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