The chemical senses
Chemical sensation: The oldest and most common of the sensory systems. Even brainless bacteria can
detect and tumble toward a favorable food source.
Gustation: Chemical sense of taste
Olfaction: Chemical sense of smell
Chemoreceptors: Chemically sensitive cells distributed throughout the body. Separate from olfaction
and gestation. Chemoreceptors serve in the conscious and subconscious report of our internal state.
1. Nerve endings in the digestive organs detect ingested substances
2. Receptors in arteries of the neck measure carbon dioxide and oxygen levels in our blood
3. Sensory endings in muscles respond to acidity as a result of physical fatigue.
Gustation and Olfaction: Detects environmental chemicals. Both are required in the perception of
Flavor. Have strong and direct connections with basic internal needs:
5. Certain forms of memory
Gustation vs Olfaction: Systems are separate and different in terms of:
1. Structures of chemoreceptors
2. Mechanisms of chemoreceptors
3. Gross organization of central connections
4. Effects on behavior
However, the neural information from each system is processed in parallel and is merged at rather high
levels in the cerebral cortex
The basic tastes:
1. Saltiness a. Salts
c. Artificial sweeteners
1. From two amino acids
1. KCl evokes both bitter and salty tastes
b. Complex organic molecules
b. MSG – Monosodium Glutamate
Factors contributing to flavor:
1. Each food activates a different combination of the basic tastes
2. Most foods have a distinctive flavor as a result of their taste and smell
a. Without smell, a bite of onion can be mistaken for a bite of an apple
3. Other sensory modalities contribute to food tasting experience:
Therefore, to distinguish unique flavor of a food, the brain uses information about taste, smell and feel.
The organs of taste
Important structures and organs of taste:
3. Pharynx 4. Epiglottis
5. Nasal cavity
6. Olfactory receptors
Tongue: The tip of the tongue is most sensitive to sweetness. The back of the tongue is most sensitive to
bitterness. The sides of the tongue are most sensitive to saltiness and sourness. However, most of the
tongue is sensitive to ALL basic tastes.
Papillae: Latin for ‘bumps’. Small projections scattered about the surface of the tongue. They are of
1. Foliate papillae
a. Shaped like ridges
2. Vallate papillae
a. Shaped like pimples
3. Fungiform papillae
a. Shaped like mushrooms
Taste buds: Visible only with a microscope, each papilla has from one to several hundred of these taste
buds. A typical person has 2000-5000 taste buds.
Taste receptor cells: Each taste bud has 50-150 of these taste cells, arranged within the bud like the
sections of an orange. Taste cells are only about 1% of the tongue epithelium. Microvilli at the apical end
of taste cells extend into the taste pore; the site where chemicals dissolved in saliva can interact directly
with taste cells.
Basal cells: These surround the taste cells within the taste buds.
Gustatory afferent axons: Set of gustatory afferent axons that synapse with taste cells and basal cells
Threshold concentration: At some critical concentration, the stimulus will evoke a perception of taste.
At concentrations just above threshold, most papillae tend to be sensitive to only one basic taste: 1)
Sweet sensitive papillae, and 2) Sour sensitive papillae. At increasing concentrations, most papillae
become less selective (e.g. instead of only sour sensitive, it becomes sour and salt selective). This lack of
selectivity is common throughout sensory systems.
Taste receptor cells
Apical end: The chemically sensitive part of a taste receptor cell. It is located near the surface of the
tongue at the small membrane region of taste cells.
Microvilli: Thin extensions that project into the taste pore from the apical ends.
Taste pore: A small opening on the surface of the tongue where the taste cell is exposed to the contents
of the mouth Characteristics of taste receptor cells:
1. They are NOT neurons, BUT they do form synapses with the endings of the gustatory afferent
axons near the bottom of the taste bud.
2. They make both chemical and electrical synapses onto some of the basal cells
a. Some basal cells synapse onto the sensory axons, forming a simple information
processing circuit within each taste bud
3. Cells of the taste bud undergo a constant cycle of growth, death and regeneration
a. The lifespan of one taste cell is about 2 weeks
i. This process depends on the influence of the sensory nerve (if the nerve is cut,
the taste buds will regenerate).
4. More than 90% of receptor cells respond to two or more of the basic tastes.
a. However, the gustatory axons that taste cells synapse onto has a clearly defined bias to
which of the tastes it is sensitive to.
