Download The Cutaneous Senses

The Cutaneous Senses
(Cutis: Latin for skin)
ELL788
Date:26/09/2016
Cutaneous System
•  Skin - Protects the organism by keeping
damaging agents from penetrating the body
•  Weight? Is it the heaviest organ?
•  Surface area? Is it the largest organ?
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
Merkel receptor –
Ruffini cylinder branched fibers inside a
cylindrical capsule
disk-shaped receptor
located near the border
between the epidermis
and dermis
Meissner corpuscle stack of flattened disks
in the dermis just
below epidermis
Pacinian corpuscle - onionlike capsule located deep in
the skin
Properties of mechanoreceptor nerve fibers
Temporal properties - adaptation:
Slowly adapting (SA)
fibers continue to fire
during a sustained
stimulus.
Rapidly adapting
(RA) fibers fire only
at the onset and
offset of a sustained
stimulus.
Properties of Mechanoreceptor Nerve Fibers
•  Spatial properties - detail resolution
–  SA1 fibers (Merkel receptor) respond to
patterns of grooves
–  RA2 fibers (Pacinian corpuscle) do not
respond to the details of these stimuli
(a) The firing of Merkel receptors/SA1 fiber signals the grooved stimulus pattern, but (b) the firing of the
Pacinian corpuscle/RA2 fiber does not. Results such as these indicate that the Merkel receptor/SA1 fiber
signals details
What happens beyond the skin?
Pathways from Skin to Cortex
peripheral
nerves
proprioceptive and
touch information
Travel to contralateral
Thalamus/cortex
temperature and
pain information
Maps of the Body on the Cortex
•  Signals travel from the thalamus to the
somatosensory receiving area in the cortex
•  Body map (homunculus) on the cortex shows
more cortical space allocated to parts of the
body that are responsible for detail
•  Plasticity in neural functioning leads to
changes in how cortical cells are allocated to
body parts
Body map on the somatosensory cortex
The sensory homunculus of the somatosensory cortex
Plasticity of the
somatosensory map
(a) Each numbered zone represents
the area in the somatosensory
cortex that represents one of the
monkey’s 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.
Any evidence of this in humans?
Maps of the Body on the Cortex - continued
Focal hand dystonia in musicians (“musician’s cramp”), is a greatly
feared condition that leads to reduced performance levels, very often resulting
in the termination of a musician’s career. This neurological disorder is
characterized by a loss of control over individual finger movements. The
Symptoms usually only occur when patients perform certain tasks such as
playing an instrument.
A
B
Why does focal dystonia happen?
Research examining the cortex has found that
musicians with this disorder have “fused” cortical
areas belonging to the affected hand
Fortunately this only happens in about 1% of
musicians
Cross section of the cortex, showing where the five fingers (1-5) are represented in the left and right
hemispheres in a musician with focal dystonia (musician’s cramp). The areas representing all the fingers of
the left hand (on the right side of the brain) are separated, but the areas representing fingers 3, 4, and 5 on
the right hand (left side of brain) are superimposed. This superimposition is associated with the dystonia of
the right hand.
Maps of the Body on the Cortex
•  Signals travel from the thalamus to the
somatosensory receiving area in the cortex
•  Body map (homunculus) on the cortex shows
more cortical space allocated to parts of the
body that are responsible for detail
•  Plasticity in neural functioning leads to
changes in how cortical cells are allocated to
body parts
How do we quantify tactitle acuity?
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
Receptor Mechanisms for Tactile Acuity
•  There is a high density
of Merkel receptor/SA1
fibers 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
Correlation between density of Merkel receptors/SA1 fiber
density and tactile acuity
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
Receptive fields of cortical neurons are smallest on the monkey’s fingers and become
larger on the hand and the forearm.
Two-point thresholds across the male body
Putting tactile perception to use:
Perceiving textures
Perceiving objects
Perceiving Texture
•  Is texture perception just a matter of resolving
the spatially separated bumps on a surface?
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
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
(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.
Experiment by Hollins et al.
