I - Olfactory Flashcards
Describe the olfactory epithelium
The olfactory epithelium consists of three cell types: supporting cells, basal cells, and olfactory receptor cells.
Supporting cells - columnar epithelial cells lined with microvilli at their mucosal border and filled with secretory granules.
Basal cells - base of the olfactory epithelium and are undifferentiated stem (mitotic).
Olfactory receptor cells - primary afferent neurons, are the site of odorant binding, detection, and transduction.
Free endings of man
Describe olfactory transduction
Involves the conversion of a chemical signal into an electrical signal that can be transmitted to the CNS
1. Odorant molecules bind to specific olfactory receptor proteins located on the cilia of olfactory receptor cells.
2. Bound odorant activates G protein which activates adenylyl cyclase.
3. Adenylyl cyclase catalyzes the conversion of ATP to cAMP. Intracellular levels of cAMP increase, which opens cation channels in the cell membrane of the olfactory receptor that are permeable to Na+, K+,and Ca2+.
4. The receptor cell membrane depolarizes, and action potentials are then generated and propagated along the olfactory nerve axons toward the olfactory bulb.
Outline the olfactory pathway
Axons from the receptor cells leave the olfactory epithelium, pass through the cribriform plate, and synapse on apical dendrites of mitral cells (the second-order neurons) in the olfactory bulb.
These synapses occur in clusters called glomeruli.
The granule and periglomerular cells are inhibitory interneurons that make dendrodendritic synapses on neighboring mitral cells that offer lateral inhibition.
Mitral cells of the olfactory bulb project to higher centers in the CNS:
The lateral olfactory tract synapses in the primary olfactory cortex, which includes the prepiriform cortex.
The medial olfactory tract projects to the anterior commissure and the contralateral olfactory bulb.
Explain the adaptation of odor perception?
Decrease and cessation of odor perception after continuous exposure to a specific odor. This is may be due to:
Calcium-binding protein (calcium/calmodulin) that binds to the receptor channel protein to lower its affinity for cyclic nucleotides.
Short-term adaptation - which occurs in response to cAMP and implicates a feedback pathway involving calcium/calmodulin-dependent protein kinase II acting on adenylyl cyclase.
Long-term adaptation - which includes activation of guanylate cyclase and cGMP production. A Na+/Ca2+ exchanger to restore ion balance also contributes to long-term adaptation.
Describe the pathologies associated with the olfactory nerve
Congenital anosmia: Autosomal dominant condition characterized by lifelong loss of the sense of smell, minus any other conditions. When accompanied by hypogonadotropic hypogonadism it is referred to as Kallman syndrome.
Tumors: Tumors can affect the olfactory bulb and tract via compression and mass effect. E.g., meningioma (most common), esthesioneuroblastoma, primitive neuroectodermal tumors (rare, exclusive to pediatric population).
Post-traumatic anosmia: Post-traumatic olfactory dysfunction can occur via these mechanisms; sinus or nasal tract disruption, shearing of olfactory nerve fibers at the cribriform plate and localized hemorrhage/contusion within the olfactory bulb or primary olfactory cortex.
Hyperosmia: A heightened or increased sense of smell. Can occur in pregnancy, hormone changes, or certain medical conditions
Alzheimer dementia: The olfactory deficits observed in patients with AD are similar in severity to the ones observed in IPD.
Huntington disease: Associated with moderate hyposmia.
Other neurodegenerative diseases: Friedreich ataxia and spinocerebellar ataxia types 2 and 3 also show impaired olfactory function.
Motor neuron diseases: Hyposmia has been described in Creutzfeldt-Jakob disease. Data on prion diseases is insufficient (note that prion protein has a role in olfactory processing).
