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B.3: Development of the Olfactory System (Neurogenesis in the OE…
B.3: Development of the Olfactory System
IMPORTANCE OF EARLY OLFACTION: Most important sense to be ready from birth in mammals.
Babies can't see/hear/touch very well - need to be able to smell.
Important for some species - e.g. rabbits, only breastfeed for 5mins a day - therefore strong pheromone signal to suckle.
Mice are similar - paper from Logan et al, 2012. Cues come from time in the womb.
= important behavioural consequences from birth - need a working system ready to go.
EARLY STAGE OF OLFACTORY PROCESSING:
Olfactory epithelium - external signals become electrical signal to send to brain. Project from top of nose to top of bone - axons go through the cribiform plate to olfactory bulb.
O.B. = under frontal lobes and small in humans; bulky in mice/rats - ease of access
Smells and relationship, obv, to romance etc - but aggression??
Insect olfaction is fairly similar to mammals
Rodents have accessory olfactory system in form of social cues. Has disappeared in our evolution.
Most plastic sensory system; most capable of regeneration in brain - development is lifelong!
Neurogenesis in the OE
Key plays in Olf epith = OSNs - do everything in one cell: detect odour - turn into electrical signal - sends to brain. Only sense to do this.
Dendrite linked to surface of epithelium.
Exposed to external environment - dangerous as exposure to toxins/viruses/pathogens etc. SO - ongoing process of neurogenesis which replaces all cells on a three monthly basis. This process of ongoing neuroGen = same as when first developing.
GBC (globo-basal cells??) - stem cells in OE
HBC (horizontal-basal cells) - under emergency conditions = can produce more GBC and repopulate in the OE. -- emergency backstop.
OLFACTORY RECEPTORS
Sequencing RNA of olfactory receptors: paper = Saraiva et al, 2015.
Initial steps in OSN axon guidance: epith and bulb dvp at same time in early development.
In mouse E13-17 = axon relation between OE and OB.
Axon guidance in OE - gradient from dorsal to ventral. In brain, are repelled by slit in brain and OB connects just with OB in dorsal. Later, ventral axons push more to ventral side. = 1 dimension from d to v.
Medial lateral mapping works in same way with different molecules.
Olfactory Receptor molecule go out to dendrites and wave around in nose - but also stimulate axon guidance in nose mm away.
Anterior posterior third dimension = based on cyclicAMP levels. Not dependent on them binding to...didn't hear!
Neuronal activity in glomerular refinement
Blocking sensory imput in one nostril. Projection is ipsolateral in nose (same side of nose to same side of bulb). (Mice can't breathe through mouth).
Critical period in first Post natal month.
Critical period for navigator axons - in first few weeks - and then disappear.
DVP of OLFACTORY BULB
Mitral cells glomerular refinement - no role for sensory activity.
Paper looked at occurrence in mice that can't smell - spontaneous activity is key. Depends on how active the cells are.
Sometimes systems develop before things are needed to work.
OB cicuitry
Many more local inhibitory cells in OB than there are excitatory neurons. Flipped in comparison to cerebral cortex.
Happens throughout life in OB (unlike neurogenesis in cortex). Can produce new GABAnergic neurons in OB throughout adult life.
By continually generating new immature cells - more plastic - and keep chucking in plastic cells that allows for more change in the system than the older cells that have been there all along.
Newborn, immature cells = have different properties than older cells. Known to be more plastic.