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CHAPTER 14 Feeling, Smelling, andTasting 271
Noting specialized somatosensory areas
Primary somatosensory cortex (S1) is essentially Brodmann area 3, which receives
the largest direct thalamocortical projection, although a few direct projections
may go to Brodmann areas 1 and 2 as well. Area 3 has been subdivided into areas
3a and 3b. Neurons in 3b are selectively responsive to typical tactile, temperature,
and pain stimuli, whereas neurons in 3a appear to receive proprioceptive inputs
about joint position and muscle forces.
Brodmann areas 1 and 2 are generally now considered higher-order (association)
somatosensory areas (S2) that receive most of their input from area 3b rather than
directly from thalamus. Neurons in Brodmann area 1 respond to stimulation of
Pacinian corpuscles (refer to the “Localizing touch” section earlier in this chap-
ter) that encode some aspects of texture information. Area 2in S2 has neurons
that are involved in active touch for determining the shape of objects.
Inputs to somatosensory cortex project into cortical layer IV.
Getting complicated with pain
The primary cortical areas that receive pain receptor projections have been di-
cult to locate. The spinothalamic tract ends in the VPM, VPL, intralaminar, and
medial dorsal nuclei of the thalamus. The ventral and medial dorsal thalamic
nuclei that receive input from pain bers project directly to secondary cortical
areas. Pain information from the thalamus is transmitted directly to S1 and also to
S2, the insula and anterior cingulate cortex. The perception of pain tends to be
associated with activity in the insula and ACC.Pain axon projections also travel to
the lateral nucleus of the amygdala and the hippocampus. Activating the amyg-
dala is associated with immediate emotional pain responses.
Some ascending spinothalamic axons synapse on brainstem nuclei, including the
parabrachial nucleus (PAB), rostral ventromedial medulla (RVM), and periaque-
ductal gray (PAG). These nuclei transmit pain information to the brain and are
also involved in descending pain pathways. The PAG is largely responsible for
descending modulation of pain perception. It receives inputs from the ACC, insula,
and amygdala and has two major descending pathways involving the RVM and the
locus coeruleus.
The somatosensory cortical areas for the hands and ngers are located close to
areas for the face (refer to Figure14-2). The phenomena of what’s called “phan-
tom limb” feelings (including pain) that some people experience after losing a
limb may occur because neural projections from the face invade the neighboring
part of the cortex that was once devoted to the limb. Face sensation that activates
these inappropriate inputs may be interpreted as pain in the limb, even though
the limb is no longer there.
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272 PART 3 Understanding Perception and Cognition
Pain is also complex because psychological factors can strongly inuence it. Men
tend to be less tolerant of chronic pain than women are, though men are more
tolerant of acute pain. Athletic training and strong motivation to obtain some goal
can signicantly reduce the disabling eects of pain. Pain tolerance generally
increases with age. Dierent cultures also tend to have dierent levels of pain
tolerance— cultures that encourage emotional expression typically report lower
pain tolerance.
Sning Out the World around You
Whereas senses like vision and hearing require the detection of energy (light pho-
tons and sound waves), olfaction (smell) involves detecting actual chemicals,
odors, from the world. You smell using your nose, which also lters, warms, and
humidies the air you breathe and analyzes that air for odors. The nose has olfac-
tory receptor cells in its roof. These neurons have cilia that stick down into the
mucus that lines the roof of the nose that respond to dierent odors. Figure14-3
shows the location of the olfactory receptor cells in the nose and the olfactory bulb
that we discuss a bit later.
Olfactory receptor cells are the only type of special sense neurons that contain
both sensory dendritic endings and an axon, and they produce action potentials.
Olfactory receptor cells have a lifespan of about 60 days and are constantly
replaced from progenitor cells within the nasal epithelium throughout life. Olfac-
tory and taste receptor cells are the only sensory neurons that can be regenerated
in the adult.
FIGURE14-3:
Olfactory
receptors and the
olfactory bulb.
© John Wiley & Sons, Inc.
