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CHAPTER 14 Feeling, Smelling, andTasting 261
Chapter14
Feeling, Smelling,
andTasting
T
he skin, the largest organ in the body in terms of area, is the protective bar-
rier between you and the rest of the world. You get a lot of information
through receptors in the skin for the senses of touch, temperature, and pain
so you can interact optimally with the physical world.
Smell and taste are senses that also tell you a lot about the world. Is that food safe
to eat? Are you near a toxic substance? These senses also work together to create
avor and are strongly associated with memory.
In this chapter, we explain how these three senses work and how information
about touch, odors, and tastes from the outside world travels in the nervous sys-
tem and projects to the brain so you can perceive them.
IN THIS CHAPTER
»
Contacting the world through
the skin
»
Knowing what you’re touching
»
Smelling your way around
»
Acquiring the basics of sweet, sour,
salt, and bitter tastes
»
Giving the brain a taste of the action
»
Detecting substances that enter
the body
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262 PART 3 Understanding Perception and Cognition
Getting in Touch with the Skin
You feel all sorts of dierent touch sensations with your skin: tickle, pressure, and
movement. You also perceive three other important sensations: temperature, itch,
and pain. For these dierent somatosensory perceptions, you have dierent kinds
of skin receptors that transmit information to the brain. These sections describe
the dierent types of somatosensory receptor cells and what they respond to.
Feeling your way with mechanoreceptors
The skin senses are called somatosensation. The somatosensory receptor cells in the
skin allow you to have two senses of touch:
»
Passive touch: It’s when something contacts your skin. Passive touch
receptor cells allow you to identify this contact, such as a mosquito landing
on your arm or the feel of the back of the chair in which you’re sitting.
(Perhaps at this very moment?)
»
Active touch: Most active touch happens with your hands and ngertips. This
sense is precise enough that you can hold an egg in your hand without
damaging or dropping it. Dierent kinds of receptor cells in the skin mediate
the many active touch senses and skin perceptions you feel.
Humans and other mammals have four types of somatosensory receptor cells:
»
Mechanoreceptors are for the senses of touch.
»
Thermoreceptors are for the senses of warm and cold temperatures.
»
Pruriceptors mediate the sense of itch.
»
Nociceptors are for pain sensation.
The cell bodies of all somatosensory receptor cells are located in the dorsal root
ganglion outside the spinal cord or in sensory ganglia outside the brainstem. The
somatosensory cell body projects an axon that immediately branches. One branch,
the peripheral axon, extends to the skin while the other branch, the central axon,
extends to the spinal cord or brainstem.
The axonal endings in the skin produce action potentials when they’re stimulated
by a mechanical force, temperature, itch, or pain stimulus. The action potentials
travel toward the soma along the peripheral axon, then proceed along the central
axon toward the CNS.The central axon makes synapses with neurons in the spinal
cord or brainstem for the relay to the thalamus, then on to a cortical strip in the
parietal lobe where a somatosensory map of the body and head exists. Somatosen-
sory receptors also provide input to local reex circuits.
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CHAPTER 14 Feeling, Smelling, andTasting 263
Breaking down the skin’s structure
The skin has two major layers with dierent properties:
»
Epidermis: The outermost layer of the skin, the epidermis consists of several
layers of dead cells that provide your body with mechanical protection from
the outside world. It also contains some live cells. At the bottom of the
epidermis are stem cells that constantly divide, migrate outward, and die to
replace dead layers as they wear o.
»
Dermis: Below the epidermis is the dermis, the living layer of the skin that
contains the majority of somatosensory receptor cells. Some mechano-
receptors are located close to or at the junction between the epidermis
and dermis. The free nerve endings of temperature and pain receptor cells
often extend into the epidermis.
By dragging your ngernail lightly along your skin, you can easily remove a layer
or two of epidermis. It doesn’t hurt because these cells are dead, and this outer
layer doesn’t contain pain receptor endings.
Dierentiating quick touch versus pressure
Mechanoreceptors in the skin respond to various kinds of pressure. Dierent
types are sensitive to dierent characteristics of force applied to the skin, such as
the rate of change and intensity. Figure14-1 shows the various types of mechano-
receptors and their locations in the dermis layer of the skin. Mechanoreceptors
located closer to the epidermis have smaller receptive elds, whereas receptors
located deeper in the dermis typically have larger receptive elds.
FIGURE14-1:
Mechano-
receptors
in the skin.
© John Wiley & Sons, Inc.
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264 PART 3 Understanding Perception and Cognition
Localizing touch
In mechanoreceptors, the endings of their peripheral axon are encapsulated by
connective tissue or specialized cells. The four anatomical and functional types of
skin mechanoreceptors are as follows (refer to Figure14-1):
»
Merkel disks: These disk-shaped receptor cells for pressure are located typically
near the border between the dermis and the epidermis. The axon endings are
surrounded by a specialized cell that detects touch. They respond to relatively
constant pressure over small areas of the skin, as well as texture and skin motion.
