Download The Chemical Senses and Transduction

UNIT 2
PRINCIPLES OF SENSORY PROCESSING
CHAPTER 8
THE CHEMICAL SENSES AND TRANSDUCTION
8.1. DEFINING THE CHEMICAL SENSES
When we think of our ability to sense molecules in our environment, we immediately
think of our senses of smell and taste. In addition to these two sensory modalities that are
specialized for detecting the presence of chemical substances in external world, we have other
systems that monitor chemicals in the internal environment, inside our bodies. For example,
chemoreceptors located in the arteries measure carbon dioxide and oxygen in the circulating
blood. When the concentration of either is too high or too low, the nervous system is notified and
signals are sent out to the circulatory and respiratory systems to adjust the beating of the heart
and the rate of breathing in the appropriate direction.
Similarly, there are sensory endings in our muscles that detect lactic acid when we have
been exercising very hard. They send a pain signal to the brain that tells us it is time to stop or
slow down. Nerve endings in the skin and mucus membranes detect chemical irritants, allowing
us to avoid them if possible.
8.2. ORIGIN OF THE CHEMICAL SENSES.
The chemical senses are probably the most primitive of all our sensory systems. Virtually
every living cell is sensitive to some set of chemical substances. Even primitive one-celled
organisms such as bacteria, amoebas and paramecia have chemoreceptors on their cell
membranes so that they can approach molecules that are beneficial and avoid those that are
harmful.
It is probable that the same mechanisms that originally evolved in single-celled
organisms for the purpose of detecting external stimuli not only retain their original purpose in
the various chemoseneory systems, but have been elaborated in multicellular organisms to
provide a means of communication among the billions of cells that make up a higher organism.
Signalling mechanisms that depend on chemicals that are released from one cell and detected by
another cell include hormones in the endocrine system and neurotransmitters in the nervous
system.
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8.3. GENERAL FUNCTION OF THE CHEMICAL SENSES
In keeping with its "primitive" origins, chemoreception is closely related to basic
motivational drives including feeding, drinking, sex, and emotion.
Memories of tastes and smells are some of the most persistent and provocative of all
sensory stimuli. A specific taste paired with nausea or other noxious stimulus leaves a lasting
aversion for the taste. This type of learning is referred to as a conditioned taste aversion.
8.4. WHY ARE THERE SEPARATE SYSTEMS FOR SMELL AND TASTE?
Smell (olfaction) and taste (gustation) both allow us to detect and discriminate among
different classes of molecules. What we commonly call "tastes" are really a combination of
olfactory and gustatory information. Nevertheless, taste and smell serve somewhat different
functions in that they allow us to sense a wider variety of molecules than either could alone. The
following table summarizes the differences between taste and smell
Olfaction
Taste
airborne
molecules
waterborne
molecules
lipid-soluble
molecules
water-soluble
molecules
locating food
testing food
We can smell many more classes of stimuli than we can taste, due to the wide variety of
different receptor sites that are present in the olfactory system.
8.5. THE BASIC PROCESS OF TRANSDUCTION
Before any neural processing can take place, there must be some means of detecting the
presence of information in the environment, collecting the different forms and patterns of energy
that represent this information, and converting the physical energy into a form that can be acted
upon and utilized by the nervous system. The process through which a specific pattern of
information (energy) in the environment (e.g., light, vibrations, dissolved chemicals or airborne
chemicals) is converted to a pattern of electrical activity in the nervous system is called
transduction..
Beginning with the physical energy of an environmental stimulus, there are several
processes that must occur before a pattern of neural activity is generated. The steps leading to
transduction include the following:
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The stimulus energy must reach specialized receptor cells. In some cases (e.g., taste,
touch) this process is relatively simple and straightforward. In other systems (e.g., hearing,
vision), it is quite complicated.
The receptor cells must be activated. The activation process involves opening of ion
channels to cause a change in the cell's membrane potential. In different sensory systems,
different mechanisms cause ion
channels to open in response to a stimulus.
The opening of ion channels creates a receptor potential. Like an EPSP in a neuron, the
receptor potential is graded; its size reflects in some way the properties of the stimulus. In
general, the larger the magnitude of the stimulus, the larger the receptor potential.
The receptor potential causes release of neurotransmitter onto the dendrite of the primary
afferent (nerve fiber projecting to the central nervous system). The larger the receptor potential,
the greater the quantity of neurotransmitter released. If enough excitatory neurotransmitter is
released to bring the primary afferent neuron to threshold, it will fire an action potential.
Figure 8-1. Diagram summarizing the events that take place in transduction.
8.6. A SIMPLE EXAMPLE: TRANSDUCTION IN THE GUSTATORY (TASTE)
SYSTEM.
8.6.1. Peripheral mechanisms.
In order for us to taste a substance, it must be in the mouth, contacting the tongue (or, to
a lesser extent, other parts of the mouth). The experience that we commonly think of as "taste" is
actually a complex interaction resulting from stimulation of several sensory systems including
gustatory (sweet, salty, etc) , olfactory (e.g. coffee, apple, or onion aroma), tactile (e.g.,smooth
or rough texture) temperature (hot or cold), and even pain (e.g., hot chili peppers).
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The tongue is covered with small bumps called papillae. Different types of papillae are
concentrated on different parts of the tongue. The transduction process takes place in the taste
buds, specialized concentrations of receptor cells located on the papillae.
Figure 8-2. Interaction between taste and smell occurs when the liquid in the cup is taken into the mouth. The
lliquid that contacts the tongue stimulates taste receptors; the vapors that enter the nasal cavity through the nostrils
and/or through the back of the throat (arrows) stimulate olfactory receptors.