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CHAPTER 14 Feeling, Smelling, andTasting 273
Most mammals use a larger percentage of their brain for olfactory processing
compared to humans. A dog has about 300 million olfactory neurons, compared to
about 2.5 billion total brain neurons. Humans have 5 to 10 million olfactory recep-
tor cells in an 86 billion neuron brain. Here we describe olfactory receptor cells
and how they transduce odor sensation.
Nosing around: Olfactory receptors
Odor molecules, also called odorants, are small volatile chemicals that pass through
the nasal passages and dissolve in the mucus that lines it. Odorants can be classi-
ed as either pleasant or oensive, and into categories such as fragrant/oral,
fruity, citrus, grassy, herbal, sweet, and so forth.
Olfactory receptor cells express odorant receptors on their sensory dendritic end-
ings. Odorant receptors are G protein-coupled receptors. There are about 1,000
dierent odorant receptor genes, the largest receptor gene family.
Binding of odorants to odorant receptors leads to the ring of action potentials in
the olfactory receptor cell. This occurs through a G protein and second messenger
pathway. In mammals, odorants activate a G protein called G
olf
. G
olf
stimulates the
enzyme adenylyl cyclase to synthesize cAMP, which is a second messenger. cAMP
binds to and opens a cyclic nucleotide gated (CNG) ion channel. Opening this
channel produces an inux of cations leading to a depolarization of the neuron.
The calcium inux opens a calcium-activated chloride channel, leading to an
eux of chloride that depolarizes the neuron and triggers an action potential in
the axon.
Humans have approximately 1,000 dierent odorant receptor genes in our
genome. Each olfactory receptor cell expresses only one type of odorant receptor.
However, each odorant can bind to several types of odorant receptors to produce a
unique code of olfactory receptor responses. The brain identies the odor by com-
paring the relative activity from dierent olfactory receptor neurons (like color
vision, but with much higher dimensionality).
Exploring the olfactory bulb
Olfactory receptor cells project their axons, which form the olfactory nerve, to the
olfactory bulb in the brain, just above the back of the nose (refer to Figure14-3).
Within the olfactory bulb are about 1,000 to 2,000 dierent small zones called
olfactory glomeruli. There, synapses form between the terminals of the olfactory
receptor axons and the dendrites of neurons called mitral, periglomerular, and
tufted cells.
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274 PART 3 Understanding Perception and Cognition
Each glomerulus receives input from olfactory receptor cells expressing only
one type of olfactory receptor. A large degree of convergence occurs, with
approximately 25,000 axons synapsing on 25 or so mitral cells; each mitral cell
can project dendrites to multiple glomeruli. Mitral cells also receive information
from periglomerular cells and granular cells that inhibit the mitral cells
surrounding it, through lateral inhibition. The mitral cell bodies and tufted cell
bodies are located outside the glomeruli and their axons are the main outputs
from the olfactory bulb.
Most odors that humans detect actually contain a complex mix of odors. Coee
aroma, for example, contains at least 100 dierent kinds of odor molecules, mean-
ing that the odor of coee generates more than 100 dierent signatures simulta-
neously in the olfactory glomeruli. Your brain still needs to do a bit of processing
before you can recognize what you’re drinking.
The problem of multiple odorants is mitigated somewhat by the fact that dierent
odorants have dierent transit times across the olfactory mucosa. Each sni pro-
duces a synchronized input to the nose so that the odorant responses are spread
out over time. This is sometimes referred to as the chromatographic theory of odor
perception. Proteins in the mucosa called olfactory binding proteins help some non-
soluble odorants enter and transit the mucosa to reach the receptor cilia.
Reaching the cortex and amygdala
The axonal projections from the olfactory bulb are unique among the sensory sys-
tems because they travel directly to several areas of the cortex without rst relay-
ing through the thalamus. The mitral cell and tufted cell axons leave the olfactory
bulb in the lateral olfactory tract. Mitral or tufted cell axons synapse on the pri-
mary olfactory cortex in the inferior temporal lobe. The olfactory cortex includes
the piriform cortex, entorhinal cortex, periamygdaloid cortex, anterior olfactory
nucleus, and olfactory tubercle.
