SKIN AND BODY SENSES
OVERVIEW OF QUESTIONS
• Are there specialized receptors in the skin for
sensing different tactile qualities?
• What is the most sensitive part of the body?
• Is it possible to reduce pain with your
thoughts?
TOUCH THERAPY OF NEWBORNS
• Touch Research Institute
• Established in 1992 by
Director Tiffany Field, Ph.D. at
the University of Miami School
of Medicine.
• Premature infants got little
tactile stimulation.
• Trained senior citizens to give
massages.
• Many benefits to babies and
seniors.
BENEFITS OF MASSAGE
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Compared with controls.
Massaged premature babies:
Were more alert and active
Gained more weight
Went home 3-6 days earlier (huge savings in costs)
Most likely helps emotional bonding.
Need for tactile contact when we are young.
And older, too.
Seniors gave each other massages.
Less stress and sense of purpose.
CULTURAL DIFFERENCES
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USA culture somewhat aversive to touch.
Compared with South Americans and Europeans.
French touch children much more than USA parents.
Okay for straight men to walk hand in hand?
Some cultures accept.
Lots of jokes about Bush and Saudi Prince.
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
CUTANEOUS SYSTEM
• Skin - heaviest organ in the body
• Funtion: Protects the organism by keeping
damaging agents from penetrating the
body.
• Layers: Epidermis is the outer layer of the
skin, which is made up of dead skin cells.
• Dermis is below the epidermis and contains
mechanoreceptors that respond to stimuli
such as pressure, stretching, and vibration.
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
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.
MECHANORECEPTORS – GOING DEEPER
• 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
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.
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.
• These cross over to the opposite side of the body
and synapse in the thalamus.
Touch
Pain
The fiber carrying signals from a receptor in the finger enters the
spinal cord through the dorsal root and then travels up the spinal cord
in two pathways: the medial lemniscus and the spinothalamic tract.
These pathways synapse in the ventrolateral nucleus of the thalamus
and then send fibers to the somatosensory cortex in the parietal lobe.
MAPS OF THE BODY ON THE CORTEX
• Signals travel from the thalamus to the
somatosensory receiving area (S1) and the
secondary receiving area (S2) in the parietal
lobe.
• 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.
(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.
SENSORY
HOMUNCULUS
SENSORY RECEPTORS FOR
MOVEMENT
Also on the sensory strip: Body awareness
KW 10-23
VESTIBULAR SYSTEM: BALANCE
Also important
for body sense
The labyrinth of the inner ear. It contains the semicircular canals
(brown), which detect acceleration in all directions; and the
otolithic organs (in the blue/purple pouches), which head
orientation. Output interacts with eye movements.
MOTION SICKNESS
• A condition in which a disagreement exists
between visually perceived movement and the
vestibular system's sense of movement.
• Motion is sensed by the vestibular system and
hence the motion is felt, but no motion or little
motion is detected by the visual system.
• Car (as passenger), planes and ships. Also films.
• Brain mechanism to reject poisons (nausea-vomit).
• Treatment to reduce nausea: dramamine, ginger
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
Methods for determining tactile acuity (a) two-point threshold;
(b) grating acuity.
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.
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.
Two-point thresholds for males. Touch most sensitive at periphery.
Two-point thresholds for females follow the same pattern.
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.
(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.
PERCEIVING TEXTURE 1
• 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
PERCEIVING TEXTURE 2
• 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.
(a) Participants in Hollins and Reisner’s (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.
PERCEIVING OBJECTS 1
• Humans use active rather than passive touch
to interact with the environment.
• Haptic perception is the active exploration of
3-D objects with the hand.
• It uses three distinct systems
• Sensory system
• Motor system
• Cognitive system
PERCEIVING OBJECTS 2
• Psychophysical research shows that people
can identify objects haptically in one to two
seconds.
• Klatzky et al. have shown that people use
exploratory procedures (EPs)
• Lateral motion
• Contour following
• Pressure
• Enclosure
Some of the exploratory procedures (EPs) observed by Lederman
and Klatzky as participants identified objects.
PAIN PERCEPTION
• Pain is a multimodal phenomenon containing a
sensory component and an affective or emotional
component.
• Three types of pain
• 1) 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.
• More pain receptors in center of body
TYPES OF PAIN
• 2) Inflammatory pain - caused by damage
to tissues and joints or by tumor cells
• 3) 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
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.
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 person’s mental state.
• Pain can occur when there is no stimulation of the
skin.
• Pain can be affected by a person’s attention.
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
EMOTIONAL DISTRACTION
• Content of emotional distraction - participants
could keep their hands in cold water longer
when pictures they were shown were positive
The results of deWied and Verbaten’s (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.
BRAIN STRUCTURES AND PAIN
• Subcortical areas including the
hypothalamus, limbic system, and the
thalamus.
• Cortical areas including S1 and S2 in the
somatosensory cortex, the insula, and the
anterior cingulate and prefrontal cortices.
• These cortical areas taken together are called
the pain matrix.
Figure 14.26 The perception of pain is accompanied by activation of a
number of different areas of the brain. All of these areas, taken
together, are called the pain matrix.
CHEMICAL OPTION: OPIOIDS AND PAIN
• Brain tissue releases neurotransmitters called
endorphins.
• Evidence shows that endorphins reduce pain.
• Injecting naloxone (opiate antagonist)
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. Ex: women giving birth
(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.
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.
TWO COMPONENTS OF PAIN
• Where am I hurt?
• How much do I care?