SMELL Flashcards
Smell basics
Our olfactory system detects volatile substances in our environment to:
– Identify food (pleasant smells)
– Warn of poisons (unpleasant smells)
– Communicate? (pheromones)
Estimated that we can detect several hundred thousand chemicals:
– About 20% pleasant smells
– Majority unpleasant smells
Sniffing draws air through nasal passages to sample odorants
– Odorants are substances with a smell
– Sniffing faster allows more rapid stimulus acquisition
Olfactory epithelium at roof of nasal cavity:
– Contains olfactory receptor cells which detect odorants
Size of olfactory epithelium indicates animal’s olfactory acuity
– Human (weak smeller): surface area about 10cm2
– Dog: surface area may be over 170cm2
Olfactory epithelium
Olfactory receptor cells develop and die every 4-8 weeks. Thus, theses cells need to be replaced regularly.
Olfactory epithelium contains olfactory receptor cells:
– Cilia of olfactory receptor cells immersed in mucus
– Odorants dissolve in mucus and contact cilia
– Axons of olfactory receptor cells cross the bony cribriform plate towards the brain
Olfactory epithelium also contains supporting cells and basal cells:
– Supporting cells are like glia and help generate mucus among other functions
– Basal cells are the source of new olfactory receptor cells
Olfactory receptor cell
Severing the olfactory nerve (e.g., after head trauma) results in anosmia – inability to smell.
Odorants bind to membrane receptor proteins → Stimulate G-protein (Golf) → Activate adenylyl cyclase →
Form cAMP → cAMP binds to cAMP-gated cation channels → Open cation channels → Na+ and Ca2+ influx →
Open Ca2+-activated Cl- channels → Cl- efflux → Current flow depolarizes membrane (receptor potential)
Cell has one dendrite which ends in a knob
Axons from all olfactory receptor cells form olfactory nerve (cranial nerve I)
Odorant-evoked responses from olfactory receptor cell
Receptor potential propagates along dendrite and triggers action potentials in cell body
Response adapts usually in about one minute, even if odorant is still present
Olfactory receptor proteins
Olfactory receptor proteins on receptor cells bind odorants
– Many different proteins allow detection of many odorants
About 350 genes in humans code for receptor proteins
– Olfactory receptor genes comprise 3-5% of mammalian genome
– Each of the receptor genes has unique structure giving rise to
receptor proteins that bind different odorants
Each receptor cell expresses just a few receptor genes
– Majority of olfactory receptor cells express just one receptor gene
Many different receptor cells identified by expressed gene
– Greater than 1,000 types of receptor cells in mice
Olfactory epithelium zoned based on expressed genes
– Each zone contains receptor cells expressing a different group of receptor genes
Broad tuning of individual olfactory receptor cells
Broad tuning means cell responds to multiple different stimuli
Each receptor cell expresses single receptor protein
– Receptor cell types scattered within region of epithelium
Each receptor cell responds to multiple odorants
– But the odorant preferences differ between receptor cells
Differentiate odorants based on activity of many cells
– i.e., population coding overcomes broad tuning
– e.g., Receptor 1 responds to citrus, floral and peppermint
– Receptor 3 responds to floral, peppermint and almond
– So if Receptor 1 active but not 3, then likely citrus odor
Odorant concentration coding in olfactory receptor cells
Increase concentration, increase response
– Current flow in cilia increases with concentration
– # of action potentials increases with concentration
– Although response plateaus at high concentration
Olfactory bulb
Input layer of bulb contains glomeruli
– In mice: 2,000 glomeruli, each 50-200μm across
In each glomerulus, around 25,000 axons from receptor cells converge on dendrites of about 100 second-order (mitral) cells
ORCs expressing particular receptor gene send axons to same glomeruli
ORC = olfactory receptor cell
E.g., axons from ORCs expressing receptor gene called P2 converge on only two glomeruli in the olfactory bulb
Each glomerulus receives input from ORCs expressing particular gene
Olfactory bulb contains orderly map of receptor genes expressed in olfactory epithelium
Different odorants evoke different neural activation patterns in the bulb
Activity of many cells in olfactory bulb simultaneously recorded using optical imaging (cells expressed fluorescent protein sensitive to intracellular calcium, so changes in neural activity changed light emitted)
Odorant concentration coding in olfactory bulb
Different receptors show different odorant tuning. As concentration of particular odorant increases, large # of receptors, then glomeruli, are activated. This spatial expansion of activated glomeruli may signal concentration.
Increase concentration,
increase # of action potentials
Spatial and temporal coding of odorant concentration in olfactory bulb
Shorter latency of response (action potential occurs sooner after sniff) with increasing concentration
ET, external tufted; GC, granule cell; MC, mitral cell; ON, olfactory nerve; PG, periglomerular; SA, short axon
Bypass thalamus? Where is primary olfactory cortex?
Olfactory bulb projects via olfactory tract to several cortical areas
– Main target is piriform cortex (bypasses thalamus)
– Common to refer to these cortical targets as primary olfactory cortex
Consider that olfactory bulb represents odorant features
– But other primary sensory cortical areas (e.g., V1) have representations of stimulus features?
– And so-called primary olfactory cortex integrates olfactory with behavioral and contextual information
Different odorants evoke different neural activation patterns in cortex
Activity of many cells in olfactory cortex simultaneously recorded using optical imaging (cells expressed fluorescent protein sensitive to intracellular calcium, so changes in neural activity changed light emitted).