Axons from the olfactory bulb also project directly to the amygdala. This connec-
tion provides an immediate emotional response to some odors before you’re con-
sciously aware of their identity. Figure14-4 shows the main olfactory projection
scheme (though several areas aren’t illustrated).
The olfactory bulb projects to the following areas. Most are shown in
Figure14-4:
»
The amygdala: The amygdala mediates emotional responses to odors and
projects to the hypothalamus to mediate hormonal responses to odors.
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CHAPTER 14 Feeling, Smelling, andTasting 275
»
The entorhinal cortex: The entorhinal cortex projects to the amygdala and
the hippocampus, allowing you to remember odors, particularly if they were
part of an important experience.
»
The piriform cortex: The projection to the piriform (pear-shaped) cortex is an
ancient pathway, which makes unconscious and reactive odor discrimination
possible. The piriform cortex is allocortex lying just ventral to the amygdala
and contiguous with the entorhinal cortex. It projects to the mediodorsal
thalamus.
»
The olfactory tubercle: This is a multisensory processing center. It plays a
role in the integration of olfactory information with other senses, reward
cognition, and locomotion and attentional behaviors.
»
The periamygdaloid cortex: This area conducts cognitive evaluation of
olfactory input it receives and projects back to the olfactory bulbs.
»
The anterior olfactory nucleus; This nucleus is found in front of the piriform
cortex. It plays a pivotal but relatively poorly understood role in the processing
of odor information.
The mediodorsal thalamus is reached indirectly by relay projections from the
olfactory cortex. This thalamic region projects to the piriform cortex and
orbitofrontal cortex and mediates conscious odor discrimination. Some odor
information also merges with taste here to give the perception of avor.
Smell is dierent from the other senses in that smell often intrinsically generates
an emotional response. Many smells are either immediately disgusting or dis-
pleasing (such as feces or spoiled food) or pleasing (such as owers or chocolate
cake). The olfactory system has an evolutionarily older pattern with a direct
FIGURE14-4:
Projections of the
olfactory bulb.
© John Wiley & Sons, Inc.
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276 PART 3 Understanding Perception and Cognition
projection to cortical and memory structures. This pathway mediates quick
responses to elemental smells, such as something rotting. The amygdala is located
anterior to the hippocampus and has a similar memory function to it (refer to
Chapter15), except that it processes emotionally salient memories. In the case of
olfaction, it stores memories for which a smell was the trigger for emotional
salience, such as the odor of a particular food before you get sick.
The amygdala projects to the olfactory thalamus and frontal lobe. The direct pro-
jections from the olfactory bulb to the amygdala appear to be part of a neural
circuit that mediates learning associated with approach/withdrawal behavior for
smells. A single negative incident can trigger lifelong aversion to getting sick after
eating a particular food— even if the food isn’t spoiled.
One pathway through the mediodorsal thalamus mediates responses that depend
on more nuanced identication of odors that requires higher-order cortical pro-
cessing to identity before being stored in a person’s memory. This is the smell
processing that doesn’t generate an immediate limbic-like behavioral response,
but that mediates conscious aspects of odor detection, as occurs in all other senses.
The dierence between the overall anatomical structure of the olfactory system
and other sensory systems may be due to its early evolution in mammals. Most
mammals depend more on olfaction than primates do, and olfactory-induced
behavior is generated directly. This is a proposed function of the limbic system
(refer to Chapter10). Olfactory processing that wasn’t directly linked to behavior
happened when a phylogenetically newer thalamic pathway was created. The
pathway was created by projections from olfactory cortex to thalamus, which then
projects back to cortical areas.
Smelling badly versus smelling bad
Continuous exposure to noxious substances that damage the tissue can reduce or
destroy the sense of smell. The inability to perceive odors is called anosmia and is
most often a result of damage to the olfactory mucosa and olfactory receptors.
Temporary anosmia typically is caused by mucosa inammation from infections
such as cold viruses, minor head trauma, or Covid. Olfactory neurons are among
the few CNS cells that regenerate constantly, so receptors are always being repop-
ulated. A less than total loss of smell is called hyposmia, which can occur for spe-
cic odors.
