land
- volatile hydrophobic molecules/compounds
aquatic
- water soluble molecules
amphibians
- deal with both volatile hydrophobic compounds AND water soluble molecules
olfaction
sampling the chemical composition in an environment around us
odorants
volatile, lipophylic organic compounds
- elicite an olfactory response
odours
combination of multiple odorants
how does nasal perception begin
- odour molecules enter nasal caviity and interact with nasal (olfactory) epithelium
olfactory epithelium
main site of odour detection
- olfactory sensory neurons in epithelium
- fire action potentials in response to application of odourants
- dendrites into epithelium
- epithelium is mucus filled
- cilia on dendrites in mucus
- axons of receptors converge
- axons go to brain (olfactory bulb)
why are cilia important
- house olfactory receptors
- odurant attributes transduced into a signal that can go to brain
basal (stem) cells
supporting cell in olfactory epithelium
- role is to reproduce olfactory sensory neurons
life span of olfactory sensory neurons
7 months - 1 yr
supporting cells in epithelium / sustentacular cells
- similar to glia
- offer support to neurons and help metabolize environment surrounding
lateral nasal gland
- odourant is hydrophobic but OSN live in mucus of nasal epithelium, which is aqueous, so we have proteins (OBP) that live in mucus of nasal cavity
Odourant binding proteins
- bind to odurant in the air and take to mucus to present to receptors
degrading enzymes
- break down bound odourants so we aren’t always smelling the same thing / replenishes the pool of odourants
olfaction and GPCRs - initiation of signal
- Golf (hetero dimeric G protein)
- when odourant binds, GDP->GTP
- Golf interact with adenylate cyclase
- AC turns ATP->cAMP
- cAMP activates CNGC, depolarises cells
- Ca also binds to CaCC channels to efflux
CNGC
cyclic nucleotide gated channel
- influx of cation (Na and Ca)
- depolarises neuron
CaCC - anoctamine 2 channel
Calcium gated chloride channel
- efflux of chloride amplifies signal
stopping of signal cascade, calcium calmodulin
- Ca binds to calmodulin, inhibiting CNG channels and
- (1) Ca binds to Phosphodiesterase , activating it, cAMP->ATP, reducing
- (2) also binds to CaMKII directly inhibits AC
- 1 + 2 both reduce cAMP
-important in detecting new odours
olfactory projection in the brain
- nasal epithelium and olfactory sensory neurons
- neurons cross skull and synapse at olfactory bulb
- form olfactory nerve
- via olfactory tract to olfactory cortex
- signals to hippocampus, amygdala, thalamus (frontal cortex)
olfactory cortex
(paleocortex)
odorant receptor proteins
- GPCRs recognise odourant based on amino acid sequence and shape
- each receptor can recognise multiple odorants
- each odorant can activate more than one receptor
disriminating odours
involves assessing combined activity across all receptors
transgenic Lac-Z reporter mice
- take original coding sequence and geneticall modify to form Lac Z coding sequence