»
Meissner’s corpuscles: These receptors also respond to pressure over small
areas, but they can respond to more rapid changes in pressure than Merkel
disks. They respond to light/sensitive touch and stroking movements. The
sensation evoked when the Meissner corpuscles are stimulated is called utter.
»
Runi endings: These receptors respond to cutaneous tension and skin
stretched over large receptive elds. They’re important because they detect
forces likely to tear the skin.
»
Pacinian corpuscles: These are the fastest responding of all the touch
receptors. Typically located deep within the dermis, they have a large connec-
tive tissue wrapping that allows them to respond to rapid changes in pres-
sure. They mediate the perception of tactile texture. If you drag your ngertips
across a coarse surface like sandpaper, these receptors mediate how you
perceive the coarseness of that surface.
Understanding sensory transduction in
mechanoreceptors
When pressure or touch is applied to a mechanoreceptor, the plasma membrane
becomes deformed, leading to the opening of mechanosensitive ion channels.
Most mechanically gated channels are cation channels. The transient receptor
potential (TRP) channels are a family of cationic channels that act as signal trans-
ducers in several type of sensory neurons. Dierent TRP channels can be activated
by dierent sensory stimuli, including mechanical force, temperature, or chemi-
cals. More than 50 TRP genes having been identied.
When opened, these mechanosensitive channels allow sodium and calcium ions to
ow into the axon. This resulting depolarization is called a receptor potential or gen-
erator potential. If the receptor potential is large enough, it triggers an action poten-
tial in the axon. The action potential is conducted along the peripheral axon and
central axon to the spinal cord or brainstem where the presynaptic terminus releases
glutamate at their synapses with target neurons. Mechanoreceptors may undergo
adaptation. This reduction of the response over time allows you to ignore irrelevant
constant stimuli and also increases the dynamic range for stimulus detection.
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CHAPTER 14 Feeling, Smelling, andTasting 265
Employing receptor cells for temperature,
itch, and pain
In addition to touch sensations, the skin also senses temperature, itch, and pain.
Receptor cells for these stimuli are similar to mechanoreceptors in overall cellular
morphology and location, but their axon terminals have free nerve endings that
aren’t encapsulated by specialized tissues. The free nerve endings are located
close to or in the epidermis (refer to Figure14-1).
Responding to temperature and itch
Free nerve endings for innocuous temperature have ion channels that open in
response to particular temperatures. Warmth receptors respond best to tempera-
ture ranges above body temperature, while cold receptors respond best to tem-
perature ranges below it. You can judge temperature quite precisely by sensing the
unique ratio of activity of dierent receptor types that are activated at any partic-
ular temperature.
Members of the TRP family of gated ion channels are thermosensitive channels.
An example is the TRPV1 channel, which is opened by temperatures between 50
and 55 degrees C.Interestingly TRPV1 is polymodal because it can also be activated
by the chemical capsaicin from chili peppers. That’s why eating food spiced with
chili peppers can produce a sensation of warmth or heat in your mouth. Eating
mint has the opposite eect – it gives the perception of coolness. Other TRP chan-
nel family members are activated by dierent temperatures and chemicals,
including both innocuous and noxious heat and cold.
Pruriceptors mediate the feeling of itch. Getting a mosquito bite or coming in
contact with poison ivy can lead to the release of chemicals, like histamine in the
skin. Pruriceptors detect chemicals that activate ion channels that produce a
receptor potential. Some pruriceptors can also be activated by mechanical or tem-
perature stimuli. Pruriceptors may be closely related to nociceptors, but they pro-
duce a dierent sensation and behavioral response.
LOSE IT AND USE IT
Active touch requires feedback from the skin, as well as position and force receptors
in the joints and muscles. Losing feeling in a limb is so disabling that people without
feeling from a limb refuse to use it, even if the motor neuron control circuitry is
actually intact. A treatment called constraint-induced therapy (CIT) forces a patient
to use their paralyzed arm while the other good arm is temporarily restrained in
a sling. With practice patients often achieve the necessary two hand dexterity for
common two
-armed life tasks such as tying shoes.
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266 PART 3 Understanding Perception and Cognition
Reacting to complex aspects of pain
Nociceptors are stimulated by noxious stimuli that indicate the skin has been or is
about to be harmed. They include extreme heat and cold, intense force, and nox-
ious chemicals that contact the skin or chemicals released by tissue damage. Pain
protects your body from injury.
Mechanosensitive nociceptors are activated by intense force. They express
mechanically gated ion channels that produce receptor potentials.