Congenital anosmia is a lack of the sense of smell from birth, and it’s usually
genetic. Having a greater than normal sense of smell is referred to as hyperosmia.
Women typically have a better sense of smell than men do.
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CHAPTER 14 Feeling, Smelling, andTasting 277
Permanent loss of smell may occur from death of olfactory receptor precursor
neurons. Like other senses, it also can occur from damage to the olfactory nerve
or damage to olfactory brain areas. Schizophrenia, Parkinson’s, and Alzheimer’s
diseases that aect the frontal lobe are sometimes associated with hyposmia or
anosmia.
Communicating with pheromones
Smell contributes to sexual behavior both consciously and unconsciously. For
example, the menstrual cycles of women who live together or near each other tend
to synchronize unconsciously through smell. Odors induce an attraction toward
other individuals, as the multibillion-dollar perfume industry proves.
The nose contains a specic organ of smell near the vomer bone called the vomer-
onasal organ. This organ appears to have receptors for human sexual odors that are
unconsciously processed by nonthalamic odor pathways. The biological term for a
communication odor that inuences species-specic behavior in animals is a
pheromone. Humans produce pheromone-like odors via glands similar to those in
other mammals; these odors act on similarly located receptors, have similar cen-
tral projections, and result in similar unconscious behavioral eects.
Tasting Basics: Sweet, Sour, Salty,
and Bitter Receptors
Humans, like all animals, must be able to discriminate between nontoxic and toxic
plants, ripe and unripe fruit, and fresh and spoiled meat. Because what you taste
is already in your mouth about to be ingested, you need to make these discrimina-
tions rapidly and accurately.
As taste senses both harmful and benecial things, all basic tastes are classied as
either aversive or appetitive, depending upon the eect the things they sense have
on your body. Sweetness helps to identify energy-rich foods, whereas bitterness
serves as a warning sign of poisons. These sections discuss taste receptor cells and
mechanisms of taste sensation.
Grasping the basics of taste
Taste is the sensation produced when a chemical in the mouth activates taste
receptor cells located on taste buds in the oral cavity. Receptors for taste reside
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278 PART 3 Understanding Perception and Cognition
mostly on the tongue in structures called papillae (see Figure14-5) that appear as
little bumps on the tongue. The tongue has four types of papillae: fungiform, foli-
ate, circumvallate, and liform.
The central portion of the tongue has almost exclusively liform papillae; taste
doesn’t occur there (the liform papillae don’t have taste receptors but have a
mechanical function in some species that use rough tongues to rasp, or grate,
ingested food). Fungiform, foliate, and circumvallate papillae contain taste recep-
tor cells. These papillae are located at the tip, sides, and back of the tongue. The
tongue also has somatosensory receptors.
When you think of the sense of taste, you probably think of four basic tastes:
»
Sweet: The sweet taste comes from receptors that respond to sugars (such as
glucose and fructose), the source of energy for metabolism. Many human
taste receptors respond almost exclusively to sugar.
»
Salty: Salt (NaCl) constitutes the major solute in the uid throughout the
human body. A large number of taste receptors also respond almost exclu-
sively to NaCl salt. However, other salts, such as potassium chloride can also
activate the same receptors.
»
Sour: The sour taste is the detection of acidity, so that sour receptors react to
protons (H
+
ions). Sourness is a characteristic of foods that many people
FIGURE14-5:
Taste receptors
on the tongue.
© John Wiley & Sons, Inc.
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CHAPTER 14 Feeling, Smelling, andTasting 279
enjoy, such as lemons, but it can also indicate spoilage. So, the reaction to the
sour taste is complex and depends on learning.
»
Bitter: Bitterness, like sourness, is sometimes a sign of food being unripe or
toxic. The prototypical bitter substance is quinine. Bitterness is one of the
most sensitive of the tastes, and many perceive it as unpleasant, sharp, or
disagreeable. You learn that the occurrence of bitter taste in some foods
means you should avoid them, while other foods may contain bitter spices
but are okay to eat.
