Download Topic 9

Biology 463 - Neurobiology
Topic 9
The Tongue and Nasal
Cavity: Sites for Chemical
Sensation
Lange
Taste
Introduction
Animals depend on the chemical senses to identify nourishment, poison,
potential mate
• Chemical sensation
– Oldest and most common sensory system
• Chemical senses
– Gustation
– Olfaction
– Other chemoreceptors
Be especially aware of
the overlap that exists
between sensitivities
of different receptor
axons of different
primary stimuli.
Taste
The Basics Tastes
– Saltiness, sourness, sweetness, bitterness, umami/savory, and fat/savory
– Examples of correspondence between chemistry
• Sweet—sugars like fructose, sucrose, artificial sweeteners (saccharin
and aspartame)
• Bitter—ions like K+ and Mg2+, quinine, and caffeine
– Advantage – Survival
• Poisonous substances - often bitter
Steps in the detection of a sweet stimulus:
1. A G-protein coupled receptor for a sweet flavorant
(such as sucrose) is in the membrane of this taste
bud.
2. When a sweet flavorant stimulates the receptor,
cAMP is synthesized by the activation of Adenyl
cyclase.
3. The cAMP will activate Protein Kinase A which will
lead to the blockage of potassium.
4. This blockage will lead to a deplorization of the
region of the taste bud causing an influx of calcium
ions which will stimulate the release of a
neurotransmitter stimulating the adjacent neuron.
5. It is known that there are at least two different sweet
receptor types… T1R2 and T1R3 which are
expressed in different cells.
Currently no one definitive neurotransmitter that is
released from the taste bud, but there is strong
evidence for serotonin (see Huang et. al. (2005)).
Steps in the detection of a salty and sour
stimuli:
1. An ion-channel receptor (the Amiloridesensitive sodium channel) allows EITHER
sodium or hydrogen ions to pass into the
taste bud.
2. This ion movement will lead to a
depolarization which leads to the influx of
calcium ions, stimulating the release of
neurotrasmitter agents.
3. The hydrogen ions will additionally block
potassium channels in the membrane. It
is through this blockage (and other effects
of the change in pH) that distinguishes the
sour response from the salty.
There is as yet, no definitive understanding of
how the neurotransmitter release differs
between these two tastes categories.
Steps in the detection of bitter stimuli:
1.
Two different taste receptors may be
present. The Bitter1 receptor is a
potassium channel that is shut down (is
blocked) by the bitter flavorant.
Whereas the Bitter2 receptor is a Gprotein coupled receptor.
2.
Bitter1 activation leades to
depolarization whereas Bitter2
activation leads to Phospholipase C to
convert a precursor molecule into IP3
(IP3 is a secondary messenger
molecule used in signal transduction.)
3.
In Bitter1 the result is an influx of
calcium leading to neurotransmitter
release, whereas in Bitter2 the IP3
stimulates the release of internal stores
of calcium leading to neurotransmitter
release.
There are several potential neurotransmitter
candidates gustatory afferent neurons
for the two bitter taste buds.
In 1907, Dr. Ikeda conducted experiments that identified
the fifth taste category…. Umami.
Steps in the detection of umami stimuli:
1. Amino acids of a few specific forms (most notably
glutamate) will stimulate the activation of channels
that allow BOTH calcium and sodium ions to flood
into taste buds.
2. The influx of these ions triggers a depolarization
event which opens more calcium channels leading
to neurotransmitter release.
As only a subset of the population displays the
presence of umami tastebuds, what potential
role/value could they play for individuals with
these types of taste buds play?
The Fat/Savory Taste Bud
In November 2005, it was reported that a team of French researchers
experimenting on rodents claimed to have evidence for a sixth taste, for fatty
substances. Investigator Philippe Besnard and his team believe the CD36
receptors that they found on rodents, were important for evolutionary reasons to ensure animals ate a high energy diet when foods were scarce. It is
speculated that humans may also have the same receptors. Fat has
occasionally been raised as a possible basic taste since at least the 1800s.
As it currently stands, the mechanism of this taste bud is unclear, as is its
prevalence in the human population. Best estimates are that it is found in a
sub-population that is similar in size or smaller than that of the umami
tastebud.
See Laugerette et. al. (2005) for the landmark study identifying this
tastebud/receptor research.
The Basic Tastes
– Steps to distinguish the countless unique flavors of a food
• Each food activates a different combination of taste receptors
• Distinctive smell
• Truthfully, it is difficult for us in day-to-day living to discern our “sense
of taste” as separate from our “sense of smell” as they are intimately
intertwined.
The Organs of Taste
– Tongue, mouth, palate, pharynx, and epiglottis
Central Taste Pathways
Three different cranial nerves relay
afferent sensory information from
the tongue to the brain:
Taste
VII - Facial
IX - Glossopharyngeal
X – Vagus
Information enters the brain via the
medulla, into the thalamus and
then project to the primary
gustatory cortex on the postcentral
gyrus.
Smell
Pheromones
– a mode of communication using the sense of smell (typically
subconsciously)
– pheromonal signals are extremely important in:
• Reproductive behavior
• Territorial boundaries
• Identification
• Aggression
– potential presence and role of human pheromones
Several studies in the last two decades have suggested potential human
pheromones that will relate to selection of mates. Some have also linked
these potential pheromones to immune system diversity as an
evolutionary factor
See Thornhill & Gangestad (1999) for one of the more interesting studies.
Caspar Berthelsen Bartholin - Danish physician and theologian who wrote in
1619 one of the most widely read Renaissance manuals of anatomy.
He was first to describe the olfactory nerve (the first identified cranial nerve).
Concluding Remarks
•
•
•
•
Transduction mechanisms
– Gustation and olfaction
Similar to the signaling systems used in every cell of the body
Common sensory principles - broadly tuned cells
– Population coding
– Sensory maps in brain
Timing of action potentials
– May represent sensory information in ways not yet understood
END.