Lecture 22 - Smell - Dr Peter Brennan PDF

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Dr Peter Brennan ...

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Smell ! Why smell? In humans, the sense of smell went into evolutionary decline as speech and vision became more important. For other mammals, a good sense of smell is essential for detecting predators, prey and food, as well as for navigation and communication.! Odours Odourants are the chemicals that cause a particular smell to be perceived, most natural odours are a complex mix of many different odourants. Olfactory adaptation means response to a constant stimulus is reduced over time, this helps distinguish new odours from constant background odours. ! Olfactory systems The olfactory mucosa contains a layer of columnar epithelial cells surrounding bipolar olfactory neurones that sit on cilia. Basal cells near the lamina propria undergo differentiation and replace these neurones every 5-8 weeks. Theses neurones have an axon which projects straight up to the brain and a dendrite which projects towards the apical mucosa and ends in many cilia. Each cilia contains many olfactory receptors in their membrane, to which odourants bind.! Olfaction is the capacity of smelling, there are two types of olfaction:! - Orthonasal olfaction - odour enters the nose in inspired air and activates receptor cells in the olfactory epithelium of the nasal canal - involved in detecting smells in the world around us ! - Retronasal olfaction - odour enters nose in expired air and activates receptor cells in the olfactory epithelium of the nasal canal - involved in favour perception. Flavour perception is influences by taste receptors, the retronasal system, the motor cortex and visual inputs !

Orthonasal olfactory perceptual system

Odour in inspired air

Retronasal olfactory flavour system

Odour in expired air

Olfactory receptors Olfactory receptors are 7 transmembrane spanning G protein coupled receptors. The olfactory receptor gene family is highly diverse and contains many variable residues, this means different olfactory receptors will bind different odourants, allowing discrimination of different smells. Olfactory sensory neurones choose at random which gene to express and the rest are silenced, however they are not highly selective for individual odourants so will respond to a range of smells. Each type of receptor response to different ranges of odourants, so the pattern of activity in a group of receptors is used to distinguish the exact smell. There aren’t enough types of olfactory receptor to response to each smell individually, this use of pattern recognition means thousands of different odourants can be distinguished. ! Propagation of action potentials 1. 2. 3. 4. 5.

An odourant binds to an olfactory receptor, activating the G protein it is coupled to ! The G protein’s beta-gamma subunit dissociates from the alpha subunit ! The alpha subunit activates adenylyl cyclase ! cAMP levels increase ! Sodium and calcium ion channels are opened !

6. Sodium and calcium ions flow into the cell and a small amount of potassium ions flow out of the cell - causing a net influx of positive charge ! 7. Calcium ions activate and open chloride channels ! 8. Chloride ions move out of the cell, adding to the cell’s depolarisation ! 9. If this depolarisation is sufficient to produce an action potential, it will be sent down the olfactory nerve axon to the olfactory bulb! !

The olfactory bulb The olfactory bulb is a complex neuronal structure found in the forebrain. Axons from the olfactory nerve synapse on the mitrial and tufted cells at glomeruli (bundles of synapses), both of which project out of the olfactory bulb as the olfactory tract. Each glomerulus in the olfactory build receives input from olfactory sensory neurones expressing a single type of olfactory receptor. These glomerular patterns can be seen by applying a calcium sensitive dye that forms bright areas the calcium is released, this shows that each glomerulus responds to a range of different odours, but the pattern of glomeruli activated is specific to each odour. ! Inhibitory interneurones in the olfactory bulb

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The olfactory bulb contains two types of inhibitory interneurones that regulate projections form mitrial and tufted cells:! Granule cells - contain no axons, they for dendro-dendritic synaptic contacts with mitral cells, meaning both sides of the synapse are dendrites capable of releasing neurotransmitter. These are involved in lateral inhibition, where excited mitrial cells release glutamate onto granule cells, this excites the granule cells causing them to release GABA onto mitrial cells. GABA is an inhibitory neurotransmitter that causes mitrial cells to inhibit themselves (auto-inhibition) and neighbouring mitrial cells (lateral inhibition). This inhibition means the pattern of glomeruli activation is sharpened before being sent to higher structures! Periglomerular cells - also have inhibitory synapses on mitrial and tufted cells and are involved in lateral inhibition. !

Projections from the man olfactory bulb The piriform cortex is an olfactory structure found in the cerebrum, it contains pyramidal cells which each receive input from 200 mitral cells. While mitrial cells respond to different odourant features, pyramidal cells respond to a combination of inputs to recognise the mixture of smells as a whole. The olfactory tract also synapses into the limbic system which projects to the hypothalamus, this is important in the behavioural effects of olfaction....