Chapter 7 The Other Sensory Systems

1
Chapter 7 The Other Sensory Systems
2
Audition

• Our senses have evolved to allow us to detect and
  interpret biologically useful information from
  our environment .
• However, we do not detect all sensory information
  in the world.
• Some sensory information lies beyond our ability
  to detect it.
• We also tend to focus on information that is
  important or relevant.

3
Audition

• Audition refers to our sense of hearing.
• Audition depends upon our ability to detect sound
  waves.
• Sound waves are periodic compressions of air,
  water or other media.
• Sound waves are transduced into action
  potentials sent to the brain.

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Audition

• The amplitude refers to the height and subsequent
  intensity of the sound wave.
• Loudness refers to the perception of the sound
  wave.
• Amplitude is one factor.
• Frequency refers to the number of compressions
  per second and is measured in hertz.
• Related to the pitch (high to low) of a sound.

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Fig. 7-1, p. 196
6
Audition

• Anatomist distinguish between
• The outer ear
• The middle ear
• The inner ear

7
Audition

• The outer ear includes the pinna and is
  responsible for
• Altering the reflection of sound waves into the
  middle ear from the outer ear.
• Helping to locate the source of a sound.

8
Audition

• The middle ear contains the tympanic membrane
  which vibrates at the same rate when struck by
  sound waves.
• Three tiny bones (malleus, incus, stapes)
  transmit information to the oval window or a
  membrane in the middle ear.

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Fig. 7-2, p. 197
10
Audition

• The inner ear contains a snail shaped structure
  called the cochlea which contains three
  fluid-filled tunnels (scala vestibuli, scala
  media, and the scala tympani).
• Hair cells are auditory receptors that excite the
  cells of the auditory nerve when displaced by
  vibrations in the fluid of the cochlea.
• Lie between the basilar membrane and the
  tectorial membrane in the cochlea.

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Fig. 7-3, p. 198
12
Audition

• Pitch perception can be explained by the
  following theories
• Frequency theory - the basilar membrane vibrates
  in synchrony with the sound and causes auditory
  nerve axons to produce action potentials at the
  same frequency.
• Place theory - each area along the basilar
  membrane is tuned to a specific frequency of
  sound wave.

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Fig. 7-4, p. 199
14
Audition

• The current pitch theory combines modified
  versions of both the place theory and frequency
  theory
• Low frequency sounds best explained by the
  frequency theory.
• High frequency sounds best explained by place
  theory.

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Audition

• Volley principle states that the auditory nerve
  can have volleys of impulses (up to 4000 per
  second) even though no individual axon approaches
  that frequency by itself.
• provides justification for the place theory and

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Audition

• The primary auditory cortex is the ultimate
  destination of information from the auditory
  system.
• Located in the superior temporal cortex.
• Each hemisphere receives most of its information
  from the opposite ear.

17
Audition

• The superior temporal cortex contains area MT
  which allows for the detection of the location of
  sound.
• Area A1 of the brain is important for auditory
  imagery.
• The auditory cortex requires experience to
  develop properly.
• Auditory axons develop less in those who are deaf
  from birth.

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Fig. 7-5, p. 200
19
Audition

• The cortex is necessary for the advanced
  processing of hearing.
• Damage to A1 does not necessarily cause deafness
  unless damage extends to the subcortical areas.
• The auditory cortex provides a tonotopic map in
  which cells in the primary auditory cortex are
  more responsive to preferred tones.
• Some cells respond better to complex sounds than
  pure tones.

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Fig. 7-6, p. 201
21
Audition

• Areas around the primary auditory cortex exist in
  which cells respond more to changes in sound.
• Cells outside A1 respond to auditory objects
  (animal cries, machinery noise, music, etc.).
• Because initial response is slow, most likely
  responsible for interpreting the meaning of
  sounds.

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Audition

• About 99 of hearing impaired people have at
  least some response to loud noises.
• Two categories of hearing impairment include
• Conductive or middle ear deafness.
• Nerve deafness.

23
Audition

• Conductive or middle ear deafness occurs if bones
  of the middle ear fail to transmit sound waves
  properly to the cochlea.
• Caused by disease, infections, or tumerous bone
  growth near the middle ear.
• Can be corrected by surgery or hearing aids that
  amplify the stimulus.
• Normal cochlea and normal auditory nerve allows
  people to hear their own voice clearly.

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Audition

• Nerve or inner-ear deafness results from damage
  to the cochlea, the hair cells or the auditory
  nerve.
• Can be confined to one part of the cochlea.
• people can hear only certain frequencies.
• Can be inherited or caused by prenatal problems
  or early childhood disorders (rubella, syphilis,
  inadequate oxygen to the brain during birth,
  repeated exposure to loud noises, etc).

