14: Cutaneous Perception

Question 1

Cutaneous perception is part of the broader category of somatosensory processing. What three elements make up somatosensory processing?

Answer 1

Cutaneous Perception: processing of sensations from receptors in the skin.

Proprioception: “body sense” processing of input from skin, muscles, tendons, and vestibular systems to create a perception of the body in the environment.

Kinesthesis: sense of position and movement of the limbs.

Question 2

What are three measures with which we measure cutaneous perception?

Answer 2

Precision stimulus control.

Electrophysiological techniques.

Appropriate animal models.

Question 3

List three features of skin.

Answer 3

Heaviest organ in the human body.

Prevents bacteria, chemical agents and dirt from entering our bodies.

Preserves bodily fluids by decreasing evaporation.

Question 4

What are the four mechanoreceptors in top-to-bottom order and their associated fibers?

Answer 4

Merkel Receptor (associated with SA1 fibers).

Meissner corpuscle (associated with RA1 fibers).

Ruffini cylinder (associated with SA2 fibers).

Pacinian corpuscle (associated with RA2 fibers).

Question 5

Transducers reside in the _____ of afferent fibers.

Answer 5

Unmyelinated endings.

Question 6

List the response method, frequency response and perception specialization of each mechanoreceptor.

Answer 6

Merkel Receptor: continuous (slow adapting), 0.3-3Hz (slow pushing), fine details.

Meissner corpuscle: Responds to change (rapid adapting), 3-40Hz, flutter (“microslip,” hand-grip controls).

Ruffini cylinder: Continuous (slow adapting), 15-400Hz, stretching.

Pacinian corpuscle: Respond to change (rapid adapting), 10-500Hz (rapid vibration at upper range), vibration (texture by moving fingers).

Question 7

Response properties of each receptor are dictated by what three things?

Answer 7

Physical ultrastructure of the receptor ending.

Ion channels on the unmyelinated ends of the afferent fibers.

Physical location of the receptors.

Question 8

Touch stimuli are further processed in the _____ and _____.

Answer 8

Thalamus; cortex.

Question 9

What are the two pathways from skin to the cortex? Where do fibers decussate and where do most synapse?

Answer 9

Medial Lemniscal Pathway: large fibers, touch and proprioception signals

Spinothalamic Pathway: small fibers, temperature and pain signals

Fibers from both pathways decussate before reaching the thalamus.

Most fibers synapse in the medial or lateral portions of the ventral posterior lateral nucleus.

Question 10

Multiple representations of the body in the parietal lobe (sometimes called a homunculus) are mapped in what three areas?

Answer 10

Somatosensory receiving area (S1).

Secondary somatosensory area (S2).

Other somatosensory areas.

Question 11

What is like the magnification factor in V1?

Answer 11

Cortical representation is related to receptive field size.

Question 12

Areas of the body with numerous small receptive fields have _____ representation in the cortex. Areas of the body with fewer but larger receptive fields have _____ representation in the cortex.

Answer 12

Much; less.

Question 13

What is an example of plasticity in the somatosensory system?

Answer 13

Stimulus on fingertip becomes important for obtaining a reward → cortical representation of the fingertip expands.

Question 14

Define tactile acuity and what is used to measure it.

Answer 14

Our ability to detect details on the skin, important for coding object shape and texture.

Two-point threshold: to perceive two points, each point has to stimulate significantly different pool of receptive fields.

Question 15

Parts of the body that have the lowest two-point thresholds have what two things?

Answer 15

Smallest receptive fields, largest representation in somatosensory cortex.

Question 16

Tactile acuity can also be measured with grating acuity. Define it.

Answer 16

To perceive grating as horizontal or vertical, must detect the spacing between bars.

Question 17

_____ have ideal response properties to signal shape.

Answer 17

Merkel Receptors (SA1 afferents).

Question 18

List three ways in which Merkel Receptors are shown to be good at perceiving details.

Answer 18

Good correlation between tactile acuity and SA1 spacing.

Respond ~linearly to static skin indentation.

For stimuli smaller than the receptive field (2-5mm), spatial detail is likely detected by a single terminal branch.

Question 19

Spike rate indicates the activity of sensory afferents when different parts of the stimulus are pressed into the receptive field center. What do Merkel (SA1) and Pacinian (RA2) respond to?

Answer 19

Merkel: detects each gap/bump of the stimulus.

Pacinian: responds about equally to all parts of the stimulus.

Question 20

What are important interrelated factors when it comes to cortical mechanisms for tactile acuity? When are two stimuli more likely to be perceived as separate?

Answer 20

Receptive field sizes of cortical neurons, spacing of neurons within cortex.

More likely to be perceived as separate if they activate different receptive fields, and non-overlapping regions of cortex.

Question 21

Pacinian corpuscle (RA2 afferents) respond to vibrations of what sensitivity?

Answer 21

Can respond to 10nm of skin motion at 200Hz

Question 22

Transient nature of RA2 afferents is due to the onion-like structure of the Pacinian corpuscle. What are the layers composed of and what do they do? What happens when the “onion skin” is removed?

Answer 22

Layers filled with viscous fluid that dampen static forces.

Transient response changes to sustained response when the “onion skin” is removed.

Question 23

Touch perception depends on what two cues?

Answer 23

Spatial Cues: tactile acuity (mainly Merkel Receptors) objects press into the skin, and we detect this pressure.

Temporal Cues: objects move as we interact with them, this movement transmits vibrations through the skin.

Question 24

The combination of spatial and temporal cues for tactile perception is called the _____.

