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Chapter 7
Hearing, Balance, and the Cutaneous and
Chemical Senses
The Auditory System:
Sound - Vibrations in a material medium, such as
air, water, or metal.
 Sound waves vary along three dimensions:

• Frequency refers to the number of vibrations per
second and is measured in hertz (Hz). We perceive
the frequency of a sound as pitch.
• Amplitude refers to the loudness of a sound wave and
is measured in decibels (dB).
• Timbre refers to the combination of multiple
frequencies that make up complex sounds and give
them their characteristic qualities.
Wave Forms for the Three
Dimensions of a Sound
The
Human
Ear
The Transduction of Sound Waves
into Neural Impulses
The inner hair cells have a resting
potential of -60 mV.
 When cilia bend in the direction of the
longest cilium the membrane depolarizes.
 This leads to a rapid influx of Ca2+ ions
into the hair cells, which results in the
release of glutamate.

Auditory Pathways
Cochlea Cross Section
Pitch Perception: Early Research

Place Theory of Pitch Perception
• The view that different sounds activate nerve
fibers at different locations on the basilar
membrane.
• High-pitched sounds activate the nerve fibers at
the base of the membrane near the oval window
• Low-pitched sounds stimulate nerve fibers at the
opposite end of the basilar membrane.
Pitch Perception: Early Research

Frequency Theory of Pitch Perception
• The view that the firing rate in the auditory
nerve matches the frequency of the sound. That
is, the basilar membrane vibrates in synchrony
with the sound wave.
• We now know that the firing rate matches the
frequency because of the volley principle:
• while one group of neurons in the auditory nerve is
firing, another group is recovering from its previous
activity
• the end result being that the combined firing of all the
groups matches the frequency of the sound.
Pitch Perception: Current Theory

The current theory of pitch perception uses a
combination of the previous theories:
• From 20 Hz to 400 Hz, frequency theory accounts for pitch
perception (the firing rate of individual neurons in the auditory
nerve directly matches the frequency of the sound).
• From 400 Hz to 4 kHz, volley principle takes over.
• Beyond 4 kHz, place theory comes into play (the place of maximal
vibration on the basilar membrane determines the pitch that we
perceive).
• Additionally, both place theory and the volley principle work for
sounds from about 1 kHz to 4 kHz (may explain our greater
sensitivity to pitches within this range).
Detection of Loudness

The nervous system has two mechanisms for
determining the intensity of a stimulus:
• The rate of firing of individual neurons
• The number of neurons firing

The higher the firing
rate, or the greater the
number of neurons
firing, the more intense
the stimulus.
Detection of Sound Complexity



Pure tones are sounds of only one
frequency; complex sounds have two or
more frequencies.
Combination of frequencies produces what
we perceive as the timbre of a particular
sound.
According to the place theory, because each
sound frequency activates a specific part of
the basilar membrane, a complex sound
produces a unique pattern of neural activity.
Sound Localization
For both low-pitched sounds and high-pitched
sounds, the cues to sound localization are
based on differential time of arrival at the two
ears.
 As long as the sound does not come from the
median plane, the sound will arrive at one ear
slightly before it gets to the other ear, which
allows us to locate the direction from which a
sound comes from.

The Role of the Auditory Cortex in
Sound Recognition

Auditory receptors encode sound:
• Frequency
• Intensity
• Timbre
Receptors send this information to the
primary auditory cortex.
 In auditory cortex, some neurons respond
selectively to specific aspects of sounds;
others react to more complex aspects of
the sound stimulus.

The Role of the Auditory Cortex in
Sound Recognition
Sound is identified as the neural
information moves from the primary
auditory cortex to the anterior part of the
lateral surface of the superior temporal
gyrus
 Sound is localized as it moves to the
posterior part of the superior temporal
gyrus and then to the parietal cortex

Cochlea Implants
The Vestibular Sense

The sense responsible for maintaining
balance.
• Enables us to walk on two feet, keep our head
upright, and adjust our eye movements to
compensate for our head movements.
Phillippe Petit
Components of the Vestibular
System

Vestibular sacs - Provide information
about the position of the head relative to
the body.
• Utricle and saccule -The two vestibular sacs
containing the vestibular receptor cells, or
hair cells.

