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Physiology of Behavior
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Neil R. Carlson • Melissa A. Birkett
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Chapter Opener captions: Ch. 1: The human nervous system contains billions of neurons; Ch. 2: Neurons are the cells of the
nervous system that are specialized for communication; Ch. 3: The structures of the human nervous system are made up
of billions of neurons that make trillions of synapses; Ch. 4: Cross-section of the vagus nerve of the peripheral nervous
system; Ch. 5: Neurons in the cortex labeled with a fluorescent dye; Ch. 6: Cross-section of a retina. Photoreceptor cells are
visible at the top of the image; Ch. 7: Confocal microscopy image of neurons (green) and glia (red) in the vestibular pathway;
Ch 8: Cross-section of the cerebellum; Ch. 9: Cross-section of the hypothalamus of a mouse; Ch. 10: Cross-section of the
pituitary gland (left) attached to the hypothalamus (right); Ch. 11: Example of pyramidal neurons found in the hippocampus;
Ch. 12: Color-enhanced transmission electron micrograph of portions of two adipose cells and associated connective tissue
in a rat; Ch. 13: New neurons in the mouse hippocampus are labeled with green fluorescence; Ch. 14: Scanning electron
microscope image of a neuron in the cortex; Ch. 15: Neurons derived from mouse embryonic stem cells. Tyrosine hydroxylase
(TH, a dopamine-synthesizing enzyme) is labeled in red; TH-containing neurons degenerate in Parkinson’s disease; green
labels a protein that’s found in all neurons; blue labels the nuclei of all cells; Ch. 16: Neurons in the mouse hippocampus;
Ch. 17: Cross-section of the adrenal medulla; Ch. 18: Neurons in the CA1 region of the hippocampus from a transgenic
mouse stained for the CB1 cannabinoid receptor (red) and cell nuclei (blue).
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Authorized adaptation from the United States edition, entitled Physiology of Behavior, 12th edition, ISBN 978-0-13-408091-8, by Neil R.
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Audition, the Body Senses, and the Chemical Senses
aware of the information received from these organs. This
section describes the vestibular apparatus and the vestibular pathway in the brain.
Anatomy of the Vestibular Apparatus
LO 7.11 Identify the structures of the vestibular apparatus.
The vestibular system has two components: the vestibular
sacs and the semicircular canals. They represent the second and third components of the labyrinths of the inner ear.
(We just studied the first component, the cochlea.) The vestibular sacs respond to the force of gravity and inform the
brain about the head’s orientation. The semicircular canals
respond to angular acceleration—changes in the rotation of
the head—but not to steady rotation. They also respond (but
rather weakly) to changes in position or to linear acceleration.
Figure 7.19 shows the labyrinths of the inner ear, which
include the cochlea, the semicircular canals, and the two vestibular sacs: the utricle (“little pouch”) and the saccule (“little
sack”). The semicircular canals approximate the three major
planes of the head: sagittal, transverse, and horizontal. Receptors in each canal respond maximally to angular acceleration
in one plane. The semicircular canal consists of a membranous canal floating within a bony one; the membranous canal contains a fluid called endolymph. An enlargement called
the ampulla contains the organ in which the sensory receptors reside. The sensory receptors are hair cells similar to those
found in the cochlea. Their cilia are embedded in a gelatinous
mass called the cupula, which blocks part of the ampulla.
To explain the effects of angular acceleration on the semicircular canals, we will first describe an “experiment.” If we
place a glass of water on the exact center of a turntable and
then start the turntable spinning, the water in the glass will, at
first, remain stationary (the glass will move with respect to the
water it contains). Eventually, however, the water will begin
rotating with the container. If we then stop the turntable, the
water will continue spinning for a while because of its inertia.
The semicircular canals operate on the same principle.
