OLFACTION AS AN ORAL SENSE AND AS A TELECEPTIVE SENSE
“O God, make me to smell the odours of Paradise, and bless me
with its delights, and make me not to smell the smell of the
fires of hell.”
-Koran, quoted in Moncrieff (1967)
The sense of smell has a vividness and evocative quality that
are different from any other sense. This may result from the
intimate connections of the olfactory system with the paleocortex,
or evolutionarily older part of the brain, and with the
hypothalamus. It is thus considered to be able to evoke memories
which may be processed in the paleocortex (hippocampus and
prepyriform cortex), and to have some very primitive functions, in
connection with eating and reproduction, that are mediated by the
hypothalamic/pituitary system.
Most of our subjective judgments of odors have to do with
their pleasantness or unpleasantness. This is useful, since things
that may be poisonous to eat, such as spoiled meat or rotten eggs,
have offensive odors. As mentioned in Chapter 7, estimates of the
flavor of food are mostly based on odors, and less on tastes.
Olfaction has been the subject of studies in a wide variety
of vertebrates and invertebrates (Gesteland et al., 1965; Bang and
Wenzel, 1985; Lancet, 1986; Ache, 1987). Much has been written
about the role of pheromones which serve as odorous attractants or
repellants in insects and higher species (Schneider, 1971).
Whether humans consider sweat or other odors attractive is a
matter of debate. However, the use of perfumes, which are often
floral extracts combined with animal musk, has been practiced for
centuries.
The olfactory apparatus functions both as an exquisitely
sensitive detector of remote smells - for instance in wild
animals, to detect the presence of enemies - and also as a method
of quality control for foods about to be consumed. The French
gastronomist Brillat-Savarin in 1825 wrote that the nose "acts as
the first sentinel, crying out, 'Who goes there?'."
Anatomy of Olfactory Sensory Cells
We do not smell with our noses, but with our olfactory
epithelium. As shown in Figure 1, this is where the sensory cells
for olfaction in higher vertebrates are located, in the upper and
middle conchae of the nasopharynx. The area of this sensitive
region varies from a few cm2 in man to 100 cm 2 in dogs (Moulton and
Beidler, 1967; Graziadei. 1971).
A drawing of the structure of the olfactory epithelium is
shown in Figure 2. There are basal cells, supporting
(sustentacular) cells, and olfactory sensory (receptor) cells.
Olfactory cilia project from the sensory cells into a layer of
mucus approximately 35 m thick which covers the sensory
epithelium. Molecules of an odorant substance must diffuse across
this mucus layer before they can come in contact with the membrane
of the olfactory cell, located in the cilia. The main source of
the mucus is considered to be the sustentacular cells and the
acinar cells of Bowman's glands (Getchell and Getchell, 1987). The
olfactory sensory cells are primary sensory neurons. (In the taste
system, by contrast, endodermally-derived sensory cells transduce
taste stimuli into secretory activity that excites the connected
sensory axons.) Electrophysiological and biochemical studies have
been done with single isolated olfactory sensory cells and will be
described below.
There are also endings of the trigeminal system present in
the nasal mucosa; these are thought to be the pathway for
irritating or "stinging" sensations from the nose.
Neural Pathways for Olfactory Signals
Which is the first cranial nerve?
The axons of the sensory cells are called the fila olfactoria, and
pass through the cribriform plate of the ethmoid bone to reach the
olfactory bulb.
The relay points for the sensory axons of the fila olfactoria are
glomeruli in the olfactory bulb, as shown in Figure 5. The
glomeruli (a, b, c) contain the dendrites of the large mitral
cells (layer C), which are the main afferent pathway for olfaction. Axons of the mitral cells turn in the granular layer (E) and
become the olfactory tract. Other important cells are granule
cells (I, J), which connect to the mitral cells, and tufted cells
(d), whose axons also exit in the olfactory nerve.
Some interconnections of the cell types in the olfactory bulb
are shown in Figures 6 and 7. The fila olfactoria (FO) synapse
with mitral cells (M) and periglomerular cells (PG), in the
lowest, or glomerular layer of the bulb. PG-cell axons then
synapse on other mitral cells. Granule cells (GR) receive
efferent, or centrifugal inputs from the anterior olfactory
nucleus (AON) on the same side, and synapse on mitral cells.
Mitral cell axons then exit the bulb as the olfactory tract (OT).
Recent estimates indicate that about 2000 olfactory sensory
cells, coding for a single odorant receptor type, project into a
single glomerulus (see for example Ressler et al., 1994). With
granule and periglomerular cells, there are many synaptic
interactions and possibilities for modifications of the neural
olfactory code at the level of the olfactory bulb.