•  Adaptation experiment - participants’ skin was adapted with either:
–  10-Hz stimulus for 6 minutes to adapt the RA1/Meissner corpuscle
–  250-Hz stimulus for 6 minutes to adapt the RA2/Pacinian corpuscle
•  Results showed that only the adaptation to the 250-Hz stimulus affected
the perception of fine textures
Perceiving Objects
•  Humans use active rather than passive touch
to interact with the environment
(demo)
•  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 - continued
•  Psychophysical research shows that people
can identify objects haptically in 1 to 2 sec
•  Klatzky et al. have shown that people use
exploratory procedures (EPs)
–  Lateral motion
–  Pressure
–  Enclosure
–  Contour following
(texture assessment)
(material assessment)
(shape assessment)
Some of the exploratory procedures (EPs) observed by Lederman and Klatzky as participants identified
objects
The Physiology of Tactile Object Perception
•  The firing pattern of groups of mechanoreceptors
signals shape, such as the curvature of an object
(a) Response of SA1 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.
The Physiology of Tactile Object Perception
•  Neurons further upstream
become more specialized
–  Monkey’s thalamus shows
cells that respond to centersurround receptive fields
An excitatory-center, inhibitory-surround receptive field
of a neuron in a monkey’s thalamus.
The Physiology of Tactile Object Perception
Somatosensory cortex shows cells that
respond maximally to orientations and
direction of movement
Receptive fields of neurons in the monkey’s somatosensory cortex. (a) This neuron responds best when a
horizontally oriented edge is presented to the monkey’s hand. (b) This neuron responds best when a
stimulus moves across the fingertip from right to left.
The Physiology of Tactile Object Perception
- continued
•  Monkey’s 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
The response of a neuron in a monkey’s parietal cortex that fires when the monkey grasps a ruler but that
does not fire when the monkey grasps a cylinder.
Pain Perception
•  Pain results from exogenous factors (e.g. a pin prick)
and endogenous ones (e.g. emotions)
•  There are three types of pain:
–  Nociceptive - signals impending damage to the
skin
•  Nociceptors in the skin respond to heat,
chemicals, severe pressure, and cold
•  The threshold at which nociceptors respond
should be ‘just right’. Too low would cause
normal activity to result in pain; too high would
take away from the early warning aspect of
pain (the skin would already be damaged
before we sensed pain).
Types of Pain
–  Inflammatory pain caused by damage
to tissues and joints
that releases
chemicals that
activate nociceptors
–  Neuropathic pain caused by damage
to the nervous
system, such as:
•  Nerve damage
due to amputation
•  Spinal cord injury
Pain can be created in a number of different ways.
Pain Perception - continued
•  Signals from nociceptors travel up the
spinothalamic pathway and activate various
subcortical and cortical areas (which are
together are called the pain matrix)
PAIN MATRIX: The perception of pain is accompanied by activation of a number of different areas of the
brain, including the somatosensory cortex, thalamus, anterior cingulate cortex, amygdala, hippocampus,
and thalamus. The perception of pain and corresponding brain responses
can be affected by both sensory and cognitive factors.
Experiment by Hoffauer et al.
•  Participants were presented with potentially
painful stimuli and asked:
–  To rate pain intensity (sensory aspect)
–  To rate pain unpleasantness (affective aspect)
•  Brain activity was measured while they placed
their hands into hot water
•  Hypnosis was used to increase or decrease the
sensory and affective components
Experiment by Hoffauer et al. - continued
• 
Results showed that:
–  Hypnotic suggestions to change the intensity led to changes in those
ratings as well as those of unpleasantness and brain responses in
somatosensory cortex S1
–  Hypnotic suggestions to change the unpleasantness of pain did not affect
the intensity ratings but did change:
•  Ratings of unpleasantness
•  Activation in the anterior cingulate cortex
Cognitive and Experiential Aspects of Pain
(contd.)
•  Expectation - when surgical patients are told what to expect,
they request less pain medication and leave the hospital earlier
•  Shifting attention - virtual reality technology has been used to
keep patients’ attention on other stimuli than the pain-inducing
stimulation
•  Content of emotional distraction - participants could keep their
hands in cold water longer when pictures they were shown were
positive
Cognitive and Experiential Aspects of Pain continued
Pain in Social Situations
•  Singer’s expts on empathic and sensory pain
•  Experiment by Eisenberger et al.
–  Participants watched a computer game
–  Then 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
Given how debilitating pain can be, has the body
evolved any mechanisms to overcome it? Opioids and Pain
•  Brain tissue releases
neurotransmitters called
endorphins (one of the class of
opioid chemicals)
–  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
(a) Naloxone reduces the effect of heroin by occupying a
receptor site normally stimulated by heroin. (b) Stimulationproduced analgesia (SPA) may work by causing the release of
endorphins, which stimulate opiate receptor sites. (c) Naloxone
decreases the pain reduction caused by SPA and placebos by
displacing endorphins.
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?
•  Do all people experience pain in the same
way?