Thermosensitive nociceptors are activated by extreme heat or cold. Chemosensi-
tive nociceptors are activated by noxious chemicals, like acids or bases that come
in contact with the skin. This type of nociceptor can also be activated by chemicals
that are released inside the skin by tissue damage, including bradykinin, prosta-
glandins, potassium, and substance P.
Thermoreceptors, pruriceptors, and nociceptors have neuronal morphologies that
are very similar to mechanoreceptors (except at their endings). Dierent somato-
sensory neurons respond to dierent types of stimuli, but they all use a similar
mechanism of sensory transduction. The stimulus leads to the opening of a gated
ion channel, which increases the inux of cations that generate the receptor
potential. Depolarization triggers action potentials that are conducted along the
axons to the spinal cord or brainstem, and they release glutamate at synapses
with target neurons.
Although pain is usually associated with something dangerous happening to a
particular part of your body, pain can arise from other, unknown causes. Chronic
pain that isn’t associated with any identiable physical source can cause long-
lasting depression. The brain has no known map for pain, like the maps in the
brain for other skin senses.
FEELING NO PAIN
Attitude and mood can strongly inuence the perception of pain. Distraction or neces-
sity can cause you to ignore or not even feel pain, while anxiety can intensify the percep-
tion of pain associated with stimuli that cause it. Being able to inuence pain sensation
with the mind is well documented in cases such as soldiers ignoring signicant wounds,
yogis tolerating extreme cold, and women feeling reduced pain in childbirth from
hypnosis.
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CHAPTER 14 Feeling, Smelling, andTasting 267
Reducing pain with endogenous opioids
The pain system in the CNS involves a unique set of neurotransmitters in the
modulation of pain transmission and perception. Endogenous opioids are neuro-
peptides called endorphins (“endogenous morphine”), enkephalins, and dynorphins.
Endogenous opioids bind to opioid receptors. Located in both the brain and spinal
cord, opioid receptors can modulate the transmission of pain signals and decrease
pain perception.
Opioids— like morphine, heroin, oxycodone, and fentanyl— reduce pain and are
analgesics. They work by mimicking the action of endogenous opioids. Opioid
drugs like heroin bind the same opioid receptors and, at normal physiological
doses, produce similar analgesic eects. When ingested in large doses, opioid
drugs produce a high and can be addictive. The drug naloxone antagonizes the
eects of opioids and is often given to people who have overdosed to reverse their
life-threatening eects.
Endogenous opioids are also responsible for some types of the pain placebo eect.
The pain placebo eect is when a patient is given an inert substance, but because
the patient believes it’s a real drug, they perceive a decrease in pain. Naloxone
reduces the placebo eect. The pain placebo eect isn’t just psychological; it has
a physiological component that involves cognitive stimulation— from belief— of
the body’s endogenous opioid production.
The endogenous cannabinoid (endocannabinoid) system has also been identied
as a key endogenous system involved in regulating pain sensation, with modula-
tory actions at multiple stages of pain processing. Endocannabinoids include
anandamide, an unconventional neurotransmitter released during synaptic trans-
mission that activates cannabinoid receptors. The main psychoactive chemical
derived from the marijuana/cannabis plant, delta9THC, binds to cannabinoid
receptors and mediates its analgesic eects.
Easing pain with distraction
Cognitive distraction— focusing your mind on something else— can often reduce
pain. The Melzack and Wall gate theory says that ordinary mechanoreceptors in
the skin activate some of the same relay neurons in the spinal cord that receive
inputs from pain receptors. When only pain receptors are activated, the pain sig-
nals pass through the gate and reach the brain. However, if mechanoreceptors are
suciently activated, they can block the neural gate and suppress the pain signal
to the brain. These gate neurons also receive descending inputs from cognitive
activity (that is, by distracting yourself) that can close the pain gate.
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268 PART 3 Understanding Perception and Cognition
Losing the pain sense
Pain is an unpleasant subjective experience with a sensory and an emotional com-
ponent. Would you be better o without it? Denitely not. Pain is necessary
because of what happens to people who have lost this sense. For example, in
peripheral neuropathy, pain receptors in the peripheral nervous system become
damaged, often as a result of vascular problems associated with diabetes. People
with peripheral neuropathy tend to injure themselves without knowing. They may
burn themselves severely while cooking or break bones during routine physical
activity. People who can’t feel pain can also ignore skin lesions until they become
serious infections.
Locating your limbs with skin, muscle,
and joint receptors
How does your brain know where your limbs are and what they’re doing? And how
do you maintain balance and posture and coordinate movements without thinking
about it? These senses and abilities occur through proprioception, the term for
knowledge of limb position, while the sense of kinesthesia concerns information
about limb movement, and both can be conscious and unconscious.
