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Why do we have a sense of taste?
•
protection
palatable or poisonous?
•
pleasure
taste and olfaction = flavour
•
communication
olfaction important, taste less so
Sensors (taste buds) located in
immediate vicinity of mouth
Tongue
Palate
Pharynx, oesophagus, epiglottis
Papillae on tongue bear taste buds
From Gray’s Anatomy
Usually 2000-5000 taste buds in
humans
LM section of taste buds
light, dark and basal cells
From UT Houston Medical School
Taste buds
structure
Orange/onion structure
cell types
EM appearance - four types of cells
cell lineage
3H thymidine studies for determining
cell “birthdays”
cell turnover
Very high
10-13 days most species
cell number
Originally thought to be low
Now thought to be 50-100 cells per taste
bud
Cell types in taste buds
Type I (dark cells) 55-75%
Extend base to apex of bud
Have dense granules in cytoplasm,
partciularly apically
Irregular nucleus
Rough endoplasmic reticulum
Long branched apical microvilli
Type III (intermediate cells) 515%
Extend base to apex of bud
Similar to Type II, with dense cored
vesicles in cytoplasm particularly at
base of cell
Synapses with afferent nerves
Type II (light cells) 20%
Type IV (dark cells)
Extend base to apex of bud
Do not reach taste pore
Large oval nucleus
Dense core vesicles
Smooth endoplasmic reticulum
Rough endoplasmic reticulum
Short apical microvilli
Precursor cell for other types?
interneuron? or mechanoreceptor?
How do we taste?
1. Transduction of taste stimuli by taste cells.
Taste buds found throughout oral
cavity, oropharynx and epiglottis.
Processing of taste stimuli not
simple - different tastes detected
by different mechanisms, and
processed in the taste bud before
afferent nerves are stimulated.
Actually at least 5 tastes (if not
more) and some are detected in
more than one way.
Two new tastes? Or more?
umami - “deliciousness”
glutamate acts on a mutant glutamate receptor
found in taste cells (found February 2000)
first identified as a taste in 1908
Fats
free fatty acids in mouth close K+ channel
specific fatty acid transporters directly increase
[Ca2+]I
“thermal” tastes (2000)
How do we taste?
2. Communication of taste signals to the nervous system.
Innervation of taste buds
Oropharynx, epiglottis - X
Posterior 1/3 - IX
Anterior 2/3 - VII (in chorda tympani)
Somatosensory afferents - V
Efferent innervation
Innervation of taste buds
Taste bud - 50-100 cells, 5-15
afferents
primary afferent
 Each afferent innervates 5-10
taste cells
papilla
papilla
Afferents may innervate multiple
taste buds, in multiple papillae.
How is the information decoded
into taste?
Taste bud
Taste cells
Taste cells
Complex signalling in taste transduction
Taste cells - don’t know specificity of taste transduction
Taste buds - single buds respond to more than one taste stimulus
Primary afferents - respond best to one taste but may respond to
all
NTS and thalamic neurons - also multiresponsive, but may be
more “tuned” to one stimulus
Tastes
1
2
3/4
1o afferent
30%
44%
26%
NTS
30%
44%
26%
Thalamus
40%
25%
35%
Cortex
48%
52%
Olfaction
Olfactory epithelium
specialised epithelium above turbinate cartilage
receptor cells:
bipolar neurons - unmyelinated axons
cilia form dense mat above cells
new cells generated every 60 days from basal cells
Odorant detection
molecules dissolve in mucus film
diffuse to cilia/bind to carrier protein
bind to specific receptors on olfactory neurons (cells may
respond to more than one odorant)
L-carvone = carraway
D-carvone = spearmint
specific anosmias
Odorant binds to receptor = cAMP generation leading to receptor
potential due to direct opening of Na+ channels
Ability to  cAMP directly related to ability to generate action
potentials
Olfaction
Specific stimuli activate specfic receptors
One cell = one stimulus = one response - probably not
Projection to higher centres directly to cortex, then to limbic regions and other cortical
areas