The endolymph within these canals, like the water in the glass,
resists movement when the head begins to rotate. This inertial
Figure 7.19 Receptive Organ of the Semicircular Canals
Semicircular
canals
Vestibular sacs
(utricle and saccule)
Semicircular
canals
Vestibular
nerve
Cochlea
Section of
ampulla
221
Cupula
Filled with
endolymph
Hair
cells
Axons of
ampullary nerve
222 Chapter 7
Figure 7.20 Receptive Tissue of the Vestibular Sacs: The Utricle and the Saccule
Efferent
axon
Hair cell
Vestibular
nerve
Semicircular
canals
Filamentous
base
Afferent
axon
Otoconia
Utricle
Saccule
Supporting
cell
Otolithic
membrane
resistance pushes the endolymph against the cupula, causing
it to bend, until the fluid begins to move at the same speed as
the head. If the head rotation is then stopped, the endolymph,
still circulating through the canal, pushes the cupula the other
way. Angular acceleration is thus translated into bending of
the cupula, which exerts a shearing force on the cilia of the
hair cells. (Of course, unlike the glass of water in the example,
we do not normally spin around in circles; the semicircular canals measure very slight and very brief rotations of the head.)
With this explanation in mind, what might be responsible for
the perception of movement after a person stops spinning?
The vestibular sacs (the utricle and saccule) work very
differently. These organs are roughly circular, and each contains a patch of receptive tissue. The receptive tissue is located on the “floor” of the utricle and on the “wall” of the
saccule when the head is in an upright position. The receptive tissue, like that of the semicircular canals and cochlea,
contains hair cells. The cilia of these receptors are embedded in an overlying gelatinous mass, which contains something rather unusual: otoconia, which are small crystals of
calcium carbonate. (See Figure 7.20.) The weight of the crystals causes the gelatinous mass to shift in position as the orientation of the head changes. Thus, movement produces a
shearing force on the cilia of the receptive hair cells.
The hair cells of the semicircular canal and vestibular
sacs are similar in appearance. Each hair cell contains several cilia, graduated in length from short to long. These hair
cells resemble the auditory hair cells found in the cochlea,
and their transduction mechanism is also similar: A shearing
force of the cilia opens ion channels, and the entry of potassium ions depolarizes the ciliary membrane. All three forms
of hair cells employ the same receptor molecules: TRPA1,
Cilia
which we described earlier in this chapter. Figure 7.21
shows two views of a hair cell of a bullfrog saccule made by
a scanning electron microscope.
The Vestibular Pathway
LO 7.12 Outline the vestibular pathway.
The vestibular and cochlear nerves constitute the two
branches of the eighth cranial nerve (auditory nerve). The
bipolar cell bodies that give rise to the afferent axons of the
Figure 7.21 Saccular Hair Cells
These scanning electron microscope views of hair cells of a bullfrog
saccule show (a) an oblique view of a normal bundle of vestibular hair
cells and (b) a top view of a bundle of hair cells from which the longest
has been detached.
(From Hudspeth, A. J., and Jacobs, R., Stereocilia mediate transduction in
vertebrate hair cells, Proceedings of the National Academy of Sciences, USA,
1979, 76, 1506–1509. Reprinted with permission.)
(a)
(b)
Audition, the Body Senses, and the Chemical Senses
vestibular nerve (a branch of the eighth cranial nerve) are located in the vestibular ganglion, which appears as a nodule
on the vestibular nerve.
Most of the axons of the vestibular nerve synapse
within the vestibular nuclei in the medulla, but some axons
travel directly to the cerebellum. Neurons of the vestibular nuclei send their axons to the cerebellum, spinal cord,
medulla, and pons. There also appear to be vestibular projections to the temporal cortex, but the precise pathways
have not been determined. Most investigators believe that
the cortical projections are responsible for feelings of dizziness; the activity of projections to the lower brain stem can
produce the nausea and vomiting that accompany motion
sickness. Projections to brain stem nuclei controlling neck
223
muscles are clearly involved in maintaining an upright position of the head.
