Olfactory Flashcards
summary of olfaction
neuroepithelium contains olfactory sensory neurons. sensory neurons contain and are responsible for receiving odour stimulation. there is a layer of mucus within olfactory system and within the mucus sit cilia and these contain receptor sites for odorant molecules.
The apical dendrite of sensory neurons projects through to nasal cavity and is capped by dendritic cilia projecting into specialised mucus in the cavity.
volatile odours (odorant molecules) dissolve into mucus layer and will contact receptors.
olfactory neuron axons are found at basement membrane of olfactory system and these axons leave epithelium and extend to olfactory bulb. the olfactory bulb receives electrical signals from sensory neurons.
the surface of OB contains zones called glomeruli. action potentials are produced that are transferred to mitral cells. mitral cells are a major output channel of olfactory bulb sending signals to various olfactory cortical structures.
stimuli called odorants interact with olfactory receptor neurons in an epithelial sheet called olfactory epithelium that lines interior of the nose.
axons arising from the receptor cells project directly to neurons in olfactory bulb which projects in turn to pyriform cortex in the temporal lobe. the olfactory system is unique as unlike other systems it does not entail a thalamic relay en route to primary cortical region that processes the sensory information. the olfactory tract projects to other targets in forebrain such as hypothalamus and amygdala.
where is olfactory system
the roof of the nasal cavity at the cribform plate - a perforated portion of the ethmoid bone separating frontal lobe of cerebrum from nasal cavity.
what is the cribform plate?
axons have to go through this part of the skull to access olfactory bulb. it is a bony structure with tiny holes separating from the brain. OSN axons pass through these holes to enter brain.
what does the olfactory receptor do
detects receptors and transduced into an electrical signal resulting in generation of an action potential.
binding of odorant molecules to this receptors causes signalling mechanism which will generate changes in electrical ions which generates membrane potential resulting in AP and AP travels down axons and within the glomerulus the transmitter will release glutamate.
describe signal transduction
olfactory receptors are GPCRs which activate adenyl cyclase type-III via G protein (Golf) which converts ATP to cAMP.
cAMP opens cation channel, causing sodium and calcium to enter into olfactory sensory neuron. entry of cations depolarises membrane, and olfactory receptor boosts signal further as the calcium that enters triggers Ca2+ activated Cl- channel.
intracellular chloride levels are v high due to the membrane ump NKCC1, so when calcium activates Cl channel, chloride leaves the ORN causing further depolarisation and provides amplification. AP generation in ORNs releases glutamate.
since intracellular Cl levels are v high they are not inhibitory like most neuronal Cl channels. thus the OSN maintains its own CL battery incase Na+ gradient in mucus is insufficient to to support a threshold current and use it to boost the response
what are odorant responses
each receptor responds to a family of chemically similar molecules.
an odour is encoded by specific combination of responding neurons.
describe olfactory bulb circuitry
glomeruli are the first processing station in the brain. they are spherical structures containing the incoming axons of the OSNs.
mitral cells receive odour information from a receptor/sensory neuron, refine the signal and amplify it from inputs from periglomerular cells and granule cells, then relay the message.
where are signals projected from olfactory bulb?
mitral cells are activated, impulses flow from olfactory bulb , the lateral olfactory tracts to the thalamus, hypothalamus, amygdala and other regions of the limbic system.
in thalamus, signal is sent to piriform lobe of olfactory cortex and part of frontal lobe where smells are consciously interpreted and identified.
when impulses are sent to hypothalamus, the amygdala and other parts of the limbic system. this is where the smell innervates and emotional responses. smells that are associated with danger, trigger the sympathetic fight or flight response.
how do sensory neurons transmit and detect signals
each sensory neuron expresses only a single receptor.
cells expressing the same receptor converge on only 1 or few glomeruli.
each mitral cell innervates a single glomerulus via its apical dendrites.
cells for detecting odour are dispersed in epithelium and all detection of the odour is gathered and summated into specific cluster of olfactory bulb neurons.
what is monoallelic expression of OR genes
expression of a gene from single allele against background of a diploid heterozygous genome.
each OSN chooses, for expression, one functional OR gene in a monoallelic manner.
organising sensory info- generated by ORs is challenging, so monoallelic expression overcomes this.
the single OR expressed by each OSN determines the neurons odour sensitivity and the axonal connections it will make to downstream neurons in olfactory bulb.
what is the locus control region?
its a a 2kb was sufficient to allow expression of OR genes on transgenic chromosomes
how can YAC be manipulated to identify expression determinants in the promoter?
YAC has its own centromere and origins of replication so large fragments of DNA may be manipulated and propagated.
YAC contains MOR 28 locus which can be manipulated to contain IRES-tau-lacZ after it.
what is the current model for OR gene expression?
stochastic selection of an OR by the LCR.
describe anatomical features of olfaction system.
cells expressing the same receptor converge on on only 1 or 2 glomeruli.
each mitral cell innervates a single glomerulus via its apical dendrites; the axons project out of the bulb.
therefore, cells for detecting an odour are dispersed in the epithelium, and all detection of the odour is gathered and summated into a specific cluster of olfactory bulb neurons
why is the shape of an OR relevant to the odorant?
like lock and key fit but may extend to multiple receptors. many similar shaped odorants can fit and activate the same OR, but result in different timing and amplitude.