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Audition

• Tinnitus is a frequent or constant ringing in the
  ears.
• experienced by many people with nerve deafness.
• Sometimes occurs after damage to the cochlea.
• axons representing other part of the body invade
  parts of the brain previously responsive to
  sound.
• Similar to the mechanisms of phantom limb.

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Audition

• Sound localization depends upon comparing the
  responses of the two ears.
• Humans localize low frequency sound by phase
  difference and high frequency sound by loudness
  difference.

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Audition

• Three mechanisms
• High-frequency sounds (2000 to 3000Hz) create a
  sound shadow, making the sound louder for the
  closer ear.
• The difference in the time of arrival at the two
  ears is most useful for localizing sounds with
  sudden onset.
• Phase difference between the ears provides cues
  to sound location for localizing sounds with
  frequencies up to 1500 Hz.

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Fig. 7-7, p. 202
29
Fig. 7-8, p. 203
30
Fig. 7-9, p. 203
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The Mechanical Senses

• The mechanical senses include
• The vestibular sensation
• Touch
• Pain
• Other body sensations
• The mechanical senses respond to pressure,
  bending, or other distortions of a receptor.

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The Mechanical Senses

• The vestibular sense refers to the system that
  detects the position and the movement of the
  head.
• Directs compensatory movements of the eye and
  helps to maintain balance.
• The vestibular organ is in the ear and is
  adjacent to the cochlea.

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• The vestibular organ consists of two otolith
  organs (the saccule and untricle) and three
  semicircular canals.
• The otolith organs have calcium carbonate
  particles (otoliths) that activate hair cells
  when the head tilts.
• The 3 semicircular canals are oriented in three
  different planes and filled with a jellylike
  substance that activates hair cells when the head
  moves.

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The Mechanical Senses

• The vestibular sense is integrated with other
  sensations by the angular gyrus.
• Angular gyrus is an area at the border between
  the parietal and temporal cortex.

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The Mechanical Senses

• The somatosensory system refers to the sensation
  of the body and its movements.
• Includes discriminative touch, deep pressure,
  cold warmth, pain, itch, tickle and the position
  and movement of the joints.
• Touch receptors may be simple bare neurons, an
  elaborated neuron ending, or a bare ending
  surrounded by non-neural cells that modify its
  function.

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Fig. 7-11, p. 207
37
The Mechanical Senses

• The pacinian corpuscle is a type of touch
  receptor that detects sudden displacement or
  high-frequency vibrations on the skin.
• Mechanical pressure bend the membrane.
• increases the flow of sodium ions and triggers an
  action potential.

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Fig. 7-12, p. 207
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• Information from touch receptors in the head
  enters the CNS through the cranial nerves.
• Information from receptors below the head enter
  the spinal cord and travel through the 31 spinal
  nerves to the brain.

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The Mechanical Senses

• Each spinal nerve has a sensory component and a
  motor component and connects to a limited area of
  the body.
• A dermatome refers to the skin area connected to
  a single sensory spinal nerve.
• Sensory information entering the spinal cord
  travel in well-defined and distinct pathways.
• Example touch pathway is distinct from pain
  pathway.
• Spino-thalamic tract, etc.

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Fig. 7-14, p. 208
42
The Mechanical Senses

• Various aspects of body sensations remain partly
  separate all the way to the cortex.
• Various areas of the thalamus send impulses to
  different areas of the somatosensory cortex
  located in the parietal lobe.
• Different sub areas of the somatosensory cortex
  respond to different areas of the body.
• Damage to the somatosensory cortex can result in
  the impairment of body perceptions.

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The Mechanical Senses

• Various aspects of body sensations remain partly
  separate all the way to the cortex.
• Various areas of the thalamus send impulses to
  different areas of the somatosensory cortex
  located in the parietal lobe.

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The Mechanical Senses

• Pain depends on several types of axons, several
  neurotransmitters, and several brain areas.
• Mild pain triggers the release of glutamate while
  stronger pain triggers the release of glutamate
  and substance P.
• Substance P results in the increased intensity of
  pain.
• Morphine and opiates block pain by blocking these
  neurotransmitters.

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Fig. 7-15, p. 210
46
The Mechanical Senses

• Opioid mechanisms are systems that are sensitive
  to opioid drugs and similar chemicals.
• Activating opiate receptors blocks the release of
  substance P in the spinal chord and in the
  periaqueductal grey of the midbrain.
• Enkephalins refer to opiate-type chemical in the
  brain.
• Endorphins- group of chemicals that attach to the
  same brain receptors as morphine.