The olfactory tract eventually leads to the olfactory bulb on
the opposite side, to the prepyriform area and the pyriform lobe,
the hippocampus, and, via the amygdaloid complex to the autonomic
nuclei of the hypothlamus, where signals may influence the release
of hormones involved in reproduction.
It is possible to obtain electrical recordings of nerve activity at various parts of the olfactory pathway: Electrodes
placed against the surface of the olfactory epithelium record the
summed activity of many sensory cells when odorants are applied to
the area; this negative-going wave is called the
electroölfactogram or EOG. Summed responses of the olfactory nerve
may likewise be observed when stimulants are applied to the
olfactory epithelium (Adrian, 1950; Gesteland, 1976; Hornung and
Mozell, 1981). Action potentials may be recorded extracellularly
from single olfactory receptor neurons (Juge et al., 1979a, b).
One may also place microelectrodes inside single olfactory
receptor cells, and record generator potentials and action
potentials when odorants are applied to the preparation (Getchell,
1977a, Suzuki, 1977.
Organization of Olfactory System Studied with In-situ
Hybridization
One may wonder why the neuronal organization of this chemical
sense differs from that of taste: there is no analogue of the fila
olfactoria in the taste system, but rather a single taste nerve on
each side of the mouth. This difference might have to do with the
necessity of the olfactory sensory axons to pass through bone
before reaching the olfactory bulbs. The complex structure of the
olfactory bulb likewise is not seen in the first relay station for
taste, the nucleus of the tractus solitarius. With some recent
experiments tracing the distribution of the genes for specific
receptor molecules in the olfactory system, the function of the
bulbs is starting to become clearer:
In 1991 Buck and Axel identified proteins in the olfactory
epithelium that were coded by a large multigene family, as
putative olfactory receptors. In addition, most members of the
multigene family were shown to belong to subfamilies ranging in
size from one to about 20 genes (Buck, 1993). The individual
proteins coded by each subfamily were almost identical in amino
acid sequence. The suspicion was, that different subfamilies might
recognize different structural classes of odorants.
The next development was the finding that different receptor
subfamilies were expressed in one of four distinct regions of the
olfactory epithelium (Ressler et al., 1993; Vassar et al., 1993).
In-situ hybridization methods were used, in which probes are
constructed having the same structure as known olfactory receptor
molecules, and then applied to sections of the olfactory mucosa or
bulb. If the probes find complementary RNA in the sections, they
bind by Watson-Crick base pairing. The identified areas of
receptor protein in the section are then revealed by a visible or
radioactive marker when the probe is washed off. These studies
showed that neurons containing the same receptor molecules lay
only in one of four different expression zones. This is
illustrated in the next slide.
Part A shows the turbinates of the rat olfactory epithelium, and
Part B indicates the bilaterally symmetrical expression of
receptor genes.
Several different types of receptors may be produced in a
given expression zone, but these types do not occur in other
zones. Also, receptors of a given type are not clumped together,
but are randomly distributed throughout each single expression
zone. The next major finding having to do with connections of the
fila olfactoria was somewhat amazing: By looking at hybridization
in the olfactory bulb, it was found that specific odorant receptor
gene probes hybridize into small and distinct sets of olfactory
bulb glomeruli (Ressler et al., 1994; Vassar et al., 1994). This
is illustrated in the next slide.
Part A shows the hybridization of olfactory marker protein, which
binds to all olfactory tissue. B is a magnified view, showing
hybridization to specific glomeruli. Then in part E the applied
probe is made from olfactory receptor protein M50. Here the probe
hybridizes only to certain areas of the olfactory epithelium and
only in a few distinct glomeruli. This result, and others with
different olfactory receptor proteins, led to the conclusion that
while individual sensory neurons expressing a given receptor are
distributed throughout the expression zone of the epithelium, they
all project to one or a few glomeruli in the olfactory bulb.
This indicates there may be a receptor map in the olfactory bulb.
By looking at the activity of specific glomeruli, the nervous
system is able to identify the stimulus.
Chemical Structure and Odor
In an analogous manner to the efforts of taste researchers in
identifying four (or five or six) basic tastes (Chapter 7), some
schemes have been presented wherein classes of odors are related
to shapes of odorant molecules (Amoore et al., 1964). One often
sees lists of "compounds and their odors" such as the following:
Table I.
Types of compounds producing various odors.
