Chapter 15 - The Cutaneous Senses Flashcards
1- Ian Waterman story
Ian Waterman story
* Worked as an apprentice in a butcher shop.
* He had previously gotten a small cut in his finger and most likely the cut had
developed into an infection.
* What started out as a common cold would prove to be something much worse.
* Ian gradually lost control over his limbs and ended up lying in bed without
conscious control over any part of his body from his neck down.
* His muscles still worked and his brain was receiving signals from his body
conveying sensations such as pain and differences in temperature. But the brain
seemed to have lost the notion of where the different parts that it was supposed
to move were located.
Nerve fibre:
* Can be either a sensory fibre or motor fibre.
* The motor fibres sends signals to our muscle fibres telling them to contract.
* The sensory fibres starts either in the skin or in the muscle and come in
different sizes.
* The largest ones convey information concerning touch, muscle sensitivity or
sense of movement.
* The smallest ones convey information concerning muscle fatigue, temperature and certain forms of pain.
2- The Somatosensory System
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.
2- Overview of the Cutaneous System
Skin: heaviest organ in the body
* Protects the organism by keeping damaging
agents from penetrating the body (viruses, bacteria, toxins)
* 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
-fine details and textures, shapes
* Meissner corpuscle fires only when a stimulus is first applied and when it is
removed.
- Responsible for controlling hand-grip
-motion across skin
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 (by moving fingers)
4- 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…dorsal root…spinal cord…medial lemniscus or spinothalamic tract…thalamus….ventrolateral nucleu…somatosensory cortex
The Somatosensory
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.
(see homunculus map)
5- 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.
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
Two-Point Thresholds on Different Parts of the Male Body:
Fingers= 4mm (until you can feel that there are two needles for example)
Upper lip=8mm
Big toe=9mm
Back=42mm
Thigh=44mm
Upper arm=46mm
In the brain, smaller threshold on body= more spaced out in brain (can distinguish two points better)
and opposite when bigger threshold on body=more close in brain (can’t distinguish two points as well)
(see image)
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.
6- Vibration of the Skin
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.
(see image)
7- Surface Texture
Katz (1925) proposed that perception of texture depends on two cues.
* Spatial cues
- determined by the size, shape, and distribution of surface elements.
- bumps and grooves.
*Temporal cues
- determined by the rate of vibration as skin is moved across finely textured
surfaces.
- Fine textures
- Could only be perceived as fingers move across the surface.
* Duplex theory of texture perception
-Two receptors may be responsible for this
process.
Spatial and Temporal Cues
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.
Cortical Responses to Surface Texture
Lieber and Sliman Bensmaia (2019)
* These patterns show that
- (1) different textures caused
different ????????????
- (2) different neurons
responded differently to the
same texture
* Lieber and Bensmaia
* Cortical neurons that
fired best to coarse textures received input from SA1
neurons in the skin
(Merkel receptors)
* Neurons that fired
best to fine textures
received input from
PC receptors
(Pacinian corpuscles).
8- Object Perception
- Active touch
- touch in which a person actively explores an object, usually with fingers and hands.
- Passive touch
- occurs when touch stimuli are applied to the skin, as when two points are pushed onto the skin to determine the two-point threshold.
- Haptic perception
- perception in which three-dimensional objects are explored with the fingers and hand
- a number of different systems are interacting with each other.
- (1) the sensory system, which was involved in detecting cutaneous sensations such as touch,
temperature, and texture and the movements and positions of your fingers and hands. - (2) the motor system, which was involved in moving your fingers and hands.
- (3) the cognitive system, which was involved in thinking about the information provided by the
sensory and motor systems. - Psychophysical research has shown that people can accurately identify
most common objects within 1 or 2 seconds using active touch. - people use a number of distinctive movements, which they called exploratory procedures (EPs), and that the types of EPs used
depend on the object qualities the participants are asked to judge
Ex: lateral motion, pressure, enclosure, contour following
9- The Cortical Physiology of
Tactile Object Perception
- Cortical Neurons Are Specialized Neurons in the ventral posterior nucleus
- tactile area of the thalamus.
