Transcript Chapter 08: The Chemical Senses

CHEMICAL SENSES
TASTE
Slide 1
Neuroscience: Exploring the Brain, 3rd Ed, Bear, Connors, and Paradiso Copyright © 2007 Lippincott Williams & Wilkins
Introduction
Animals depend on the chemical senses to identify
i) nourishment, ii) noxious substance, and iii)
potential mate
Chemical sensation
Oldest and most common sensory system
Essential to monitor changing environment both
internally and externally
Chemical senses
Gustation - Mostly for sensing outside environment,
reporting to our basic instincts such as thirst,
Olfaction hunger, emotion, sex
Chemoreceptors - internal probes (gut, muscle, blood..)
Slide 2
Taste
Inborn vs. Learned preference
The Basics Tastes
Saltiness, sourness, sweetness, bitterness, and umami
Simple or complex relationship between chemicals and
taste
Sour-acids, Salty-salts
Sweet
sugars like fructose and sucrose
Proteins such as monellin
artificial sweeteners (saccharin and aspartame)
Bitter
ions like K+ and Mg2+, quinine, and caffeine
Mostly perceived at a very low concentrations :
survival advantage (many poisonous substances are
often bitter)
Slide 3
Taste
The Basic tastes and Unique Tastes
A different combination of basic taste
Distinctive flavor (taste + smell)
Other sensory modalities (texture,
temperature, pain…)
Slide 4
Taste
The Organs of Taste
Tongue, palate, pharynx, and epiglottis
Slide 5
Taste
The Organs of Taste
Areas of sensitivity on the tongue
Tip of the tongue
Sweetness
Back of the tongue
Bitterness
Sides of tongues
Saltiness and sourness
Most of the tongue are sensitive to all basic tastes
Slide 6
Taste
•One to several hundred
taste buds/papilla
•50-150 taste receptor cells
/ taste bud
• 2000-5000 taste buds /
person
• Taste tuning (specificity)
at papilla level is present
but not absolute
exclusiveness (at higher
concentrations of taste
stimuli)
Slide 7
Taste
Tastes Receptor Cells
Apical ends’ membrane extensions :
Microvilli project to taste pore
Not neurons by standard histological
criteria but do form synapses to gustatory
ganglion neurons (as well as to basal cells)
Receptor potential: Voltage shift occur
when taste cells are activated by
chemicals
Slide 8
Taste
Taste Receptor Cells
- More than 90% of
receptor cells respond to
two or more of the basic
tastes
Slide 9
Taste
Mechanisms of Taste Transduction
Transduction
Process by which an environmental
stimulus causes an electriacl response in
a sensory receptor cell
Taste stimuli (tastants)
Pass directly through ion channels
(salt and sour)
Bind to and block ion channels (sour)
Bind to G-protein-coupled receptors
(bitter, sweet, umami)
Slide 10
Taste
Mechanisms of Taste Transduction
Saltiness
Salt-sensitive taste cells have a
special Na+ selective channel
• Blocked by the drug amiloride
• Open all the time
• Requires the concentration to be
quite high (at least 10mM)
Anions of salts affect saltiness - anion
inhibition (the larger anions inhibit
the salt taste of cations)
Some anions could produce their own
taste
Slide 11
Taste
Mechanisms of Taste Transduction
Sourness
Sourness- acidity – low pH
Protons causative agents of acidity
and sourness
Amiloride-sensitive sodium
channels allow the influx of protons
- inward current produce
membrane depolarization
Hydrogen ions bind and block K+ selective channels - depolarization
Slide 12
Taste
Mechanisms of Taste Transduction
Families of taste receptor
genes - TIR and T2R discoveries
shone a light into the study of
bitterness, sweetness and
umami
GPCRs- seem to use the same
secondary messenger pathway
Slide 13
Taste
Mechanisms of Taste Transduction
Bitterness
~30 T2R receptors ;
vast array of bitterness
detection = for safer life
Distinguishing among
bitterness is hard
because many (most)
T2Rs are expressed in
the same cell
Slide 14
Taste
Mechanisms of Taste Transduction
Sweetness
Sweet tastants natural and
artificial
Sweet receptors
T1R2+T1R3
Expressed in different
taste cells from bitter
receptors
Slide 15
Taste
Mechanisms of Taste
Transduction
Umami
Umami receptors:
Detect amino acids
(monosodium
glutamate; MSG)
T1R1+T1R3
Slide 16
Taste
Central Taste Pathways
VII; facial nerve
IX; glossopharyngeal nerve
X; vagus nerve
Mostly ipsilateral!
Slide 17
Taste
Central Taste Pathways
Localized lesions to VPM or gustatory cortex
Ageusia- the loss of taste perception
Conscious experience of taste
Gustation and behavior
Important to the control of feeding and
digestion – involve additional taste pathways
Intramedullar projections : swallowing,
salivation, gagging, vomiting…
Hypothalamus
Motivation to eat
Basal telencephalon
Slide 18
Taste
The Neural Coding of Taste
Labeled line hypothesis
Individual taste receptor cells for each
stimuli
In reality, neurons broadly tuned
Multiple transduction systems in a
receptor cell
Convergence of receptor cell inputs
onto afferent axons
Population coding
Roughly labeled lines
Activation of different sets of neurons
encode different taste
Slide 19