Download Olfaction

• 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.
Pheromones
Species 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
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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
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Taste bud
• Taste pore
• Taste cells