• Some behaviour associated with olfaction

• Two olfactory subsystems – Main– Vomeronasal

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Olfaction

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Physiological and behavioural responses to odours.

• Visceral responses: Smell food--> salivation and gastric motility– Noxious smell-->gag

Physiological and behavioural responses to odours.

• Reproductive and endocrine functions– Women housed together synchronize

menstral cycles– Smelling gauze pads from underarms of

women also synchronizes menstral cycles.

Physiological and behavioural responses to odours.

• Infants recognize mothers by scent• Mothers can recognize the scent her

baby.

PheromonesSpecies specific odorants.

Some pheromones stimulate the vomeronasal organ

VNO--> accessory olfactory bulb-->hypothalamus.

(Found in 8% of human adults), VNO receptors are pseudogenes in humans.

Olfactory receptor (sensory) neuron

• In the olfactory epithelium• Have cilia projecting into the nasal

cavity mucus• These cells become damaged, and

turnover.

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Transduction

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Odorant Receptors

• Homologous to a large family of G protein coupled receptors.

• G proteins interact with the carboxyl terminal

• Membrane spanning regions differ.

Odorant Receptors

• The largest known gene family• Between 3% and 5% of all genes.• In humans, 60% of the odorant

receptors are not transcribed.

Odorant Receptors

• Have been expressed in olfactory sensory neurons with reporter proteins.

• Each olfactory sensory neuron expresses only one or at most a few odorant receptor genes.

• Different odors must activate a subset of olfactory sensory neurons.

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Olfactory Coding

• Each olfactory sensory neuron responds to a subset of odorants.

• Threshold values vary.• Number of ligands vary.

Olfactory Coding

• I7 receptor

• N-octanol (cut grass)

• The I7 olfactory receptors are spatially coded in the olfactory epithelium and in the olfactory bulb.

Olfactory Coding

• Temporal coding.– MAYBE information conveyed by timing.

– In insects (now also in fish) brain neurons sychronize responses. (Gilles Laurent)

Olfactory signals in the brain.

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Olfactory bulb

• Glomerular subsets receive input from olfactory sensory neurons that express distinct odorant receptor molecules.

• These glomeruli seem to be selective for odors.

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Mitral cell projections

• Mitral cell axons form the lateral olfactory tract.

• Projects to accessory olfactory nuclei, olfactory tubercule, entorhinal cortex, amygdala, pyriform cortex.

• Pyriform cortex axons project to thalamus, hippothalamus, hippocampus, amygdala.

Taste System

• Taste cells• Taste buds• Peripheral cells, a number of central

pathways.

Taste cells synapse onto primary sensory axons of:

• Cranial nerves:– VII (facial nerve branches)– IX (glossopharyngeal nerve branches)– X (vagus nerve branches)

Projections of taste neurons

• Cranial nerves VII, IX and X project to the solitary nucleus of the brainstem (gustatory nucleus)

• Topography of the cranial nerve input to the gustatory nucleus.

• Integration of visceral and gustatory input.

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Projections from the gustatory nucleus

• Thalamus--> cortex

• Hypothalamus (homeostasis), amygdala

Human taste perception

• Soluble in saliva• NaCl (electrolyte balance)• Glutamate (amino acids)• Sugars (glucose)• Acids (palatability)• Plant alkaloids (bitter, poison indicating)

Threshold concentrations

• NaCl, 2 mM

• Sucrose 10 mM

• Quinine 0.008 mM, strychnine 0.0001 mM

• Gustatory sensitivity decreases with age.

Human taste

• Response thresholds vary in different parts of the tongue.

• Taste sensations as well: fat, spicey, metallic, taste mixtures.

Sweet

• Saccharides - glucose, sucrose, fuctose, cAMP pathway

• Organic anions - saccharin

• Amino acids - aspartame, activate IP3 pathways

• People can discriminate these.

Peripheral organization

• Papillae– Fungiform– Circumvallate– Foliate QuickTime™ and a

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Taste bud

• Taste pore• Taste cells