20.6
Récepteurs olfactifs : emplacement et structure
Le processus de l’olfaction, également connu sous le nom d’odorat, est un système de réponse chimique sophistiqué. Les neurones sensoriels spécialisés…
L’organe olfactif est situé sur le toit de la cavité nasale. Il recouvre la conque nasale supérieure de chaque côté de la cloison nasale.
Il se compose du tissu conjonctif appelé lamina propria et de l’épithélium olfactif, une zone de cinq centimètres carrés riche en neurones sensoriels olfactifs.
Chaque neurone olfactif est un neurone bipolaire. Leurs dendrites apicales se poursuivent dans un bulbe et forment plusieurs cils olfactifs non mobiles rayonnants qui contiennent des protéines réceptrices.
Les neurones olfactifs sont entrecoupés de cellules de soutien cylindriques et de glandes olfactives, qui produisent du mucus autour des cils olfactifs.
Le mucus aide à capturer et à dissoudre les odorisants en suspension dans l’air, ce qui permet une détection efficace par les protéines réceptrices.
De plus, l’épithélium olfactif contient également des cellules souches olfactives à la base. Ils se différencient tous les 30 à 60 jours pour remplacer les anciens neurones olfactifs.
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Q1: Where are olfactory receptors located in the nasal cavity?
Olfactory receptors are located on the roof of the nasal cavity, covering the superior nasal concha on each side of the nasal septum. This region, called the olfactory epithelium, spans approximately five square centimeters and contains olfactory sensory neurons rich in receptor proteins. The olfactory epithelium is composed of connective tissue called lamina propria and specialized sensory cells.
Q2: What is the structure of an olfactory neuron?
Each olfactory neuron is a bipolar neuron with apical dendrites that extend into a bulb and form several nonmotile olfactory cilia containing receptor proteins. These cilia are surrounded by mucus produced by olfactory glands, which helps capture and dissolve airborne odorants for efficient detection by the receptor proteins.
Q3: How do olfactory neurons detect odorants?
Airborne odorant molecules dissolve into the mucus lining the nasal cavity and bind to specific receptor proteins within the olfactory cilia's cellular membrane. These receptor proteins are G protein-coupled and generate a graded membrane potential in the olfactory neurons, initiating the smell sensation and connecting to the physiology of smell and olfactory pathway.
Q4: Why do olfactory neurons need to be replaced?
Olfactory neurons are susceptible to damage from noxious airborne substances and environmental exposure. Olfactory stem cells located at the base of the olfactory epithelium differentiate every 30 to 60 days to replace old olfactory neurons, ensuring continuous regeneration and maintenance of olfactory function.
Q5: What is anosmia and what causes it?
Anosmia is the degradation or complete loss of the sense of smell. It can result from severe facial trauma that severs olfactory tract axons, certain medications like antibiotics that eliminate olfactory neurons, inflammation from respiratory infections or allergies, or age-related decline in olfactory neuron regenerative capacity.
Q6: How does anosmia affect taste and quality of life?
Anosmia can diminish the gustatory experience by rendering food tasteless, as smell is essential to flavor perception. Individuals with compromised olfactory capacity may require augmented levels of spices and seasonings. There is a potential link between anosmia and mild depressive states due to diminished pleasure from food.
Q7: Why is olfaction unique among sensory systems?
Olfaction is the singular sensory modality that bypasses a synapse in the thalamus before interfacing with the cerebral cortex. Olfactory axons project directly to the olfactory bulb and then to the primary olfactory cortex and limbic system, enabling direct linkage of odors with memory and emotional reactions.