PHGY 210 Lecture Notes - Lecture 4: Membrane Potential, Umami, Taste Bud

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31 Jan 2013
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TASTE
The Basic Tastes
It is likely that we can recognize only a few basic tastes. Most neuroscientists put the number at
five. The four obvious taste qualities are:
Saltiness
Sourness
Sweetness
Bitterness
Umami
How do we perceive the countless flavours of food?
First, each food activates a different combination of the basic tastes.
Second, most foods have a distinctive flavour as a result of their taste and smell
occurring simultaneously
Third, other sensory modalities contribute to a unique food-tasting experience (e.g.
texture, temperature, and pain sensations)
The Organs of Taste
Although we taste with our tongue, there are other areas of the mouth (e.g. palate, pharynx, and
epiglottis) that are also involved. Odours from the food pass, via the pharynx, into the nasal
cavity, where they can be detected by olfactory receptors.
The tip of the tongue is most sensitive to sweetness, the back to bitterness and the sides to
saltiness and sourness. However, most of the tongue is sensitive to all basic tastes.
The surface of the tongue is scattered with small projections called papillae. Each papilla has
from one to several hundred taste buds (see Fig. 8.2) and each of these have 50 150 taste
receptor cells. Taste cells make about 1% of the tongue epithelium. Taste buds also have
basal cells that surround the taste cells and a set of gustatory afferent axons. A person typically
has 2000-5000 taste buds.
Tastants at very low concentrations will not be tasted, but at some critical concentration, the
stimulus will evoke a perception of taste. This is the threshold concentration. At levels just
above threshold, most papillae tend to be sensitive to only one basic taste. However, when the
concentrations of the Tastants are increased, most papillae become less selective. For
example, a papilla might have responded only to sweet when all stimuli were weak but also
responds to sour and salt if they are made stronger.
This relative lack of specificity is a common phenomenon in sensory systems. Many sensory
receptors are surprisingly indiscriminate about the things that excite them. This presents a
paradox: how can we distinguish reliably between differences as subtle as two kinds of
chocolate? The answer lies in the brain.
Taste Receptor Cells
The chemically sensitive part of a taste receptor cell is called the apical end. These ends have
thin extensions called microvilli that project into the taste pore. Taste receptor cells are not
neurons but do form synapses with the endings of the gustatory afferent axons near the bottom
of the taste bud. Taste receptor cells also make electrical and chemical synapses onto some of
the basal cells.
When a taste receptor cell is activated by an appropriate chemical, its membrane potential
changes (i.e. depolarization). This shift is called the receptor potential (see Fig. 8.3). If
receptor potential is depolarizing and large enough, most taste receptor cells may fire action
potentials.
More than 90% of the receptor cells respond to two or more of the basic tastes. Figure 8.4
shows the results of recordings from four gustatory axons in a rat. One responds strongly only
to salt and one only to sweet. Two respond to all but sweet. Why? This is because the
responses depend on the particular transduction mechanisms present in each cell.