Download chapter 9 the chemical senses, neural coding, and central nervous

CHAPTER 9
THE CHEMICAL SENSES, NEURAL CODING, AND
CENTRAL NERVOUS SYSTEM PATHWAYS
9.1. Transduction and patterns of neural activity.
The purpose of transduction is to convert a pattern of environmental information
distributed over space and time to a pattern of neural activity which is also appropriately
distributed across space (different neurons) and time.
9.1.1. Spatial patterns of activity.
9.1.1.1. Modality-specific activity. Taste stimuli applied to the tongue elicit activity in the
taste nerves that innervate the tongue, not in peripheral nerves that innervate other structures
such as the eye. Thus, the identity of the nerves that are activated conveys information about
what kind of stimulus causes the activity (e.g., chemicals in the case of the taste nerves, light in
the case of the optic nerve, etc.).
_____________________________________________________________________________
Thought question: Usually we perceive an electrical current applied to the body as an electric
shock. However, when the inner ear of a deaf individual is stimulated with an electrical current,
the perception is usually that of sound. This technique is applied clinically in the form of a
"cochlear prosthesis" to restore the hearing of patients who have lost the ability to perform
auditory transduction. Why do you think this technique works? For what sorts of patients would
it NOT work?
_____________________________________________________________________________
9.1.1.2. Transduction, sensitivity functions and population coding. In the gustatory
system, each individual receptor cell has an assortment of different ion channels that are
specifically activated by a class of molecule (e.g., salt, acid, or sugar). Some receptor cells have
more of a given type of ion chanel than do others. For example, one cell may have a high
proportion of ion channels that are sensitive to salt, and another a high proportion of ion
channels sensitive to sugar. This means that each cell will respond to both salty and sweet tasting
stimuli, but the first cell will respond more strongly to salt and the second cell will respond most
strongly to sugar.
The response profile of each cell across the entire range of stimuli that could potentially
activate it is called its sensitivity function. The fact that receptor cells in many sensory systems
have different, often overlapping, sensitivity functions is a consequence of the transduction
process. It is also the basis for population coding. Later on, we will talk much more about
sensitivity functions and population codes in the visual and auditory systems.
In its simplest form, a population code is a neural representation in which information is
conveyed by relative amounts of activity across multiple, differentially sensitive elements of an
array (in our example, the taste receptor cells). This is in contrast to a "labeled line", in which
49
information is represented unambiguously by the absolute amount of activity in each individual
element within the array.
Figure 9-1. Examples of sensitivity functions (or profiles) for two taste neurons. The "sweet-best" neuron on the
left responds to all types of stimuli, but the largest response is evoked by sweet substances. The "salt-best" neuron
on the right also responds to all types of stimuli, but the largest response is evoked by salty substances.
9.1.2. Transduction and temporal response patterns.
Any system of interactions between or among molecules, whether it be a straightforward
chemical reaction, passage of an ion through a channel, or the binding and/or disassociation of a
molecule to/from a receptor site has a characteristic time course, or kinetics. Some of these
events happen quickly, others slowly. Some happen in a continuous, smooth pattern and others
in a discontinuous, jerky pattern. Some happen in a single stage, others in multiple stages.
For a sensory receptor cell, the kinetics of receptor activation determine the time course
of changes in its membrane potential. For a taste cell, the pattern of action potentials in the nerve
fiber innervating the cell is determined by the reeptor sites and associated channels activated,
and their respective kinetics.
Figure 9-2. Temporal patterns of action potentials produced by a taste neuron in response to four different stimuli:
salty (sodium chloride, NaCl), sour (dilute hydrochloric acid, HCl), bitter (quinine, QHCl), and sweet (sucrose).
Note that the patterns of action potentials persist for a second or more, and that they are very different for each of
the four stimuli.
Within a population of neurons, information is represented in both space and time, as a
spatio-temporal pattern.