Figure 8-3. There are several different types of papillae on the surface of the tongue. The large circumvallate
papillae are located at the rear of the tongue, the foliate papillae at the sides, and the small fungiform papillae on the
middle to front of the tongue. Right: Although all parts of the tongue have some sensitivity to all taste qualities, it is
possible to identify certain parts of the tongue that have the highest sensitivity to each taste quality .
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Figure 8-4. The taste buds are located on the tops and sides of the papillae. Each papilla contains multiple taste
buds.
Each taste bud contains receptor cells which are contacted by nerve fibers of the chorda
tympani nerve (a branch of the facial nerve) or by fibers of the hypoglossal nerve. Gustatory
receptor cells, unlike most other sensory receptor cells, die and are replaced every few weeks.
Figure 8-5. The receptor cells within the taste bud are large, and elongate in shape. The top surface of each taste
receptor cell is covered with small protrusions called microvilli, which stick up into an opening at the top of the
taste bud, the taste pore. Nerve fibers (equivalent to the dendrites of the first true neurons in the system) contact the
bottom end of each taste receptor cell.
8.6.2. The transduction process.
The membranes of taste receptor cells contain a variety of ion channels, many of which
are associated with receptor sites for specific types of molecules. Nearly every taste cells has a
variety of different ion channels and receptor sites, but these are mixed together in slightly
different proportions.
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Figure 8-6. The cell membrane of every taste receptor cell contains a variety of ion channels. Each type is activated
by a different category of taste stimulus.
Transduction occurs when molecules that enter the mouth pass through ion channels or
bind to receptor sites on ion channels. Some ion channels operate by allowing ions to pass
through and depolarize the cell directly. For example, we perceive a salty taste when sodium
ions are present in high concentration in the mouth. Because the sodium concentration is higher
outside the taste receptor cell than inside it, sodium enters the cell. Because sodium has a
positive charge, the inside of the cell becomes more positive with respect to the outside, i.e., it is
depolarized. This ultimately results in neurotransmitter release.
Sour stimuli contain an abundance of positively charged hydrogen ions (H+) that act in a
manner similar to sodium ions to depolarize the taste receptor cell.
Figure 8-7. Sodium ions and hydrogen ions pass through specific classes of ion channels to depolarize the taste
receptor cell. These channels can be blocked by "drugs" such as amiloride. Depolarization activates voltage-gated
calcium channels and calcium causes neurotransmitter release from the vesicles.
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Many ion channels operate through binding of a relatively large taste stimulus molecule
to a specific site on the ion channel, causing the pore in the channel to undergo a conformational
change (a change in shape) that admits a small ion (e.g., sodium), thereby depolarizing the cell.
For example, we perceive a sweet taste when sugar molecules bind to specific receptor sites on
ion channels. The sugar does not enter the cell itself; instead, it opens channels that allow a small
positive ion (probably sodium) to enter the cell.
Bitter substances (e.g., quinine) and amino acids (e.g., glutamate) probably stimulate
taste receptor cells through specific receptor sites, in much the same way that sugars do. There
are probably many different types of receptor sites for bitter substances. Amino acids or their
salts (e.g., monosodium glutamate) elicit a taste that is unique, often called "umami".
Figure 8-8. Sweet and bitter substances are too large to pass through ion channels diretctly. Instead, they bind to
specific receptors in the cell membrane; the binding of the tastant molecule to the receptor site initiates a
complicated series of chemical reactions that ultimately lead to blocking of potassium channels, opening of voltagegated calcium channels, and neurotransmitter release from the vesicles. As a general rule, naturally occurring sweet
molecules (i.e., sugars) do not bind to bitter receptors, and vice versa.
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Thought question: Many commercial food products that we think of as "sweet" (chocolate chip
cookies, for example) contain high concentrations of salt (sodium chloride) in addition to sugar.
Given what you know about transduction in the gustatory system, why might a manufacturer add
salt to a “sweet “ product?
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8.7. GENETICS, RECEPTORS, AND TASTE PERCEPTION
Individual humans differ from one another in their ability to taste certain substances,
especially ones that are "bitter" or "sweet". It is highly probable that these individual differences
are due to the presence or absence of certain genetically determined populations of receptor sites
on their taste receptor cells. For example, many individuals find a substance called
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phenylthiocarbamide (PTC) to be extremely bitter and unpleasant tasting. Others cannot taste
PTC at all. Individuals who cannot taste PTC do not have the genetic information that would
allow them to produce the receptor site molecule that binds PTC. Artificial sweeteners are
another class of substance that elicit quite different taste sensations in different individuals.
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Thought question: Our taste and olfactory systems evolved to detect and analyze molecules that
occur in nature such as salts, sugars, and aromatic molecules produced by plants. Modern
chemistry has produced a great variety of artificial molecules. Many of these artificial
substances are used by the food industry to "trick" our tongues, noses and brains into thinking
we are eating something nutritious when, in fact, what we are eating has little or no nutritional
value. Artificial sweeteners such as saccharin are a good example of this kind of substitution.
How do you think these molecules are able to "mimic" the sensations produced by natural
substances?
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Different animals are sensitive to different sets of molecules. For example, humans eat a
wide variety of foods and perceive five or six clearly distinct classes of tastes including sweet,
sour, salty, bitter and umami. Rats are also omnivorous, eating many of the same foods that
humans do. Rats' taste perception is very similar to that of humans. Cats, on the other hand, do
not taste sweet substances; instead, they can discriminate well among different types of amino
acids and other molecules that are present in meat and cat food. The same is true of catfish,
bottom dwellers that feed on dead animals and other detritus.
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