Information about limb position comes from mechanoreceptors called proprio-
ceptors located in the skin, joints, tendons, and muscles. The skin around your
joints is stretched when your limbs move. Golgi tendon organs at the junction
between the muscle and the tendon that attaches it to a bone report on force of
muscle contraction. The muscle spindles send information on muscle stretch and
extent of muscle contraction and relaxation.
Proprioceptors send information to the spinal cord or brainstem:
»
For conscious proprioception the sensory signals are relayed to the thalamus
and somatosensory cortex.
»
Unconscious proprioception is relayed to the cerebellum where it’s integrated
with other sensory and motor signals to produce motor control for posture,
balance, and coordination.
Spinal processing and cranial nerves
Sensory input for the body is transduced by the axons of dorsal root ganglion neu-
rons that synapse in the spinal gray area or brainstem to participate in monosyn-
aptic and polysynaptic reexes (refer to Chapter7) and be relayed to the thalamus
(see the next section). Sensory receptors in the skin of the head project to the
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CHAPTER 14 Feeling, Smelling, andTasting 269
brain via cranial nerves such as the trigeminal nerve for the face, along with some
upper cervical nerves for the back of the head (refer to Chapter9).
Sending messages to the thalamus
Two major pathways project skin somatosensory information to the brain from
the body below the head to the ventral posterior nucleus (VPN) in the thalamus:
»
Spinothalamic tract: This tract is an evolutionary older pathway that
contains small diameter axons. It relays sensory information from the skin to
the brain about temperature and pain (lateral section of the tract) and coarse
touch (anterior
-ventral section of the tract). After they enter the spinal cord,
somatosensory axons decussate (cross) and synapse with secondary neurons
in the substantia gelatinosa or the nucleus proprius. Axons from these neurons
travel up the spinal cord to the brainstem where some synapse on specic
brainstem nuclei, and the majority extend to somatosensory nuclei in the VPN
of the thalamus. The thalamic nuclei for the spinothalamic pathway project to
the somatosensory cortex, cingulate cortex, and insula. In the spinal cord, the
spinothalamic tract also participates in the withdrawal reex.
»
Dorsal column medial lemniscus (DCML) tract: This tract, whose axons are
larger in diameter than the spinothalamic tract, carries discriminative (ne)
touch and conscious proprioceptive information for active touch. The axons of
this pathway are large in diameter and myelinated. The somatosensory axons
enter the spinal cord and ascend up the length of the cord, forming the DCML
tract. When the axons reach the medulla, they synapse with secondary
sensory neurons in the cuneate and gracile nuclei. Neurons in these nuclei
relay to the ventral posterolateral (VPL) nucleus in the VPN of the thalamus.
Relay from cranial nerve sensory nuclei in the brainstem occurs to the ventral
posteromedial (VPM) nucleus in the VPN of the thalamus. In the thalamus, relay
neuron responses are similar to their inputs from the spinal cord and cranial
nerves. The thalamic neurons project to the primary somatosensory cortex called
S1. Information is also transmitted to S2, the insula, and cingulate cortex.
Recognizing What You Touch at
Somatosensory Cortex
The somatosensory area (VPN) of the thalamus projects to the narrow strip of
cerebral cortex just posterior to the central sulcus at the most anterior part of the
parietal lobe (see the left side of Figure14-2). The somatosensory cortex contains
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270 PART 3 Understanding Perception and Cognition
a somatotopic map. Parts of the body closest to the ground when you’re standing
(the feet and legs) are represented most medially, while the ngers and face rep-
resentations are more lateral. The next sections describe the cortical processing of
somatosensory information.
Mapping senses with the homunculus
The topographic map of the skin on the cortex is called the cortical homunculus.
The term means “little man,” and the map reects the relative positions of the
parts of the human body. The right side of Figure14-2 shows where major body
areas are represented on a coronal section of the somatosensory cortex.
This map has several oddities. For example, the area of the homunculus that
responds to stimulation of the ngers is near that for the face. Also, the homun-
culus representation on somatosensory cortex is clearly distorted. The ngers
take up much larger areas of somatosensory cortex than, say, the skin on the
abdomen, which is a much larger skin area.
Two principles explain the distortion in the homunculus:
»
Dierent areas of the skin have very dierent receptor densities. The skin
in the ngertip, for example, has many more receptors per area than the same
areas in the skin on the stomach or back. This allows you to have high two-
point discrimination in your ngertips so you can manipulate objects precisely.
»
A given area of cortex processes inputs from about the same absolute
number of receptors. Because of the way the neocortex is organized, each
area receives a similar number of inputs that are relayed by the thalamus.
FIGURE14-2:
The somatosen-
sory cortex.
© John Wiley & Sons, Inc.
https://t.me/med1917
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