_________________________________________________________________
Odor type
Compound
_________________________________________________________________
Floral
Alcohols e.g. á-phenylethyl alcohol
Fruity
Esters e.g diethyl adipate
Minty
Esters e.g. methyl salicylate
Camphorous
1,8-cineole
Musky
Ring ketones, civetone
_________________________________________________________________
This is only an attempt to establish classes of basic odors,
as for instance both fruity and minty compounds are esters. The
more general observation is that higher vertebrates can smell a
vast array of different smells, that hardly fall into classes at
all (Reed, 1990). Humans can probably distinguish over a thousand
different odors (Shepherd, 1990).
Amoore and colleagues (1964) tried to find a relation between
the shape of an odorant molecule and its odor. This is shown in
the next figure.
The shape, defined as ratio of major to minor axis in 2
dimensions, was weakly correlated with odor (next figure).
Odorant molecules may be polar or nonpolar and may or may not
have functional groups attached; this is illustrated in Figure 15.
The compound shown in Part VI is a strong musk, whereas the
hydrocarbon in Part VII with practically the same molecular shape
is odorless. In this case the carbonyl group C=O is necessary for
the musk quality to be present. IV and V are both musks, so the
functional group is unimportant in this case.
Some evidence that suggests the presence of at least some
primary odor classes is that of specific anosmias. Some different
types of anosmia have been found in humans (Amoore, 1971); for
instance, approximately 1% of the European population is insensitive to musks. They evidently are deficient in a gene or have a
modified gene for at least one type of olfactory receptor molecule
(Lancet, 1986).
Davies and Taylor (1959) formed an early theory of olfactory
strength, or the amount of an odorant required to reach threshold.
They assumed that the threshold depended on the adsorption energy
of the odorant molecules going from the air into the lipid-aqueous
olfactory cell membranes. From this theory they calculated the
olfactory thresholds of several classes of compounds based on
known adsorption energies.
The sense of smell, working at a distance, is almost as sensitive as is theoretically possible: In some insect chemoreceptors
a single molecule of odorant per receptor cell is sufficient to be
detected. We can estimate the sensitivity in humans by knowing the
concentration of the odorant in the inspired air: For the garlicky
compound butyl mercaptan, for instance, the threshold
concentration is about 1010 molecules/liter of air. By using
estimates of the number of olfactory cells and the volume of a
sniff of air, one can calculate the threshold as about eight
molecules of mercaptan per sensory cell per sniff.
Electrophysiology of Olfactory Sensory Cells
Analysis of current flows in olfactory epithelium when
electrodes are placed at various depths has suggested that the
action potentials arise in an area of the sensory neuron that is
relatively remote from the ciliary odorant receptors (Getchell,
1977; Getchell and Getchell, 1987). The mechanism is shown in
Figure 17: The transduction current produced by the formation of
odorant-receptor complex in the cilia flows centripetally down the
sensory cell body. Action potentials are intiated in the cell-body
region of the sensory neuron, near the point where the cell tapers
in an axon hillock before the axon.
More recently, it has been possible to do "patch clamp" experiments with isolated cilia from olfactory sensory cells of
toads (Nakamura and Gold, 1987). These reveal the intimate
membrane currents produced in the active zones of olfactory cells,
when various odorants are applied to them. One interesting result
is that the nucleotide cyclic adenosine monophosphate (cAMP) has a
direct gating effect on ionic conductances in the membrane
(Lancet, 1986; Gold and Nakamura, 1987; Anholt et al., 1989).
Cyclic AMP may serve as an intracellular second messenger between
olfactory stimulants coming in contact with the sensory cell
membrane and the resulting changes in ionic conductances, as
outlined in Figure 18.
The receptor molecule (R) has a so-called constant (c) and
variable (v) region for different odorants. When an odorant occupies the receptor, a GTP-binding protein (G) is activated, which
modulates the activity of the adenylate cyclase (C) catalyzing the
production of cAMP. This intracellular messenger activates a
protein kinase to cause phosphorylation of the ion channel
polypeptides, and opening or closing the associated ion channels.
This type of cascade has also been suggested for certain taste
receptor mechanisms (Chapter 7).
It is possible to make suspensions of homogenized olfactory
epithelium (Vodyanoy and Vodyanoy, 1987) or detached olfactory
cilia (Labarca, et al., 1988) that contain both olfactory receptors and some associated ionic channels. These suspensions may
then be added to artificial lipid bilayers membranes, and the behavior of the receptor/channel complexes studied under controlled
conditions. In the case of epithelial homogenates, both ATP and
GTP must be present in the bathing solutions for the olfactant
response to occur.