Perhaps the most interesting connections are those to the
cranial nerve nuclei (third, fourth, and sixth) that control the
eye muscles. As we walk or (especially) run, the head is jarred.
The vestibular system exerts direct control on eye movement
to compensate for the sudden head movements. This process,
called the vestibulo-ocular reflex, maintains a fairly steady retinal image. Test this reflex yourself: Look at a distant object and
hit yourself (gently) on the side of the head. Note that your
image of the world jumps a bit but not too much. People who
have suffered vestibular damage and who lack the vestibuloocular reflex have difficulty seeing anything while walking or
running. Everything becomes a blur of movement.
Section Review
Vestibular System
LO 7.11 Identify the structures and functions of the
vestibular apparatus.
The vestibular apparatus contains the vestibular sacs (the
utricle and saccule) and the semicircular canals of the
ear. The vestibular sacs respond to the force of gravity
and inform the brain about the head’s orientation. The
semicircular canals respond to angular acceleration and
changes in position or linear acceleration.
LO 7.12 Outline the vestibular pathway.
From the hair cells, vestibular information is relayed to the
brain via the vestibular and cochlear nerves. The vestibular
nerve projects to the medulla, which sends information
to cerebellum, spinal cord, pons, and to other regions of
Somatosenses
This chapter began with the case of Ashlyn, who had a
congenital lack of functional pain receptors. This case
highlights the important role of somatosenses in influencing our behavior. The somatosenses provide information
about what is happening on the surface of our body and
inside it. The cutaneous senses (skin senses) are the most
studied of the somatosenses and include several submodalities commonly referred to as touch. Proprioception
and kinesthesia provide information about body position
and movement. We will describe the contribution of sensory receptors in the skin to these perceptual systems in
this section. The muscle receptors and their role in feedback from limb position and movement are discussed in
this section and in Chapter 8. The organic senses arise
from receptors in and around the internal organs. (See
Table 7.2.)
the medulla. The cranial nerve relays information to the
eye muscles to compensate for sudden head movements.
There also appear to be vestibular projections to the
temporal cortex, but the precise pathways have not been
determined.
Thought Question
Persistent dizziness has a lifetime prevalence of approximately 25 percent and represents a significant risk factor
for falls among older adults. Select one structure involved
in vestibular perception and explain how damage or dysfunction in this structure could contribute to the experience
of dizziness (even if the exact cortical pathways are not yet
known).
Table 7.2 Somatosenses
Somatosense
Function
Cutaneous Senses
Provide information from the surface of the body.
Proprioception
Provide information about location of body in space.
Kinesthesia
Provide information about movement of body though
space.
Organic Senses
Provide information from in and around internal organs.
The Stimuli
LO 7.13 Provide examples of stimuli that activate
receptors for the somatosenses.
The cutaneous senses respond to several different types of
stimuli: pressure, vibration, heating, cooling, and events that
cause tissue damage (and hence pain). Feelings of pressure
224 Chapter 7
environment by the skin’s outer layers. The skin participates
in thermoregulation by producing sweat, thus cooling the
body, or by restricting its circulation of blood, thus conserving heat. Its appearance varies widely across the body, from
mucous membrane to hairy skin to the smooth, hairless skin
of the palms and the soles of the feet, which is known as
glabrous skin. Skin consists of subcutaneous tissue, dermis, and epidermis and contains various receptors scattered
throughout these layers. Glabrous skin contains a dense,
complex mixture of receptors, which reflects the fact that
we use the palms of our hands and the inside surfaces of
our fingers to actively explore the environment: We use our
hands and fingers to hold and touch objects. In contrast, the
rest of our body most often contacts the environment passively; that is, other things come into contact with it.
Figure 7.22 shows the appearance of free nerve endings
and the four types of encapsulated somatosensory receptors
(Merkel’s disks, Ruffini corpuscles, Meissner’s corpuscles, and Pacinian corpuscles). The locations and functions
of these receptors are listed in Table 7.3.
