Chapter 14: The Cutaneous Senses

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Chapter 14 The Cutaneous Senses
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Somatosensory System

• There are three parts
• Cutaneous senses - perception of touch and pain
  from stimulation of the skin
• Proprioception - ability to sense position of the
  body and limbs
• Kinesthesis - ability to sense movement of body
  and limbs

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Mechanoreceptors

• Two types located close to surface of the skin
• Merkel receptor fires continuously while stimulus
  is present.
• Responsible for sensing fine details
• Meissner corpuscle fires only when a stimulus is
  first applied and when it is removed.
• Responsible for controlling hand-grip

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• Figure 14.1 A cross section of glabrous (without
  hairs or projections) skin, showing the layers of
  the skin and the structure, firing properties and
  perceptions associated with the Merkel receptor
  and Meissner corpuscle - two mechanoreceptors
  that are near the surface of the skin.

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Mechanoreceptors - continued

• Two types located deeper in the skin
• Ruffini cylinder fires continuously to
  stimulation
• Associated with perceiving stretching of the skin
  
• Pacinian corpuscle fires only when a stimulus is
  first applied and when it is removed.
• Associated with sensing rapid vibrations and fine
  texture

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• Figure 14.2 A cross section of glabrous skin,
  showing the structure, firing properties and
  perceptions associated with the Ruffini cylinder
  and the Pacinian corpuscle - two mechanoreceptors
  that are deeper in the skin.

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Pathways from Skin to Cortex

• Nerve fibers travel in bundles (peripheral
  nerves) to the spinal cord.
• Two major pathways in the spinal cord
• Medial lemniscal pathway consists of large fibers
  that carry proprioceptive and touch information.
• Spinothalamic pathway consists of smaller fibers
  that carry temperature and pain information.

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Maps of the Body on the Cortex

• Body map (homunculus) on the cortex in S1 and S2
  shows more cortical space allocated to parts of
  the body that are responsible for detail.
• Plasticity in neural functioning leads to
  multiple homunculi and changes in how cortical
  cells are allocated to body parts.

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• Figure 14.4 (a) The sensory homunculus on the
  somatosensory cortex. Parts of the body with the
  highest tactile acuity are represented by larger
  areas on the cortex. (b) The somatosensory cortex
  in the parietal lobe. The primary somatosensory
  area, S1 (light shading), receives inputs from
  the ventrolateral nucleus of the thalamus. The
  secondary somatosensory area, S2 (dark shading),
  is partially hidden behind the temporal lobe.
  (Adapted from penfield Rasmussen, 1950).

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• Figure 14.5 (a) Each numbered zone represents the
  area in the somatosensory cortex that represents
  one of the monkeys five fingers. The shaded area
  on the zone for finger 2 is the part of the
  cortex that represents the small area on the tip
  of the finger shown in (b). (c) The shaded region
  shows how the area representing the fingertip
  increased in size after this area was heavily
  stimulated over a 2-month period. (From Merzenich
  et al., 1988)

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Perceiving Details

• Measuring tactile acuity
• Two-point threshold - minimum separation needed
  between two points to perceive them as two units
• Grating acuity - placing a grooved stimulus on
  the skin and asking the participant to indicate
  the orientation of the grating
• Raised pattern identification - using such
  patterns to determine the smallest size that can
  be identified

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• Figure 14.7 Methods for determining tactile
  acuity (a) two-point threshold (b) grating
  acuity.

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Receptor Mechanisms for Tactile Acuity

• There is a high density of Merkel receptors in
  the fingertips.
• Merkel receptors are densely packed on the
  fingertips - similar to cones in the fovea.
• Both two-point thresholds and grating acuity
  studies show these results.

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Cortical Mechanisms for Tactile Acuity

• Body areas with high acuity have larger areas of
  cortical tissue devoted to them.
• This parallels the magnification factor seen in
  the visual cortex for the cones in the fovea.
• Areas with higher acuity also have smaller
  receptive fields on the skin.