Semicircular canals - Fluid-filled canals
that provide information related to head
movements or rotations.
• Ampulla, crista, cupula
Vestibular Pathways

Vestibular hair cells
• convert information about passive head movement and
active head rotation into an increase or decrease in
neurotransmitter release
• synapse with bipolar neurons

Cell bodies of bipolar neurons form:
• vestibular ganglia (receive input from vestibular hair
cells)
• axons of the vestibular ganglia become the vestibular
nerve (combine with cochlear nerve fibers to form the
auditory nerve)
Motion Sickness
Feelings of dizziness and nausea; occur when the
body is moved passively without motor activity
and corresponding feedback to the brain.
 Two types of motion sickness:

• Detects movements but motor actions that could have
produced the movement have not occurred
• Detects movement
inconsistent with the
information about movement
sensed by the eyes
The Somatosenses
The skin sensations of touch, pain,
temperature, and proprioception.
 Proprioception -The somatosense that
monitors body position and movement,
acts to maintain body position, and ensures
the accuracy of intended movements

• located in the muscles, tendons, and joints
• essential to the control of movement.
Skin Receptors
•
The functions of the skin include:
• protecting the internal organs from injury
• helping regulate body temperature by producing
sweat, which cools the body when it becomes
too hot
• providing a first line of defense against invading
microorganisms.
Receptive Fields and Adaptation Rates
of Touch Receptors
Skin Receptors
Skin Receptors:Glabrous Skin
Somatosensory Pathways
•
The dorsal column-medial lemniscal system
• begins in the spinal cord and transmits information
about touch and proprioception to the primary
somatosensory cortex.
•
The anterolateral system
• begins in the spinal cord and transmits information
about temperature and pain to the brain stem,
reticular formation, and the primary and secondary
somatosensory cortices.
•
The spinocerebellar system
• begins in the spinal cord and transmits
proprioceptive information to the cerebellum.
The
Somatosensory
Cortex
The Experience and Control of
Pain

Pain has both negative and positive
functions:
• Chronic pain can be the bane of a person’s
existence.
• However, under ordinary circumstances, pain is
extremely useful, warning us of potential injury
and inducing us to seek appropriate treatment.
Gate-Control Theory of Pain
Melzack & Wall (1965)
 Gate-control theory of pain - Input from pain
receptors will produce the perception of pain
only if the message first passes through a “gate” in
the spinal cord and lower brain stem structures.
 Theory emphasizes that messages from the brain
can open or close the spinal cord gate, helping us
to understand the psychological nature of pain why our sensation of pain can be affected by our
thoughts and feelings.

Gate-Control Theory of Pain
Neuromatrix Theory of Pain
Melzack (1999)
 Neuromatrix theory of pain - A theory
that accounts for types of pain unexplained
by the gate-control theory of pain.

• Severe, chronic pain existing in the absence of
injury or disease.
The Chemical Senses
Chemical senses include the gustatory
and olfactory systems.
 Both are intermingled in our eating
experiences, in that much of what we
report as the taste of food actually comes
from its odor.

Taste and Smell
Gustation
Gustatory sense -The sense of taste.
 Tastes can be classified according to four
primary sensations:

•
•
•
•
Sweet (stimulate sugar receptors)
Sour (stimulate H+ receptors)
Bitter (stimulate alkaloid compound receptors)
Salty (stimulate NaCl receptors)
Taste Receptors
Papilla - A small, visible bump on the
tongue that contains taste bumps.
 Taste bud - A cluster of taste receptors
that lie either near or within a papilla.
 Three kinds of papillae contain taste buds:

• Foliate
• Circumvallate
• Fungiform
Types of
Papillae and
Distribution
of Taste
Receptors
Genetics of Taste


People differ in their sensitivity to bitter and
some sweet tastes.
These individual differences appear to be partly
related to the number of taste buds on the
tongue:
• Supertasters (25% of people) have the most taste
buds - about 425 per square cm on the tongue tip.
• Medium tasters (50% of people) have about 184
taste buds per square cm.
• Non-tasters (25% of people) have about 96 per
square cm.
Mechanisms of Taste Reception

Mechanism differs for each of the four basic
tastes:
• Salty food activates a taste receptor by causing Na+ ions
to move through Na+ ion channels in the cell membrane.
• H+ ions in sour foods and sugar molecules in sweet
foods close the K+ ion channels in receptor membranes,
preventing K+ ions from leaving the cell.
• In bitter foods, alkaloid compounds trigger the
movement of Ca2+ ions into the cytoplasm from storage
sites in the taste receptor, increasing the release of
neurotransmitters.
Gustatory
Pathways
Olfaction
The sense of smell.
 Habituation - can occur quickly with smells.
Whether pleasant or unpleasant, we rapidly
“get used to” smells.
 This sensory adaptation is caused by
decreased responding by receptors when
they are exposed to the same stimulus for a
continuous period of time.

Olfactory Receptors

Olfactory epithelium - The mucous
membrane in the top rear of the nasal
passage; lined by olfactory receptors.
• Humans have approximately 50 million
olfactory receptors that detect smell
• other species, such as dogs, may have up to 20
times as many, with each cell having more than
10 times as many cilia.
Olfactory
Receptors
and Pathways