- center-surround receptive
fields that are similar to the center-surround receptive fields in the lateral geniculate
nucleus, which is the
visual area of the
thalamus. - In the cortex, we find some neurons with center-surround receptive fields and others that respond to
more specialized stimulation of the skin. - Neurons in the monkey’s somatosensory cortex
(e.g., parietal) that respond when the monkey grasps a specific object. - Cortical Responding Is Affected by Attention.
- Steven Hsiao and coworkers
- recorded the response of neurons in areas S1 and S2 to raised letters that were
scanned across a monkey’s finger. - Tactile-attention condition
- monkey had to perform a task that required focusing its attention on the letters being presented to its fingers.
- Visual-attention condition, the monkey had to
focus its attention on an unrelated visual
stimulus.
Results:
Firing rate of neurons much higher in tactile-attention condition
10- Pain
- Pain is a multimodal phenomenon containing a sensory component and an affective or emotional component
- Inflammatory pain
- caused by damage to tissue or inflammation of joints or by tumor cells.
- Neuropathic pain
- caused by lesions or other damage to the nervous system.
Examples of neuropathic pain are carpal tunnel syndrome, which is caused by repetitive tasks such as typing; spinal
cord injury; and brain damage due to stroke. - Nociceptive pain
- caused by activation of receptors in the skin called
nociceptors, which are specialized to respond to tis-sue damage or potential damage. - Direct pathway model of pain
- pain occurs when nociceptor receptors in the skin are stimulated and send their signals directly from
the skin to the brain. - Situations in which pain was affected by factors in addition to stimulation of the skin.
- Soldiers’ wounds had a positive aspect: they
provided escape from a hazardous battlefield to the
safety of a behind-the-lines hospital. - Phantom limb
- people who have had a limb amputated continue
to experience the limb. - signals are sent from the part of the limb that remains after amputation.
- cutting the nerves that used to transmit signals from the limb to the brain does not
eliminate either the phantom limb or the pain and concluded that the pain must
originate not in the skin but in the brain. - Cannot be explained by the direct pathway model!!
Gate Control Model of Pain:
* begins with the idea that pain signals enter the spinal cord from the body and are then transmitted
from the spinal cord to the brain.
* there are additional pathways that
influence the signals sent from the spinal cord to the brain.
* signals from these additional pathways can act to open or close a
gate, located in the spinal cord, which determines the strength of the signal leaving the spinal cord.
* Nociceptors
- Fibers from nociceptors activate a circuit
consisting entirely of excitatory synapses,
and therefore send excitatory signals to the
transmission cells.
* Mechanoreceptors
- Fibers from mechanoreceptors carry
information about nonpainful tactile
stimulation.
- e.g., signals sent from rubbing the skin.
* Central control
- These fibers, which contain information related to cognitive functions such as
expectation, attention, and distraction, carry signals down from the cortex.
11- Top-Down Processes
- Expectation
- Placebo
- a pill that contains no active ingredients.
- Placebo effect
- decrease in pain from a substance that has no
pharmacological effect. - Expectation
*-Participants rated the pain in a condition in which a saline solution was
presented by infusion (baseline) - three conditions in which the analgesic drug remifentanil was presented, but the participants were told
(1) that they were still receiving the saline solution (no expectation)
(2) that the drug was being presented (positive expectation)
(3) that the drug was going to be discontinued in order to investigate the possible increase in pain that would occur (negative expectation). - Nocebo effect
- negative effect caused by the negative expectation.
Results:
See table and understand!! - Attention
- one way to decrease pain is to distract a person’s attention from the source of the pain.
- Emotions
- Minet de Wied and Marinis Verbaten (2001).
- Viewing positive and negative pictures
- hand immersed in cold.
- Roy and coworkers (2008)
- Pleasant and unpleasant music
- Heat stimulus
Results: Unpleasant music: no effect
Pleasant music: decreased intensity rating and unpleasantness rating
