Page 84 - 84 human physiolofy part-1

Chapter 5

can distinguish between the many radically different compounds which produce a generally
“bitter” response. This may be because the sense of bitter taste is so important to survival, as
ingesting a bitter compound may lead to injury or death. Bitter compounds act through structures
in the taste cell walls called G-protein coupled receptors (GPCR’s). Recently, a new group of
GPCR’s was discovered, known as the T2R’s, which is thought to only respond to bitter stimuli.
When the bitter compound activates the GPCR, it in turn releases gustducin, the G-protein it was
coupled to. Gustducin is made of three subunits. When it is activated by the GPCR, its subunits
break apart and activate phosphodiesterase, a nearby enzyme. It then converts a precursor within
the cell into a secondary messenger, which closes potassium ion channels. This secondary
messenger can stimulate the endoplasmic reticulum to release Ca2+, which contributes to
depolarization. This leads to a build-up of potassium ions in the cell, depolarization, and
neurotransmitter release. It is also possible for some bitter tastants to interact directly with the G-
protein, because of a structural similarity to the relevant GPCR.

Sweet
Like bitter tastes, sweet taste transduction involves GPCR’s. The specific mechanism depends on
the specific molecule. “Natural” sweeteners such as saccharides activate the GPCR, which
releases gustducin. The gustducin then activates the molecule adenylate cyclase, which is already
inside the cell. This molecule increases concentration of the molecule cAMP, or adenosine 3',
5'-cyclic monophosphate. This protein will either directly or indirectly close potassium ion
channels, leading to depolarization and neurotransmitter release. Synthetic sweeteners such as
saccharin activate different GPCR’s, initiating a similar process of protein transitions, starting
with the protein phospholipase A, which ultimately leads to the blocking of potassium ion
channels.

Umami
Umami is a Japanese word meaning "savory" or "meaty". It is thought that umami receptors act
much the same way as bitter and sweet receptors (they involve GPCR’s), but not much is known
about their specific function. We do know that umami detects glutamates that are common in
meats, cheese and other protein-heavy foods. Umami receptors react to foods treated with
monosodium glutamate (MSG). This explains why eating foods that have MSG in them often
give a sense of fullness. It is thought that the amino acid L-glutamate bonds to a type of GPCR
known as a metabotropic glutamate receptor (mGluR4). This causes the G-protein complex to
activate a secondary receptor, which ultimately leads to neurotransmitter release. The
intermediate steps are not known.

Disorders of the Tongue

Loss of taste
You may lose your sense of taste if the facial nerve is damaged. Then there is also Sjogren's
Syndrome where the saliva production is reduced. In most cases the loss of taste is typically a
symptom of anosmia - a loss of the sense of smell.

Sore tongue
It is usually caused by some form of trauma, such as biting your tongue, or eating piping-hot or
highly acidic food or drink.
If your top and bottom teeth don’t fit neatly together, tongue trauma is more likely.
Some people may experience a sore tongue from grinding their teeth (bruxism).
Disorders such as diabetes, anemia, some types of vitamin deficiency and certain skin diseases

84 | Human Physiology