Hearing,%20Balance,%20and%20the%20Cutaneous%20and%20Chemical%20Senses

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Chapter 7

• Hearing, Balance, and the Cutaneous and Chemical
  Senses

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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.

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Wave Forms for the Three Dimensions of a Sound
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The Human Ear
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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.

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Auditory Pathways
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Cochlea Cross Section
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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.

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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.

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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).

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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.

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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.

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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.

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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.

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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

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Cochlea Implants
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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
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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

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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)

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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

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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.

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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.

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Receptive Fields and Adaptation Rates of Touch
Receptors
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Skin Receptors
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Skin ReceptorsGlabrous Skin
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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.

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The Somatosensory Cortex
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The Experience and Control of Pain

• Pain has both negative and positive functions
• Chronic pain can be the bane of a persons
  existence.
• However, under ordinary circumstances, pain is
  extremely useful, warning us of potential injury
  and inducing us to seek appropriate treatment.

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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.

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Gate-Control Theory of Pain
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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.

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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.

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Taste and Smell
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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)

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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

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Types of Papillae and Distribution of Taste
Receptors
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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.

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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.

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Gustatory Pathways
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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.

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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.

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Olfactory Receptors and Pathways