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• Figure 14.10 Two-point thresholds for males.
  Two-point thresholds for females follow the same
  pattern. (From S. Weinstein, 1968.)

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• Figure 14.11 Receptive fields of monkey cortical
  neurons that fire (a) when the fingers are
  stimulated (b) when the hand is stimulated and
  (c) when the arm is stimulated. (d) Stimulation
  of two nearby points on the finger causes
  separated activation on the finger area of the
  cortex, but stimulation of two nearby points on
  the arm causes overlapping activation in the arm
  area of the cortex. (From Kandel Jessell, 1991
  (a-c).

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Perceiving Vibration

• Pacinian corpuscle (PC) is primarily responsible
  for sensing vibration.
• Nerve fibers associated with PCs respond best to
  high rates of vibration.
• The structure of the PC is responsible for the
  response to vibration - fibers without the PC
  only respond to continuous pressure.

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• Figure 14.12 (a) When a vibrating pressure
  stimulus is applied to the Pacinian corpuscle, it
  transmits these pressure vibrations to the nerve
  fiber. (b) When a continuous pressure stimulus is
  applied to the Pacinian corpuscle, it does not
  transmit the continuous pressure to the fiber.
  (c) Lowenstein determined how the fiber fired to
  stimulation of the corpuscle (at A), and to
  direct stimulation of the fiber (at B) (Adapted
  from Lowenstein, 1960)

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Perceiving Texture

• Katz (1925) proposed that perception of texture
  depends on two cues
• Spatial cues are determined by the size, shape,
  and distribution of surface elements.
• Temporal cues are determined by the rate of
  vibration as skin is moved across finely textured
  surfaces.
• Two receptors may be responsible for this process
  - called the duplex theory of texture perception

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Perceiving Texture - continued

• Past research showed support for the role of
  spatial cues.
• Recent research by Hollins and Reisner shows
  support for the role of temporal cues.
• In order to detect differences between fine
  textures, participants needed to move their
  fingers across the surface.

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• Figure 14.13 (a) Participants in Hollins and
  Reisners (2000) experiment perceived the
  roughness of two fine surfaces to be essentially
  the same when felt with stationary fingers, but
  (b) could perceive the difference between the two
  surfaces when they were allowed to move their
  fingers.

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• Figure 14.16 (a) Response of fibers in the
  fingertips to touching a high-curvature stimulus.
  The height of the profile indicates the firing
  rate at different places across the fingertip.
  (b) The profile of firing to touching a stimulus
  with more gentle curvature. (From Goodwin, 1998)

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The Physiology of Tactile Object Perception -
continued

• Monkeys somatosensory cortex also shows neurons
  that respond best to
• grasping specific objects.
• paying attention to the task.
• Neurons may respond to stimulation of the
  receptors, but attending to the task increases
  the response.

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• Figure 14.18 Receptive fields of neurons in the
  monkeys somatosensory cortex. (a) This neuron
  responds best when a horizontally oriented edge
  is presented to the monkeys hand. (b) This
  neuron responds best when a stimulus moves across
  the fingertip from right to left. (From Hyvarinin
  Poranen, 1978)

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• Figure 14.19 The response of a neuron in a
  monkeys parietal cortex that fires when the
  monkey grasps a ruler but that does not fire when
  the monkey grasps a cylinder. The monkey grasps
  the objects at time 0. (From Sakata Iwamura,
  1978)

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• Figure 14.20 Firing rate of a neuron in area S1
  of a monkeys cortex to a letter being rolled
  across the fingertips. The neuron responds only
  when the monkey is paying attention to the
  tactile stimulus. (From Hsiao, OShaughnessy,
  Johnson, 1993)

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Pain Perception

• Pain is a multimodal phenomenon containing a
  sensory component and an affective or emotional
  component.
• Three types of pain
• Nociceptive - signals impending damage to the
  skin
• Types of nociceptors respond to heat, chemicals,
  severe pressure, and cold.
• Threshold of eliciting receptor response must be
  balanced to warn of damage, but not be affected
  by normal activity.