50
9.2. SENSORY INFORMATION FLOW IN THE CENTRAL NERVOUS SYSTEM
In most sensory systems, information is transmitted in the same general pattern within the
central nervous system. The sensory nerve projects in a more or less direct way to a specialized
region of the thalamus, sometimes via one or more groups of cells in the brainstem. From the
thalamus, information is sent to a specialized area of the cerebral cortex, the primary sensory
area. The primary sensory area of the cortex gives rise to direct and indirect projections to
secondary sensory areas, polysensory areas, and ultimately to motor areas and other brain areas
associated with memory, emotion, etc. At some point, "sensory" information is transformed into
something else - for example, a motor action, emotional feeling, or abstract idea.
Figure 9-3. Diagram showing the typical pattern of information flow in the central nervous system.
9.2.1. The central gustatory pathways.
The taste pathways are typical of the general sensory plan outlined above.
Information about taste is first relayed via three separate nerves to the nucleus of the
solitary tract (NTS), a group of cells located in the brainstem. Cells in the brainstem taste regions
have outputs to motor circuits for taste-evoked facial responses as well as to regions involved in
emotional responses and feelings of hunger/satiety. From the brainstem, information is
transmitted to a taste area in the thalamus (the ventral posterior medial nucleus) which, in turn,
projects to a taste area in the cortex located in the parietal lobe, mainly within a sulcus. The
primary taste area in turn projects to a secondary polysensory area that also receives input from
the olfactory system.
Figure 9-4. The central pathway for taste.
51
9.3. THE OLFACTORY (SMELL) SYSTEM
9.3.1. The olfactory receptor cells.
We are able to discriminate among an extremely large number of odorants, most of which
are actually mixtures of different molecules. Odors reach the receptor cells in the olfactory
epithelium through the nose or through the throat. The olfactory epithelium lies at the top of the
nasal cavity. It contains receptor cells and supporting cells.
Figure 9-5. The roof of the nasal cavity is lined with tissue called the olfactory epithelium. The olfactory receptor
cells are long and thin. The exposed end of the cell faces downward into the nasal cavity, and is covered with long
hairlike processes called cilia that extend down into a layer of mucus.
Olfactory receptor cells, unlike other sensory receptors, are true neurons. They are also
the only neurons that regularly die (life span 5-8 weeks) and are replaced.
The dendrites of olfactory receptors have cilia (small hairlike structures) that stick out
into a layer of mucus. Odorants, once inhaled, are picked up by special protein molecules in the
mucus layer and transported to receptor sites on the cilia.
52
Figure 9-6. Left: A more intimate view of olfactory receptor cells and supporting cells showing the cilia and the
axons of the receptor cells exiting to terminate in the olfactory bulb. Right: Odorant molecules in the inhaled air are
trapped in the mucus, where they bind to special molecules called odorant binding proteins. These proteins then
transport the odorants to the cilia, where transduction takes place.
9.3.2. Olfactory transduction
Like taste receptor cells, each olfactory receptor neuron has multiple types of receptor
sites for different molecules. The various types of receptor sites are distributed in different
proportions on different olfactory receptor neurons.
Binding of odorants to receptor sites associated with ion channels causes a receptor
potential at the dendrites, and action potentials at the cell body and axon. The axons of olfactory
receptor cells project directly to the brain via the olfactory nerve.
9.4. THE CENTRAL OLFACTORY PATHWAYS
The olfactory system is the only major sensory system that deviates from the standard
pattern of information flow in that information is transmitted to the cortex before it reaches the
thalamus.
53
Figure 9-7. The olfactory transduction process is similar to the taste transduction process for sugars and bitter
substances. Odorant molecules bind to specific receptor sites in the cell membrane of the cilia. These receptor sites
are associated with ion channels that are activated when an odorant molecule is bound. Binding of an odorant
molecule to the receptor site causes various reactions to occur, culminating in depolarization of the cell (the receptor
potential). If this depolarization causes the cell to reach its threshold, an action potential is generated at the cell body
and transmitted along the olfactory nerve fiber to the olfactory bulb.