Figure 7.22 Cutaneous Receptors
LO 7.14 Describe the anatomy and somatosensory
receptors of the skin.
The skin is a complex and vital organ of the body—one that
we often tend to take for granted. We cannot survive without it; extensive skin burns are fatal. Our cells, which must
be bathed by a warm fluid, are protected from the hostile
Glabrous Skin
Hairy Skin
Ruffini
corpuscles
Sweat
gland
Epidermis
Anatomy of the Skin and Its Receptive
Organs
Hair
Free nerve
endings
Merkel's
disks
Meissner’s
corpuscle
Pacinian
corpuscle
Dermis
are caused by mechanical deformation of the skin. Vibration occurs when we move our fingers across a rough surface. Thus, we use vibration sensitivity to judge an object’s
roughness. Sensations of warmth and coolness are produced
by objects that raise or lower skin temperature. Sensations of
pain can be caused by many different types of stimuli, but it
appears that most cause at least some tissue damage.
One source of kinesthesia is the stretch receptors found
in skeletal muscles that report changes in muscle length to the
central nervous system. Receptors within joints between adjacent bones respond to the magnitude and direction of limb
movement. However, the most important source of kinesthetic feedback appears to come from receptors that respond
to changes in the stretching of the skin during movements of
the joints or of the muscles themselves, such as those in the
face (Johansson and Flanagan, 2009). Muscle length detectors, located within the muscles, do not give rise to conscious
sensations; their information is used to control movement.
These receptors will be discussed separately in Chapter 8.
We are aware of some of the information received by
means of the organic senses, which can provide us with unpleasant sensations such as stomachaches or gallbladder attacks, or pleasurable ones such as those provided by a warm
drink on a cold winter day. We are unaware of some information, such as that provided from receptors in the digestive system, kidneys, liver, heart, and blood vessels that are
sensitive to nutrients and minerals. This information, which
plays a role in the control of metabolism and water and mineral balance, is described in Chapter 12.
Subcutaneous
fat
Artery
Vein
Table 7.3 Categories of Cutaneous Receptors
Size and Nature of
Receptive Field
Identity of Receptor
Location of Receptor
Function of Receptor
Small, sharp borders
Merkel’s disks
Hairy and glabrous skin
Detection of form and roughness, especially by fingertips
Large, diffuse borders
Ruffini corpuscles
Hairy and glabrous skin
Detection of static force against skin; skin stretching; proprioception
Small, sharp borders
Meissner’s corpuscles
Glabrous skin
Detection of edge contours; Braille-like stimuli, especially by fingertips
Large, diffuse borders
Pacinian corpuscles
Hairy and glabrous skin
Detection of vibration; information from end of elongated object being
held, such as tool
Hair follicle ending
Base of hair follicle
Detection of movement of hair
Free nerve ending
Hairy and glabrous skin
Detection of thermal stimuli (coolness or warmth), noxious stimuli
(pain), tickle
Free nerve ending
Hairy skin
Detection of pleasurable touch from gentle stroking with soft object
Audition, the Body Senses, and the Chemical Senses
Perception of Cutaneous Stimulation
LO 7.15 Describe receptors involved in the perception of
touch, temperature, pain, and itch.
The three most important qualities of cutaneous stimulation are touch, temperature, and pain. These qualities, along
with itch, are described in the sections that follow.
Stimuli that cause vibration in the skin or
changes in pressure against it (tactile stimuli) are detected
by mechanoreceptors—the encapsulated receptors shown
in Figure 7.22 and some types of free nerve endings. Most
investigators believe that the encapsulated nerve endings
serve only to modify the physical stimulus transduced by
the dendrites that reside within them. But what is the mechanism of transduction? How does movement of the dendrites of mechanoreceptors produce changes in membrane
potentials? It appears that the movement causes ion channels to open, and the flow of ions into or out of the dendrite
causes a change in the membrane potential. You will recall
that TRPA1, a member of the TRP (transient receptor potential) family of receptor proteins, is responsible for transduction of mechanical information in auditory and vestibular
hair cells.