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Types of Pain

• Inflammatory pain - caused by damage to tissues
  and joints or by tumor cells
• Neuropathic pain - caused by damage to the
  central nervous system, such as
• Brain damage caused by stroke
• Repetitive movements which cause conditions like
  carpal tunnel syndrome

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• Figure 14.21 Nociceptive pain is created by
  activation of nociceptors in the skin that
  respond to different types of stimulation.
  Signals from the nociceptors are transmitted to
  the spinal cord and then from the dorsal root of
  the spinal cord in pathways that lead to the
  brain.

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Direct Pathway Model of Pain Perception

• Early model that stated nociceptors are
  stimulated and send signals to the brain
• Problems with this model
• Pain can be affected by a persons mental state.
• Pain can occur when there is no stimulation of
  the skin.
• Pain can be affected by a persons attention.

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Gate Control Model of Pain Perception

• The gate consists of substantia gelatinosa
  cells in the spinal cord (SG- and SG).
• Input into the gate comes from
• Large diameter (L) fibers - information from
  tactile stimuli
• Small diameter (S) fibers - information from
  nociceptors
• Central control - information from cognitive
  factors from the cortex

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Gate Control Model of Pain Perception - continued

• Pain does not occur when the gate is closed by
  stimulation into the SG- from central control or
  L-fibers into the T-cell.
• Pain does occur from stimulation from the
  S-fibers into the SG into the T-cell.
• Actual mechanism is more complex than this model
  suggests.

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Cognitive and Experiential Aspects of Pain

• Expectation - when surgical patients are told
  what to expect, they request less pain medication
  and leave the hospital earlier
• Placebos can also be effective in reducing pain.
• Shifting attention - virtual reality technology
  has been used to keep patients attention on
  other stimuli than the pain-inducing stimulation

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Cognitive and Experiential Aspects of Pain -
continued

• Content of emotional distraction - participants
  could keep their hands in cold water longer when
  pictures they were shown were positive
• Experiment by Derbyshire to investigate
  hypnotically induced pain.
• Participants had a thermal stimulator attached
  the to palm of their hand.

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Experiment by Derbyshire et al. - continued

• Three conditions
• Physically induced pain
• Hypnotically induced pain
• Control group that imagined painful stimulation
• Both subjective reports and fMRI scans showed
  that hypnosis did produce pain perception.

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• Figure 14.24 The results of deWied and Verbatens
  (2001) experiment showing that participants kept
  their hands in cold water longer when looking at
  positive pictures than when looking at neutral or
  negative pictures.

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Opioids and Pain

• Brain tissue releases neurotransmitters called
  endorphins.
• Evidence shows that endorphins reduce pain.
• Injecting naloxone blocks the receptor sites
  causing more pain.
• Naloxone also decreases the effectiveness of
  placebos.
• People whose brains release more endorphins can
  withstand higher pain levels.

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• Figure 14.28 (a) Naloxone reduces the effect of
  heroin by occupying a receptor site normally
  stimulated by heroin. (b) Stimulating sites in
  the brain that cause the release of endorphins
  can reduce the pain by stimulating opiate
  receptor sites. (c) Naloxone decreases the pain
  reduction caused by endorphins, by keeping the
  endorphins from reaching the receptor sites.

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Pain in Social Situations

• Experiment by Eisenberger et al.
• Participants watched a computer game.
• Then, they were asked to play with two other
  players who did not exist but were part of the
  program.
• The players excluded the participant.
• fMRI data showed increased activity in the
  anterior cingulate cortex and participants
  reported feeling ignored and distressed.

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Pain in Social Situations - continued

• Experiment by Singer et al.
• Romantically involved couples participated.
• The womans brain activity was measured by fMRI.
• The woman either received shocks or she watched
  while her partner received shocks.
• Similar brain areas were activated in both
  conditions.