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Fig. 7-16, p. 211
48
The Mechanical Senses

• Discrepancy in pain perception can partially be
  explained by genetic differences in receptors.
• Gate theory suggests that the spinal cord areas
  that receive messages from pain receptors also
  receive input from other skin receptors and from
  axons descending from the brain.
• These other areas that provide input can close
  the gates and decrease pain perception.

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The Mechanical Senses

• Special heat receptors account for the pain
  associated with a burn.
• Heat receptors can also be activated by acids.
• Capsaicin is a chemical found in hot peppers that
  directly stimulates these receptors and also
  triggers an increase in the release of substance
  P.

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The Mechanical Senses

• Pain activates the hypothalamus, amygdala, and
  cingulate cortex.
• results in an emotional component to pain.
• A placebo is a drug or other procedure with no
  pharmacalogical effect.
• Placebos decrease pain perception by decreasing
  the brains emotional response to pain perception
  and not the sensation itself.

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Fig. 7-17, p. 211
52
The Mechanical Senses

• Mechanisms of the body to increase sensitivity to
  pain include
• Damaged or inflamed tissue releases histamine,
  nerve growth factor, and other chemicals that
  increase the number of sodium gates in nearby
  pain receptors.
• Pain responses are thus magnified.
• Certain receptors become potentiated after an
  intense barrage of painful stimuli.
• leads to increased sensitivity or chronic pain
  later.

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The Mechanical Senses

• Pain is best controlled by preventing the brain
  from being bombarded with pain messages.
• Bombarding the brain with pain messages results
  in the increased sensitivity of the pain nerves
  and their receptors.

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The Mechanical Senses

• Morphine and other opiates are the primary drugs
  for controlling serious pain.
• Morphine inhibits slow, dull pain carried by thin
  unmyelinated axons.
• Sharp pain is conveyed by thicker myelinated
  axons.
• Not influenced by morphine and endorphins.
• Ibuprofen, an anti-inflammatory drug, controls
  pain by reducing the release of chemicals from
  damaged tissues.

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The Mechanical Senses

• The release of histamines by the skin produce
  itching sensations.
• The release of histamine by the skin activates a
  distinct pathway in the spinal cord to the brain.
• Impulses travel slowly along this pathway (half a
  meter per second).
• Pain and itch have an inhibitory relationship.
• Opiates increase itch while antihistamines
  decrease itch.

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The Chemical Senses

• Coding in the sensory system could theoretically
  follow
• The labeled-line principle in which each receptor
  responds to a limited range of stimuli and sends
  a direct line to the brain.
• Across-fiber pattern in which each receptor
  responds to a wider range of stimuli and
  contributes to the perception of each of them.

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The Chemical Senses

• Vertebrate sensory systems probably have no pure
  label-lined codes.
• The brain gets better information from a
  combination of responses.
• Example auditory perception and color perception
  both rely on label-lined codes.
• Taste and smell stimuli activate several neurons
  and the meaning of the response of a single
  neuron depends on the context of responses by
  other neurons.

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The Chemical Senses

• Taste refers to the stimulation of the taste
  buds.
• Our perception of flavor is the combination of
  both taste and smell.
• Taste and smell axons converge in the
  endopiriform cortex.

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The Chemical Senses

• Receptors for taste are modified skin cells.
• Taste receptors have excitable membranes that
  release neurotransmitters to excite neighboring
  neurons.
• Taste receptors are replaced every 10 to 14 days.

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The Chemical Senses

• Papillae are structures on the surface of the
  tongue that contain the taste buds.
• Each papillae may contain zero to ten taste buds.
• Each taste bud contains approximately 50
  receptors.
• Most taste buds are located along the outside of
  the tongue in humans.

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Fig. 7-18, p. 217
62
The Chemical Senses

• Procedures that alter one receptor but not others
  can be used to identify taste receptors.
• Adaptation refers to reduced perception of a
  stimuli due to the fatigue of receptors.
• Cross-adaptation refers to reduced response to
  one stimuli after exposure to another.

63
The Chemical Senses

• Western societies have traditionally described
  sweet, sour, salty and bitter tastes as the
  primary tastes and four types of receptors.
• Evidence suggests a fifth type of glutamate
  receptor.

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The Chemical Senses

• The saltiness receptor permits sodium ions to
  cross the membrane.
• results in an action potential.
• Sourness receptors close potassium channels when
  acid binds to receptors.
• results in depolarization of the membrane.
• Sweetness, bitterness, and umami receptors
  activate a G protein that releases a second
  messenger in the cell when a molecule binds to a
  receptor.