Most information about tactile stimulation is precisely
localized—that is, we can perceive the location on our skin
where we are being touched. However, a case study by
Olausson et al., (2002) discovered a new category of tactile
sensation. Read the case study below to learn more about a
unique case of cutaneous stimulation.
TOuCH
Patient G. L., a 54-year-old woman, experienced
a permanent loss of afferent neurons involved in
somatosensation. G. L. lost the ability to perceive tickle
but retained the ability to perceive temperature, pain,
and itch (Olausson et al., 2002, pp. 902–903).
When the hairy skin on her forearm or the back of her
hand was stroked with a soft brush, she reported a faint,
pleasant sensation. However, she could not determine
the direction of the stroking or its precise location. An
fMRI analysis showed that this stimulation activated the
insular cortex, a region that is known to be associated
with emotional responses and sensations from internal
organs. The somatosensory cortex was not activated.
When regions of hairy skin of control participants were
stimulated this way, fMRI showed activation of the
primary and secondary somatosensory cortex as well
as the insular cortex because the stimulation activated
both large and small axons. The glabrous skin on the
palm of the hand is served only by large-diameter,
myelinated axons. When this region was stroked with
a brush, G. L. reported no sensation at all, presumably
225
because of the absence of small, unmyelinated axons.
The investigators concluded that, besides conveying
information about noxious and thermal stimuli, smalldiameter unmyelinated axons constitute a “system for
limbic touch that may underlie emotional, hormonal and
affiliative responses to caresslike, skin-to-skin contact
between individuals” (Olausson et al., 2002, p. 900).
And, as we saw, G. L. could no longer perceive tickle.
Tickling sensations, which were previously believed to
be transmitted by these small axons, are apparently
transmitted by the large, myelinated axons that were
destroyed in patient G. L.
Olausson and his colleagues (Löken et al., 2009)
note that the sensory endings that detect pleasurable
stroking are found only in hairy skin, and that stroking
of glabrous skin does not provide these sensations.
However, we can think of pleasurable tactile stimuli
that can be experienced through the glabrous skin of
the palms and fingers—for example, those provided
by stroking a warm, furry animal or touching a loved
one. When our hairy skin contacts the skin of another
person, it is more likely that that person is touching
us. In contrast, when our glabrous skin contacts the
skin of another person, it is more likely that we are
touching them. Thus, we might expect receptors in
hairy skin to provide pleasurable sensations when
someone caresses us but expect receptors in glabrous
skin to provide pleasurable sensations when we caress
someone else.
Our cutaneous senses are often used to analyze shapes
and textures of stimulus objects that are moving with respect
to the surface of the skin. Sometimes, the object itself moves,
but more often, we do the moving ourselves. If an object is
placed in your palm and you are asked to keep your hand
still, you will have a great deal of difficulty recognizing the
object by touch alone. If you are then allowed to move your
hand, you will manipulate the object, letting its surface slide
across your palm and the pads of your fingers. You will be
able to describe the object’s three-dimensional shape, hardness, texture, slipperiness, and so on. In order to describe
it, your motor system must cooperate, and you need kinesthetic sensation from your muscles and joints, in addition to
the cutaneous information. If you squeeze the object and feel
a lot of well-localized pressure in return, it is hard. If you
feel a less intense, more diffuse pressure in return, it is soft.
If it produces vibrations as it moves over the ridges on your
fingers, it is rough. If very little effort is needed to move the
object while pressing it against your skin, it is slippery. If it
does not produce vibrations as it moves across your skin,
but moves in a jerky fashion, and if it takes effort to remove
your fingers from its surface, it is sticky. Thus, our somatosenses work dynamically with the motor system to provide