Chapter 8 Notes: The Chemical Senses 03:27
In this chapter, we will basically discuss gustation (taste) and olfaction (smell).
However, there are other chemical sensors such as chemoreceptors, which warn us
of irritating chemicals in our system and they are distributed throughout the body.
Gustation and Olfaction have similar tasks: to detect chemicals in our environment
but they are structurally different and have different effects on our behavior.
As humans we have an innate preference for sweetness but can grow accustomed to
bitter food as well such as coffee. Also the body has the ability to recognize the
deficiency of certain key nutrients and develop a craving for them.
Most neuroscientists have determined that there are five main categories of taste
known to mankind and those are saltiness, sourness, sweetness, bitterness and
umami (Japanese for delicious). Umami is defined by the taste of amino acid
glutamate and MSG.
Most acids taste sour, most salts taste salty, sugars taste sweet and bitter
substances can be detected even in very small amounts (down to nanomolar range)
and is beneficial to mankind because most poisons are bitter.
In order to perceive a wide variety of foods, each food activates a range of the basic
tastes to make it unique. Also, most foods have a specific combination of taste and
smell that also help make it unique. Texture and temperature of the food as well as
other modalities help contribute to the uniqueness of the food.
We experience the taste of foods with different areas of the mouth such as the
palate, the pharynx and the epiglottis.
On the surface of the tongue are small projections called papillae which contains 1
to several hundred taste buds. Each taste bud has 50-150 taste receptor cells but
they comprise of only 1% of the tongue epithelium.
When concentrations are just above the threshold, most papillae become sensitive to
only one basic taste; however, when the concentration of the taste stimuli are
increased, the papillae become less selective.
The chemically sensitive part of a taste receptor cell is its apical end (a small
membrane region), near the surface of the tongue.
Taste-receptor cells are not neurons by histological criteria; but they do form
synapses on afferent gustatory axons and make electrical and chemical synapses
onto some basal cells.
Depolarization of the receptor membrane causes voltage-gated calcium channels
open; calcium enters the cytoplasm which triggers the release of transmitter
www.notesolution.com How is there distinction between the number of chemicals that a taste receptor cell
will respond to? It is because of the particular transduction mechanisms present in
MECHANISMS OF TASTE TRANSDUCTION:
Transduction- the process by which an environmental stimulus causes an
electrical response in a sensory receptor cell.
Taste transduction can involve several processes to detect taste such as:
Tastants directly pass through ion channels (salt and sour)
Bind to G-protein coupled receptors in the membrane that activate second
messengers and in turn open ion channels (bitter, sweet and umami)
Bind to and block ion channels (sour)
Saltiness- salt-sensitive taste cells have a special Na+-selective channel (amiloride-
sensitive sodium channel). Unlike voltage-gated channels this taste channel is not
voltage sensitive; it stays open all the time. When concentration of Na+ rises outside
the receptor cell the Na+ ions move inside the receptor cells down its concentration
gradient. Once inside, the cell is depolarized which triggers the opening of voltage-
gated sodium and calcium channels to open near synaptic vesicles, which causes the
release of neurotransmitters on to gustatory afferent axons.
Sourness- Foods taste sour due to their high acidity. These types of tastes can
affect sensitive taste receptors in two ways: 1) H+ can permeate the amiloride-
sensitive sodium channel, which causes an influx of H+ and depolarized the cell; 2)
hydrogen ions can bind to and block K+-selective channels. When the permeability
of K+ is decreased, the cell depolarizes.
Bitterness- bitter substances are detect by 30 or so different types of T2R
receptors. Bitter receptors in animals cant really tell the difference between
different bitter tastes because each cell can only send one type of signal to its
afferent nerve. Different chemicals that bind to the different receptors will still
generate the same response. Bitter receptors use the secondary messenger pathway
in order to signal their afferent gustatory axon. The pathway for signaling in
bitterness are similar to those of sweetness and umami: G-protein coupled receptor
stimulated stimulation of phospholipase C increased production of IP3
activation of special taste ion channel opening of Na+ channels(depolarization)
voltage-gated calcium channels to open.
