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Chapter 16
The Special Senses
Smell and Taste
Gustation
• Taste receptors
• Clustered in taste buds
• Associated with lingual papillae
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Taste buds in lingual papilla
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• Fungiform
• small, ~5 taste buds
• most numerous
• tongue and throughout oral cavity
• Foliate
• Leaf-shaped. In folds on the sides of the tongue.
• Contain most sensitive taste buds. Decrease in number
with age.
• Vallate (circumvallate) - large, ~100 taste buds form V
shape pattern
Filiform papillae
• Filiform papillae
• most numerous papillae
• do not contain taste buds
• give tongue rough surface
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Taste buds
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• Contain basal cells which appear to be stem cells
• Gustatory cells (epithelial cells) extend specialized
microvilli (taste hairs) through a narrow taste pore
Gustatory Reception (also see Saladin fig 16.6)
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Figure 15.23
Gustatory discrimination
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• Primary taste sensations
• Sweet, sour, salty, bitter, umani
• Receptors also exist for umami (amino acids) and
water?
• Bitter> acid>> salt or sweet
• Taste sensitivity shows significant individual
differences, some of which are inherited e.g.
phenylthiourea
• Interaction with smell
Taste receptor activation
salt
acid
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sweet, bitter, umami
Gustatory pathways
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• Taste buds are monitored by cranial nerves VII, IX, X
(first order neurons)
• Synapse within the solitary nucleus of the medulla
oblongata (second order neurons)
• Then on to the thalamus via the medial lemniscus
pathway and the primary sensory cortex
• Parallel pathways go to the hypothlamus and other
brain regions and activate reflex responses (salivation,
stomach churning, etc)
Olfactory organs
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• Olfactory epithelium
• Olfactory receptor cells
• are modified bipolar neurons
• contain specialized cilia (olfactory hairs)
• Olfactory receptor cell population shows
considerable turnover
• Each cell only has one type of receptor (binds only
one type of odorant)
• basal cells
• supporting cells,
• Surfaces are coated with secretions from olfactory
glands
Olfaction
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• Olfactory reception involves detecting dissolved
chemicals as they interact with odorant binding proteins
• Can distinguish >10,000 of chemical stimuli
• Thousands of receptors in most mammals!!!!
Humans appear to have only ~350 different receptors
• CNS interprets smells by pattern of receptor activity
• Little receptor adaptation, but central adaptation occurs
• Granule cells can inhibit signals going to the
olfactory centers
Olfactory pathways
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• Do not synapse in the thalamus first (although some collateral
axons go to the thalamus)
• Olfactory bulbs are extensions of the cerebrum
• mitral cells and tufted cells synapse with bipolar
cells in glomeruli
• each glomerulus receives one type of signal
• Olfactory tracts mainly go first to the temporal lobe
(primary olfactory center)
• Collaterals go to the limbic system
• Frontal lobe allows the discrimination of different
odors and integrates smell and taste
The Olfactory Organs
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Receptor activation
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Figure 15.22