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Cranial nerve I, Some fibers from mitral and tufted exit olfactory tract…
Cranial nerve I
Pathology
- Common cold and rhinitis prevents stimuli from reaching the R cells
- Chronis smoking and viral infections can damage R
- Tumor = esthesioneuroepitheliomas
- Blow to the head
- Fracture of the cribriform plate
<=> Permanent unilateral or bilateral anosmia
Fracture of the cribiform plate = leaking CSF into the nose + anosmia
- Can become an access space for pathogens
Primary olfactory neurons regeneration
- Not 100 efficient
- Permanently altered following recovery from head trauma
Central olfactory pathway includes:
- Olfactory bulb (can be contused or lacerated from head injury)
- Olfactory tract
Susceptible to compression
- Olfactory groove meningiomas
- Aneurysms of the anterior cerebral artery
- Infiltrating tumours of the frontal lobe
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Olfactory system
- Special sensory afferent
- Act as sensory R
- Axon enters cerebral cortex through -> olfactory bulb
- Continuous tumover
- Entirely ipsilateral
Components:
- Olfactory epithelium
- Olfactory nerves
- Olfactory bulb and tract
- Olfactory cortex
Olfactory bulb and its projections
- Enlargment of olfactory tract
- Evaginate from the telencephalon
- Have cell bodied of mitral and tufted cells (2nd olfactory neurons)
- Have glomeruli (=where 1st and 2nd olfactory neurons make contact)
6 layers:
1) Nerve fiber layer
2) Glomerular layer
3) External plexiform layer = tufted cell
4) Mitral layer
5) Internal plexiform and granule cell layers
6) Nerve fiber layer of olfactory tract
Sensory neurons from the olfactory bulb to the central nervous system:
- Mitral cells --> lateral (primary) olfactory area
- Tufted cells --> anterior olfactory nucleus and to primary olfactory areas and the anterior perforated substance
- Interneuron (periglomerular and granule cells) --> interact between glomeruli
Postsynaptic fibers of mitral and tufted cells = olfactory tract and trigone
- These fibers diverge laterally in front of anterior perforated substances --> Lateral (primary) olfactory stria to the primary olfactory area for the conscious appreciation of smell
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Olfaction
- Odorants bind in plasma membrane of the cilia of the olfactory R neurons
- Depolarization of the olfactory R neuron + R potential
- AP transmitted to olfactory bulb via olfactory nerve
Sensory transduction
- Odorant binding activates olfactory specific G-protein
- Activate olfactory-specific adenylate cyclase = increase production of cAMP
- cAMP allow influx of Na+ and Ca++ = depolarization of R neuron
- Release of Ca++ increase efflux of Cl- = more depolarization
- If threshold reached = AP transmitted to olfactory bulb via olfactory nerve
Collateral branches of the axons of the secondary olfactory neurons = Anterior olfactory nucleus (=collection of nerve cell bodies located along olfactory tract)
Postsynaptic fibers from this nucleus project to the contralateral olfactory bulb via the anterior commissure
Their influence is mainly inhibitory = enhance the more active bulb and provide directional cues to the source of the olfactory stimulation
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Projections:
- Prefrontal cortex: Neurons in piriform cortex, amygdala, entorhinal cortex
- Hippocampus: Neurons in entorhinal cortex via perforant fiber pathway
- Mediodorsal thalamic nucleus: Piriform cortex
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