Activation of a taste receptor cell:
1. The taste cell is activated by an appropriate chemical
2. Results in membrane potential change (either depolarization or hyperpolarization)
a. This voltage shift is called the receptor potential.
3. If the receptor potential is depolarizing and large enough, most taste receptor cells may fire
4. Depolarization of the receptor membrane causes voltage gated calcium channels to open
a. Ca2+ enters the cytoplasm
5. Triggers the release of transmitter molecules
a. This process results in the basic synaptic transmission from taste cell to sensory axon.
b. The identity of the transmitter is unknown
i. But it is known that it excites the postsynaptic sensory axon and causes it to fire
action potentials which communicate the taste signal into the brain stem
Mechanisms of taste transduction
Transduction: The process by which an environmental stimulus causes an electrical response in a
sensory receptor cell. Transduction mechanisms make the nervous system sensitive to:
The nature of the transduction mechanism determines the specific sensitivity of a sensory system (e.g. if
we had photoreceptors in our mouth, we’d be able to see with our mouth instead of our eyes). Taste transduction: Involves several different transduction processes. Each basic taste uses one or more
of these mechanisms.
Mechanisms of taste stimuli / tastants:
1. Directly pass through ion channels (salt and sour)
2. Bind to and block ion channels (sour)
3. Bind to G-protein coupled receptors in the membrane that activate second messenger systems
that open ion channels (bitter, sweet and umami)
Saltiness: Typical salty chemical is NaCl, mostly from the cation Na+. Its concentration must be quite
high in order to taste it. Salt sensitive taste cells have a special Na+-selective channel that is common in
other epithelial cells and is blocked by the drug amiloride. The amiloride sensitive sodium channel is
insensitive to voltage and stays open all the time. When you drink something salty:
1. The Na+ concentration rises outside the receptor cell
2. The gradient for Na+ across the membrane is made steeper
3. Na+ then diffuses down its concentration gradient and flows into the receptor cell
4. The resulting inward current causes the membrane to depolarize
5. This depolarization (receptor potential) causes voltage gated sodium and calcium channels to
open near the synaptic vesicles
6. Triggers the release of neurotransmitter molecules onto the gustatory afferent axon
The anions of salts: Affects the taste of its cations:
1. The larger an anion is, the more it inhibits the salt taste of the cation
a. E.g. Sodium chloride is saltier than sodium acetate
2. As anions become larger, they tend to take on tastes of their own
a. E.g. Sodium saccharin tastes sweet because the Na+ concentrations are far too low for
us to taste the saltiness, and the saccharin potently activates sweetness receptors
Sourness: Foods taste sour because of their high acidity (protons, H+). Affect sensitive taste receptors in
1. H+ can permeate the amiloride-sensitive sodium channel
a. Causes an inward H+ current and depolarizes the cell
b. Note that the cell would not be able to distinguish a hydrogen ion from a sodium ion if
this was the only transduction mechanism available to it
2. H+ can bind to and block K+-selective channels
a. When the K+ permeability of a membrane is decreased, it depolarizes
3. Additionally, all changes of pH can affect virtually all cellular processes therefore it is the
constellation of effects that evoke the sour taste.
Bitterness: Detected by the 30 or so different types of T2R receptors. Each bitter taste cell expresses
many of the 30 bitter receptor proteins. Because each taste cell can only send one type of signal to its afferent nerve, a chemical that can bind to one of its 30 bitter receptors will trigger essentially the same
response as a different chemical that binds to another of its bitter receptors. The nervous system
apparently does not distinguish one type of bitter substance from another (only that bitter substances
are ‘bad, not to be trusted’ – associated with poison). Bitter taste mechanism:
1. Use a second messenger pathway to carry their signal to the gustatory afferent axon
a. The general pathway is similar for bitterness, sweetness, and umami.
2. When a tastant binds to a bitter (or sweet / umami) receptor, it activates its G proteins,
3. The G proteins stimulate the enzyme phospholipase C
4. Phospholipase C increases the production of the intracellular messenger inositol triphosphate
a. IP3 pathways are ubiquitous signaling systems in cells throughout the body
5. In taste cells, IP3 has two mechanisms:
a. Activates a special type of ion channel that is unique to taste cells
i. This causes it to open and allow Na+ to enter to depolarize the cell
1. The depolarization causes voltage gated calcium channels to open,
allowing Ca2+ to enter the cell.
b. Trigger the release of Ca2+ from intracellular storage sites
6. These two sources of Ca2+ help trigger neurotransmitter release, thereby stimulating the