Possible Mechanisms of Odor Discrimination
To explain the detection of specific odorants by the olfactory system, current studies have pointed toward the existence of
perhaps 1000 different genes coding for specific receptors (Buck,
2000). Each odorant receptor gene is expressed by only ~0.1% of
the olfactory sensory neuron population, suggesting that each
sensory neuron may express only a single receptor type. This
contrasts with color vision, for instance, where only three
receptor types are present in the entire population of retinal
sensory cells, and taste, where only about four different tastes
can be distinguished, suggesting that only a few receptor types
exist. The sense of smell may involve many receptors that are able
to bind with one or a small number of odorants; since there are so
many structurally diverse odorous ligands, it is likely that there
would be a large number of different receptor types.
Thus, the olfactory sense may detect up to a thousand
different odors by using individual receptors. Subfamilies of
receptors may detect structurally similar compounds. Receptors of
the same type project to only a few glomeruli in the olfactory
bulb. Each glomerulus that receives input from a given receptor
type is specific for a given epitope on an odorant. Since odorants
contain more than one epitope, a particular odorant is recognized
by the pattern of glomeruli that it excites.
In taste, where each cell expresses many types of receptors,
hundreds of different compounds may be detected, but only four of
five discriminated. Another difference is that of sensitivity; the
threshold for detection of odors is in the micromolar range, while
in taste it is in the millimolar range.
Psychophysics of Smell
Olfactory acuity may be measured by testing subjects' thresholds
for detection or recognition of odors. As with other sensory
modalities (see Chapter 1), it is possible to measure subjects'
perceived sensation magnitudes by asking them to supply a number
for each different stimulus strength tested (Cain, 1978). The
variation of sensation magnitude with odor intensity (odorant
concentration) is best described by a power function
 = KSn. The
value of n varies from odorant to odorant, between 0.2 and about
0.7. Using this measure of sensation magnitude, we can answer two
questions about the property of adaptation, or decreasing
awareness of smells even though the odorant is still present: (1)
What is the time-course of adaptation? and (2) does the sensation
decrease to zero with time or to some steady-state level greater
than zero? The result is shown in Figure 20. In this case the
odorant was n-butyl acetate, which has a fruity odor. The
perceived magnitude was the average estimate of a panel of
observers. We can see that the adaptation took place in a few
minutes, and that the final level was about 30% of the initial.
Thus, adaptation alone will not solve the problem of an unpleasant
odor.
In the United States we generally prefer our environment to
be almost odorless. This has resulted from advertising of products
to reduce or "kill" odors from foods, animals, etc. The most
common method of odor reduction is counteraction, where mixing the
malodor with a strong but acceptable smell decreases the perceived
odor of both (Cain, 1978). (The method of masking, much less
effective, is just to drown out one odor with another.) Thus,
pine-scented deodorizers counteract smells from pet stains, etc.
simply by making the olfactory apparatus less sensitive to the
offending odors.
Aging and Olfaction
The generally-stated assumption that aging is associated with
decreased ability in all the senses may be misleading. For one
thing, as mentioned in Chapter 8, the loss of appreciation of food
may be due to an impairment of smell rather than taste (Schiffman,
1979; Deems et al., 1991). The age-specific effects on olfaction
have recently been studied in a large population (Doty et al.,
1984; Doty and Snow, 1988). A survey of the results is shown in
Figure 21.
The UPSIT value on the ordinate (for University of Pennsylvania
Smell Identification Test) is a score that is inversely correlated
with recognition thresholds for a group of 40 odorants. Females in
the group outperform males at all ages, that is, they can
recognize odorants at lower concentrations. Recognition ability is
maximal from about 30 - 60 years, then declines steadily. Factors
that may contribute to this loss are (1) drying or other
alterations of the nasal epithelium, (2) death of receptor cells
(Nakashima et al., 1984), and (3) loss of neurons in the olfactory
bulb (Smith, 1942). Clearly, the biochemical mechanisms discussed
above may also be altered during life.
In comparing the chemical senses with others such as touch,
then, one is struck with the greater number of steps in the
process, both at the level of the sensory ending and of the subsequent neural pathway. Chemical stimuli must first be dissolved
in an aqueous medium before encountering the receptors on the cell
membrane. The transformed (dissolved) stimuli must then diffuse to
the cell membrane area and bind to the receptors, which must be
present. Metals or other cofactors may be required. The binding of
the ligand to the receptor probably sets off a cascade of
reactions involving an intracellular second messenger. If all goes
well, ionic channels will be opened and action potentials produced
in the sensory pathway. It is not surprising that losses occur in
these systems before they do in the skin senses.
We shall now consider some of the ways in which our senses are
blocked by general and local anesthetics.