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The Chemical Senses

• Different chemicals also result in different
  temporal patterns of action potentials and
  activity in the brain.
• Taste is a function of both the type of cell
  activity, as well as the rhythm of cell activity.

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The Chemical Senses

• Bitter receptors are sensitive to a wide range of
  chemicals with varying degrees of toxicity.
• Over 40 types of bitter receptors exist.
• Most taste cells contain only a small number of
  these receptors.
• We are sensitive to a wide range of harmful
  substances, but not highly sensitive to any
  single one.

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The Chemical Senses

• Taste coding in the brain depends upon a pattern
  of responses across fibers in the brain.
• The brain determines taste by comparing the
  responses of several types of taste neurons.
• Receptors converge their input onto the next
  cells in the taste system.
• Cells thus respond best to a particular taste but
  others as well.

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The Chemical Senses

• Different nerves carry taste information to the
  brain from the anterior two-thirds of the tongue
  than from the posterior tongue and throat.
• Taste nerves project to a structure in the
  medulla known as the nucleus of the tractus
  solitarius (NTS).
• projects information to other parts of the brain.

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Fig. 7-19, p. 219
70
The Chemical Senses

• Various areas of the brain are responsible for
  processing different taste information.
• The somatosensory cortex responds to the touch
  aspect of taste.
• The insula is the primary taste cortex.
• Each hemisphere of the cortex is also responsive
  to the ipsilateral side of the tongue.

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The Chemical Senses

• Genetic factors and hormones can account for some
  differences in taste sensitivity.
• Variations in taste sensitivity are related to
  the number of fungiform papillae near the tip of
  the tongue.
• Supertasters have higher sensitivity to all
  tastes and mouth sensations in general.

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Fig. 7-20, p. 220
73
The Chemical Senses

• Olfaction is the sense of smell and refers to the
  detection and recognition of chemicals that
  contact the membranes inside the nose.
• Olfaction is more subject to adaptation than our
  other senses.
• Olfactory cells line the olfactory epithelium in
  the rear of the nasal passage and are the neurons
  responsible for smell.

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Fig. 7-21, p. 221
75
The Chemical Senses

• Olfactory receptors are located on cilia which
  extend from the cell body into the mucous surface
  of the nasal passage.
• Vertebrates have hundreds of olfactory receptors
  which are highly responsive to some related
  chemicals and unresponsive to others.
• Olfaction processes a wide variety of airborne
  chemicals, hence the need for many different
  types of receptors.

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The Chemical Senses

• Proteins in olfactory receptors respond to
  chemicals outside the cells and trigger changes
  in G protein inside the cell.
• G protein then triggers chemical activities that
  lead to action potentials.

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Fig. 7-22, p. 222
78
The Chemical Senses

• Axons from olfactory receptors carry information
  to the olfactory bulb in the brain.
• Coding in the brain is determined by which part
  of the olfactory bulb is excited.
• Chemicals excite limited parts of the olfactory
  bulb with similar chemicals exciting similar
  parts.

79
The Chemical Senses

• The olfactory bulb sends axons to the cerebral
  cortex where messages are coded by location.
• The cortex connects to other areas that control
  feeding and reproduction.
• Both behaviors are highly influenced by smell.

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Olfaction

• Olfactory receptors are replaced approximately
  every month, but are subject to permanent
  impairment from massive damage.
• Anosmia refers to a general lack of olfaction.
• Specific anosmia refers to the inability to smell
  a single type of chemical.

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The Chemical Senses

• Individual differences in olfaction exist
  regarding olfaction.
• Women detect odor more readily than men and brain
  responses are stronger.
• The ability to attend to a faint odor and become
  more sensitive to it is characteristic of young
  adult women and thus seems to be influenced by
  hormones.

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The Chemical Senses

• The vomeronasal organ (VNO) is a set of receptors
  located near the olfactory receptors that are
  sensitive to pheromones.
• Pheromones are chemicals released by an animal to
  affect the behavior of others of the same species.

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The Chemical Senses

• The VNO and pheromones are important for most
  mammals, but less so for humans.
• The VNO is tiny in human adults and has no
  receptors.
• Humans unconsciously respond to some pheromones
  through receptors in the olfactory mucosa.
• Example synchronization of menstrual cycles in
  women.

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The Chemical Senses

• Synesthesia is the experience of one sense in
  response to stimulation of a different sense.
• Estimates suggest 1 in every 500 people (Day,
  2005).
• fMRI case studies show activity in both the
  auditory and visual cortex responsive to color
  when exposed to spoken language.
• Suggests some axons from one area have branches
  to other cortical regions.