Sweetness- sweet receptors are formed by two g-protein coupled receptors as
opposed to bitter receptors which contain one. Sweet receptor cells require T1R2 and
T1R3 receptors in order to perceive sweetness. These receptors activate the same
www.notesolution.com second messenger system as bitter receptors. The reason why we dont confuse
bitter and sweet taste is because these tastes occur in different taste cells.
Umami(amino acids)- The transduction process for umami is identical to that of
sweetness with one exception: the two protein complex consists of T1R1 an T1R3
receptors. We dont confuse umami taste with bitter or sweet for same reason that
there are different taste cells for the different tastes.
CENTRAL TASTE PATHWAYS:
Flow of taste information: taste budsprimary gustatory axonsbrain
There are three cranial nerves that relay taste information: facial nerve (receives
info from the anterior 2/3 of the tongue and palate), glossopharyngeal
nerve (receives info from posterior 1/3of the tongue) and the vagus nerve
(receives info from regions around the throat).
These nerves all bundle together in the brain stem and synapse on the gustatory
From the gustatory nucleussynapse on to the ventral posterior medial nucleus
(VPM)primary gustatory cortex (this pathway is ipsilateral to the cranial nerves
that supply them)
Lesions in the VPM thalamus and gustatory cortex can cause ageusia, the loss of
Labeled line hypothesis- shows how the modality of stimulus are encoded in the
Population coding- a scheme in which the responses of a large number of broadly
tuned neurons, rather than a small number of precisely tunes neurons, are used to
specify the properties of a particular stimulus, such as a taste.
Smell is important for distinguishing between good and bad smells which could be
harmful. It is also a mode of communication. Pheromones, which are chemicals
released by the body, are important signals for reproductive behavior.
We smell with a thing sheet of cells called the olfactory epithelium which have three
main cell types:
Olfactory receptor cells- the site of transduction; they are genuine neurons that
have axons of their own that penetrate in to the CNS.
Supporting cells- similar to glia; they help produce mucus among other things.
Basal cells- the source of new receptor cells.
www.notesolution.com Sniffing brings air in to the nasal passage and only a smell percentage of that air
passes over the olfactory epithelium. The epithelium then secretes mucus, which is
replaced every 10 minutes and is continuously flowing. The mucus is water based
and contains several mucopolysaccharides, antibodies, and odorant binding
proteins. The odorants dissolve in the mucus before they get to the receptor cells. It
is important to have antibodies in case viruses are inhaled and odorant binding
proteins to concentrate the smell.
Odorants Binding to membrane odorant receptor proteins G-protein
stimulation Activation of adenalyl cyclase Formation of cAMP
Binding of cAMP to specific cation channel Opening of cation channels
and influx of Na+ and Ca2+ Opening of Ca2+-activated chloride
channels Current flow and membrane depolarization
Olfactory axons do not come together as a single nerve like other cranial nerves.
Rather they form small clusters of axons which penetrate the cribriform plate and
then enter the olfactory bulb.
When olfactory axons are severed, it causes anosmia, which is the inability to smell.
Same as in taste, olfaction uses a similar sort of population coding scheme in which
receptors are more or less sensitive to certain odorants. The concentration of the
odorant as well as the odorant itself are important for generating stronger
Glomeruli- spherical structures that are the input layer for axons of olfactory
receptor neurons on the bulbs.
It seems that each glomerulus receives input from a large area of the epithelium
and the arrangement of input is symmetrical on both bulbs. It also seems that
glomerulus receive input from only receptor cells of one type; basically making it a
map of odor information.
How does the whole brain discriminate between odors when the single neurons
Each odor is represented by the activity of a large population of neurons
the neurons responsive to particular odors may be organized in to spatial maps
the timing of action potentials may be an essential code for particular odors.
With the use of population coding the brain can use the information provided by a
large array of neurons to determine the smell being perceived which would be
unique to only that odor.
A sensory map is an orderly arrangement of neurons that co