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Physiology of Olfaction

M.Phanindra

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FOODGAS LEAKSFIRE

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Olfactory sensory neurons are located in olfactory epithelium in

the nasal mucosa.

Area:10 cm2

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It contains 50 million bipolar olfactory sensory neurons mixed with supporting(sustentacular)cells,basal stem cellsIt is the place where nervous system is closest to the external world.New sensory neurons are generated byBasal stem cells.Epithelium is covered by mucus secreted by Bowman glands and supporting cells .

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Each sensory neuron terminates in to a knob which contain CILIA.Cilia are unmyelinated processes that protrude in to mucus.Odorant molecules dissolve in the mucus and bind to receptors.Mucus provides appropriatr molecular and ionic environment for odour detection.

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The axons pass through cribriform plate and enter the olfactory bulb.Mitral cells , tufted cells ,meet the axons in bulb to form glomeruli.

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Periglomerular cells-inhibitory neurons which connect glomeruli .Granule cells-no axons ,reciprocal connections with mitral and tufted cells.Mitral and tufted cells secrete glutamate and granule cells secrete GABA.Epithelium also contains the trigeminal pain fibers .

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OLFACTORY CORTEX

The axons of the mitral and tufted cells pass through lateral olfactory stria to terminate on the dentrides of the olfactory cortex .Olfactory cortex contains :- anterior olfactory

nucleas ,olfactory tubercle, piriform cortex , amygdala ,entorhinal cortex.

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From olfactory cortex it goes the orbitofrontal complex which is generally greater on right side thus the representation of olfaction is asymmetric.

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VOMERONASAL ORGANPHERMONESREPRODUCTIVE AND INGESTIVE BEHAVIOUR Ex:-pregnancy block in rat

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There is evidence for existence of phermones in humans and there is a close relationship between smell and sexual function.Sense of smell is more acute in females more at the time of ovulation.

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HOW IT CAN DISCRIMINATE 10000 DIFFERENT ODOURS?

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There are approxiamately 500 olfactory genes which constitute 2%of human genomeThe amino acid sequences of receptors is very diverse ,but all are G-protein coupled receptors.The GPCR contains three sub units α,β,gamma.When a chemical attaches these dissociate and α activates adenylate cyclase to catalyze the production of cAMP .

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The result is inward directed calcium current which produces graded receptor potential.This opens the calcium activated chloride channels further depolarising the cell due to high intra cellular chloride levels.If the stimulus is sufficient for the receptor potential to exceed its threshold,an action potential is triggered.Each neuron projects to one/two glomeruli.Lateral inhibition – sharpens the olfactory signals

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ODOR DETECTION THRESHOLD

ODORANT BINDING PROTEINS

ADAPTATION

Long term adaptation:cGMPShort term adaptation:cAMP

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ABNORMALITIESANOSMIA

HYPEROSMIADYSOSMIA

TREATMENT:-Antibiotics,surgery,steroids.

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