncus — the cortical olfactory area where the fibers of the olfactory pathway terminate.
- Parahippocampal gyrus (gyrus parahippocampalis) — includes the anterior part (Brodmann area 28).
It is within these gyri that the subjective perception of odors arises.
With which cells do the fibers of the olfactory nerves form synapses in the olfactory bulb?
The fibers of the olfactory nerves form synapses with dendrites of several cell types in the olfactory bulb. Excitation is transmitted to:
- Mitral cells — second-order neurons onto which impulses from numerous fibers converge.
- Tufted cells — participate in the formation of the olfactory pathway.
- Granule cells — receive signals via complex synapses.
Axon terminals form olfactory glomeruli (glomeruli olfactorii) within the bulb.
Into which structures does the olfactory tract divide in the region of the olfactory trigone?
In the region of the olfactory trigone (trigonum olfactorium), the olfactory tract divides into olfactory striae (striae olfactoriae). These include:
- Medial olfactory stria — its fibers decussate and cross to the opposite side.
- Lateral olfactory stria — runs toward the amygdala, semilunar gyrus, and ambient gyrus.
These structures conduct impulses to the cortical olfactory centers.
To memorize the epithelial cells: "Neuron, Support, and Base". Neurosensory — catch the smell, Sustentacular (support) — nourish and clean, Basal — give birth to new cells.
Class and exam questions
How does the olfactory mucosa differ from the respiratory mucosa?
It is thicker, has a yellowish-brown color, and lacks classical ciliated epithelium.
What is the function of Bowman's glands?
They produce a fluid serous secretion that dissolves odorants. Without dissolution, odor molecules cannot affect the receptors of neurosensory cells.
Does the olfactory impulse pass through the thalamus?
No. This is a unique feature of the system: from the olfactory bulb via the olfactory tract, the impulse travels to the primary cortical centers, bypassing the thalamus.
What else to study on this topic
- Mechanism of odorant detoxification by supporting cells
- Histophysiology of olfactory vesicles and receptor cilia
- Topographic anatomy of the lamina cribrosa and passage of nerve pathways
- Process and timeline of neurosensory cell regeneration from basal cells
- Localization of primary olfactory centers in the cerebral cortex
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