How is olfaction initially detected?
- epithelium in the nose is made up of receptors and supporting with cells with cilia at the surface
- air or water carries odourants over the epithelium
- proteins in the cilia catch odour molecules
- OSNs synapse directly to brain
What mapping is seen in olfactory processing?
- olfactory epithelium: organized into 4 zones by type of receptor
- main olfactory bulb: OSNs converge here in similar zones
- neighbouring areas in the MOB process similar odourants based on chemical structure
What is the general odourant pathway?
- odourant
- odourant binding protein
- main olfactory epithelium
- main olfactory bulb
- olfactory tract
- cortex (elaborate)
Where does the olfactory tract project to in the cortex?
- anterior olfactory nucleus - contralateral olfactory bulb
- lateral amygdala - hypothalamus
- entorhinal cortex - hippocampus
- piriform cortex - thalamus - orbitofrontal
- piriform cortex - frontal cortices
What is the general pheromone pathway?
- pheromone
- pheromone binding protein
- vomeronasal organ (basal and apical)
- accessory olfactory bulb (posterior and anterior)
- medial amygdala
- hypothalamus
What are the types of projection neurons? Local inhibitory neuron?
- mitral and tufted
- local inhibitory neuron called granule cells
What are neurotransmitters are used for specificity?
- GABA is released from granule cells to inhibit mitral cells
- glutamate is released from mitral cells to excite granule cells
What chemical characteristics are mitral cells sensitive to? What does this mean?
- position the molecule is in (ie. fire for para position no matter what chain length)
- chain length (ie. fire for short chain length no matter what position)
- different glomeruli detect different molecular features
What is the mechanism for plumes?
- sometimes simultaneous input is needed from two different glomeruli/mitral cells to have activation in the olfactory cortex
- input from one or the other is not sufficient
How is the accessory olfactory bulb organized/mapped?
- vomeronasal has two sections of epithelium (apical sensory neurons and basal sensory neurons)
- apical project to the anterior AOB
- basal project to the posterior AOB
- two zones in AOB
Which areas of the brain are important for what kind of processes?
- lateral amygdala: social behaviour and emotions
- hypothalamus: homeostasis and reproduction
- hippocampus: memory
- piriform cortex: identification of odours
- neocortex: conscious discrimination of odours
Where is topography present and not present in the olfactory pathway?
- topography apparent in epithelium and glomeruli of main olfactory bulb
- not much topography in cortical areas:
- anterior olfactory nucleus: preserves dorsal-ventral topograpy
- cortical amygdala: overlapping terminal fields
- piriform cortex: none
What possible role does vasopresson play?
- vasopresson neurons found in main and accessory olfactory bulbs and anterior olfactory nucleus
- vasopression is important for social recognition and complex social behaviours
- vasopressin neurons may filter out social odours
- a possible mechanism: release of primed vasopressin causes inhibition of firing for neurons representing familiar odour
What process occurs to inhibit male mating behaviour in mice?
- young mice produce exocrine-gland secreting peptide 22 (ESP22)
- secrete from lacrimal gland and tears
- activates high-affinity receptors in vomeronasal organ and downstream limbic neurons in the medial amygdala
- exerts powerful inhibitory affects on males
What is the cellular process that results from odourants?
- odourant
- odourant binding protein
- olfactory receptor
- activates g protein
- activates adenylyl cyclase
- ATP to cAMP
- binds to cation channels (CNGCC)
- influx of Ca and Na
- Ca gated chloride channel
- efflux of chloride
- membrane depolarized
What only happens in the cellular process for OSNs?
- usually we would expect to see a chloride influc because there is more chloride on outside of the membrane
- however here there is a chloride efflux that strengthens depolarization
What studies have been conducted surrounding these chloride channels?
- Pezier at al.: looked at affect of blocking the chloride channels and decreasing the external chloride concentration which caused a delay; concluded that they are used for repolarization
- Billig et al: chloride channels were identified as Anoctamin2 in the MOE and VNO, disruption in mice did not impact them greatly
What are necklace glomeruli?
- receive pheromone input from the main olfactory epithilium <1%
What is the cellular process for pheromones that bind in the main olfactory epithilium?
- pheromone
- receptor
- activates g protein
- activates guanylyl cyclase
- increase in cGMP
- cGMP gated cation channel
- Ca and Na influx
- Ca-gated chloride channel
- chloride efflux
What is the cellular process of pheromones?
- pheromone
- receptor
- activate g protein
- activate phospholipase C
- produces IP3 and DAG
- both activate cation channels
- Ca influx (possible Cl efflux)
Why is androstenone perceived differently?
- Vosshall et al.
- may act as pheromone in people
- is a break down product of testosterone and acts as pheromone in other animals
- three variants of receptor OR7D4 which responds to androstenone
- most common: sickening scent
- second: sweet smell
- third: unable to detect
What evidence has been found for vomeronasal organs in humans?
- TRPC2 receptor in some primates (replaced by trichromatic vision except in one primate with both)
- pheromone receptor gene V1rl1 found in olfactory mucus
- have looked at cadivers but no evidence
How is specificity achieved?
- relatively unknown
- odourant can bind to multiple receptors based on different characteristics (shape and ‘colour’) producing an olfactory code or reaction pattern
- characteristic pattern of hydrogen bonds between odourant and receptors
- vibrational mode (electron transport from donor to acceptor) may also play a role in recognition