In addition to the four traditional tastes, many neurobiologists now include a fth
basic taste, umami, a meaty, MSG-like taste that comes from the Japanese term
meaning “savory and pleasant taste.” This receptor responds to the amino acid
L-glutamate. Receptors may also be activated by lipids-fats, and CO
2
, activated by
carbonated liquids. Refer to the section “Understanding the umami problem” for
more about umami.
Most taste receptors respond to dierent tastes, and most taste buds have several
dierent types of receptors. The taste buds contain about 10,000 taste receptor
cells in total. Receptors for all the basic tastes are located in all non-liform
papillae types (fungiform, foliate, and circumvallate) throughout the tongue.
However, the tongue has regional dierences in the percentage of receptors,
such as relatively enhanced taste for sweet at the front of the tongue and bitter at
the back.
Transducing dierent ways
Taste receptor cells don’t re action potentials. They communicate using a nearby
gustatory neuron. Tastants depolarize the taste receptor cell, which stimulates the
release of neurotransmitter. The neurotransmitter for taste receptor cells is ATP
at the synapse onto gustatory neurons.
Saltiness is produced typically by the presence of sodium ions. A sodium channel
called ENaC in the plasma membrane allows sodium ions to enter the cell, which
depolarizes the taste cell. Sourness is the taste that detects acidity (protons). Pro-
tons directly enter through plasma membrane proton (H
+
) channels, such as the
Otop1 channel. Protons inhibit potassium channels. Together the inux of protons
directly and indirectly depolarize the cell.
Receptors for sweet, bitter, and umami are G protein-coupled receptors that share
a similar intracellular signaling cascade to transduce tastants. TAS1R family
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280 PART 3 Understanding Perception and Cognition
members constitute the sweet and umami receptors. The TAS2Rs family members
are the bitter receptors. Activation of the G protein leads to stimulation of the
phospholipase C (PLC) pathway (see Chapter5). Stimulation of PLC produces the
second messengers DAG and IP
3
. IP
3
releases calcium from the ER.The calcium
activates members of the TRP channel family, TRPM5 and TRPM4, which allow
sodium and calcium inux and depolarization of the membrane potential.
Recent research reveals a potential taste receptor called the CD36 receptor, which
is a possible lipid taste receptor because it binds to fat molecules (more speci-
cally, long-chain fatty acids).
Coding for taste: Labels versus patterns
Taste receptor cells control the output from gustatory neurons. The axons from
gustatory neurons are part of the chorda tympani (a branch of the facial nerve)
and the glossopharyngeal nerves. These nerves transmit taste information to
brainstem nuclei, which we discuss later in this section. Some brainstem neurons
receive inputs from similar receptors, such as salt bers, whereas others receive a
mix of inputs from dierent receptor cell types.
Certain receptors for saltiness and, to some extent, sweetness appear to be highly
specic, so the brain can consider activation of these receptors as labeled line recep-
tors reporting the presence of salty or sweet. However, a large percentage of taste
receptors respond to some degree to salty, sweet, sour, and bitter tastes, leaving it
to the higher brain centers to identify taste from the ratio of receptor activation
distributed across a number of receptors. What’s unique about taste is that the com-
bination of labeled line and distributed coding strategies exists in the same nerve.
Understanding the umami problem
The taste umami is based on receptors that are activated by L-glutamate. These
receptors also respond to guanosine monophosphate (GMP) and inosine mono-
phosphate (IMP). The perception generated when these taste receptors are acti-
vated is described as pleasant or meaty. However, the umami taste is pleasant only
within a narrow concentration range, which also depends on the salt concentra-
tion. Some older people like the umami taste because of their generally lowered
taste and smell sensitivity.
Although the Japanese scientist Kikunae Ikeda proposed umami as a basic taste in
1908, its status was debated until the mid-1980s. The ribonucleotides GMP and
IMP amplify the umami taste and are used to increase the palatability of many
foods because of the synergistic eect between GMP and IMP and glutamate.
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