The axons of olfactory receptor cells project to the olfactory bulb, where they terminate
in glomeruli (clusters of cells) in the olfactory bulb. The olfactory bulb projects directly to
primary olfactory cortex (also called entorhinal cortex). The primary olfactory cortex projects to
an olfactory area in the thalamus which, in turn, projects back to secondary areas of olfactory
cortex including an area called orbitofrontal cortex, which also receives input from the taste
system.
Figure 9-8. The central olfactory pathways. Axons of the olfactory receptor neurons project to the olfactory bulb
(enlarged in inset at right). Axons from the olfactory bulb form the olfactory tract, which projects to primary
olfactory cortex. The olfactory cortex projects to the thalamus, which projects back to a secondary olfactory/taste
area in the orbitofrontal cortex.
54
Figure 9-9. Schematic diagram of the central olfactory pathway. The olfactory cortex has connections with motor
systems as well as "limbic system" structures having to do with feeding, memory and emotion.
9.5. ENCODING INFORMATION ABOUT TASTE AND SMELL
Most taste receptor cells and central neurons respond to all classes of taste stimuli.
Similarly, olfactory receptor cells and central neurons respond to multiple odorants. Since there
is so little specificity in terms of which receptor cells or neurons are active, information about
taste or odor quality must be represented by activity across a population. One idea of how taste is
coded is through an across-fiber pattern, or population code, as discussed earlier. Another idea is
that taste quality is coded by the pattern of activity over time (temporal pattern). It is likely that
the neural representation of both taste and odor quality takes the form of an across-fiber pattern
that changes over time, that is, a dynamic spatio-temporal pattern.
9.6. INTERACTION BETWEEN TASTE AND SMELL
What we commonly think of as "tastes" or "flavors" are actually a combination of waterborne (taste) and airborne (smell) molecules. If we had to depend on taste alone, we would not
be able to identify many common foods and beverages such as strawberries, cherries, tea or
coffee.
_____________________________________________________________________________
Thought Question: Taste and olfactory receptor cells are constantly dying and being replaced.
Why do you think this occurs? If the neural code for taste and odor quality is activity in specific
neurons, what problems are inherent in a system with a constant turnover of receptor cells? How
do you think the chemical senses solve these problems?
_____________________________________________________________________________
55
Figure 9-8. Bar graph showing percent of subjects who correctly identified a common food flavor based on taste
alone (white bars) or using both taste and olfaction (black bars).
9.7. INDIVIDUAL DIFFERENCES IN TASTE AND SMELL
Some individuals are better at discriminating among tastes and odors than are others.
There are many factors responsible for differing abilities, and these factors are interactive. They
include:
Genetic makeup, as discussed above;
Age: Taste and odor discrimination improves during childhood, plateaus during young
adulthood to middle age, and declines rapidly with old age.
Sex: In general, women's senses of taste and smell are more sensitive than men's, but
fluctuate during the menstrual cycle or during pregnancy, indicating that hormones modulate the
acuity of the chemical senses.
Training and experience: It is possible for humans to develop extremely acute
chemosensory abilities through training. Examples are professional wine tasters and perfume
testers.
Prolonged exposure to a specific odorant often results in decreased sensitivity to that
odor.
_____________________________________________________________________________
Thought Question: If you were in the commercial food business and wanted to market a new kind
of dessert in the US, what kinds of psychophysical tests might you perform? Do you think the
tests you performed would be equally valid for markets in other countries (e.g., Finland, India or
Nigeria?) If you wanted to market the product in one of these countries, would you perform
additional tests? Why or why not?
_____________________________________________________________________________
56
Figure 9-9. Graph showing average number of odors correctly identified out of a standard set of 40 by subjects in
different age groups. Ability to discriminate odors improves up to about 15 years of age, remains relatively constant
up to about 60+ years of age, after which it deteriorates rapidly.
Figure 9-10. A rather difficult-to-read bar graph illustrating the percentage of male and female subjects who
correctly identified common odors. Each bar represents the values for males and females. For the white bars, the
lower values are for males, the higher for females. For the black bars, the lower values are for females, the higher
for males.
57