Answer 24

Duplex Theory of Texture Perception.

Question 25

What happens when frequency of vibration is 3-40Hz and 10-500Hz? What afferents detect these?

Answer 25

3-40Hz: Small microslips occur for objects being gripped in the hand. RA1 (Meissner) fibers are perfectly suited for detecting these slips.

10-500Hz: Higher frequencies are transmitted to the skin when a textured object is moved over the skin and through objects held in the hand (e.g. pen on paper). RA2 (Pacinian) fibers detect vibration.

Question 26

Are spatial cues alone good for texture discrimination? What can improve it?

Answer 26

No.

Improves when vibrations produced by hand motion are available.

Question 27

Texture discrimination is most affected when _____ are adapted

Answer 27

Pacinian corpuscle.

Question 28

What are the two types of touch? What control does the subject have and what is experienced?

Answer 28

Passive Touch: contact force is provided by outside source, objects poked/pressed into skin, experienced as sensation located on skin (e.g. I feel a prickling sensation).

Active Touch: contact force is provided by the subject, subject controls how skin is stimulated, experienced as a property of the object (e.g. I feel a pointed object).

Question 29

Haptic perception is the exploration of three-dimensional objects with the hands and uses active touch. What three things does it involve?

Answer 29

Cutaneous sensory system: Texture, pressure, temperature, finger position.

Motor system: Moving fingers and hands with sensory feedback.

Cognitive system: matching bottom up information from sensory-motor interactions with top-down information.

Question 30

People use a set of stereotypical exploratory procedures in haptic perception. What are the two?

Answer 30

Texture: lateral motion and contour following.

Shape: contour following and enclosure.

Question 31

Neurons in the _____ have center-surround receptive fields.

Answer 31

Ventral posterior lateral nucleus

Question 32

In the primate somatosensory cortex, there are neurons respond to pressure from an elongated edge of a _____. Other neurons respond to motion across the skin in a _____.

Answer 32

Specific orientation; specific direction.

Question 33

A single neuron in S1 in primate studies responds strongly to and poorly to what, respectively?

Answer 33

Responds strongly when the monkey grasps a ruler with a sharp edge.

Responds poorly when the monkey grasps a rounded cylinder.

Question 34

What modulates tactile induced activity in somatosensory cortex? What may it explain?

Answer 34

Attention.

May explain why we do not constantly feel the texture of our clothing, or the tightness of our wristwatches.

Question 35

The multimodal nature of pain is called such because? Why might this be?

Answer 35

Sensory component (prickly, sharp), affective/emotional component (annoying, nagging).

Thought because painful stimuli are so closely related to self-preservation.

Question 36

What are the three types of pain?

Answer 36

Nociceptive Pain (nociceptors): signals impending damage to the skin.

Inflammatory Pain: signals damage to tissues. Damaged cells leak “inflammatory soup” which activate or sensitize nociceptors.

Neuropathic Pain: caused by damage to the nervous system.

Question 37

Painful stimuli activate what subcortical areas?

Answer 37

Hypothalamus, thalamus.

Question 38

Painful stimuli activate what telencephalic areas?

Answer 38

Somatosensory cortex (S1, S2), insula, anterior cingulate cortex, hippocampus, amygdala.

Question 39

The 1950s Direct Pathway Model suggested what?

What do recent findings suggest?

Answer 39

Nociceptor receptors → brain = pain, purely bottom-up.

Top-down modulation of nociceptive sensations (e.g. person does not realize in pain until noticing injury; phantom limbs).

Question 40

Hypnotic suggestion to change the intensity of pain yielded what results and where did it alter activity? Suggestion to change unpleasantness?

Answer 40

Change intensity: affected subjects rating of both pain intensity and unpleasantness, altered S1.

Change unpleasantness: affected subjects rating of pain unpleasantness, but did not affect intensity. Altered anterior cingulate cortex.

Question 41

What are three aspects important to the cognitive and emotional aspects of pain?

Answer 41

Attention and distraction (saliency of distraction task is important).

Emotional content.

Expectation (e.g. placebo effect).

Question 42

What is the Gate Control Model?

Answer 42

Attempts to account for top-down (thoughts, memories, expectations, emotions) influence on pain perception. Suggested that the gate control system is located in the substantia gelatinosa of the spinal cord.

Question 43

What are the components of the Gate Control Model?

Answer 43

L-fibers: large diameter fibers (fast) carry information about non-painful tactile stimuli from mechanoreceptors.

Central control: top down inputs.

S-fibers: small diameter fibers (slow), associated with nociceptors.

Transmission-cell (T-cell): integrates inputs, so the activity level dictates the level of perceived pain.

Question 44

In the Gate Control Model, S-fibers _____. L-fibers and central control _____.

Answer 44

Increase T-cell activity; decrease T-cell activity.

Question 45

Stimulation-produced analgesia yielded what results?

Answer 45

Electrical stimulation of the midbrain (periaqueductal gray region) caused experimental animals to ignore normally painful stimuli.

Question 46

Opiate receptors are activated by opioid drugs, but also activated by endogenously produced _____.

Answer 46

Endorphines.

Question 47

What is naloxone and its function?

Answer 47

Competitive antagonist for opioid receptors. Can block the effects of placebos and stimulation-produced analgesia

Question 48

Seeing horrific injuries induced “phantom pain” in what percent of observers? Same subjects also had more activation of what areas?

Answer 48

~30%.

Activation of S2 and insula.

Question 49

Subjects with higher empathy scores in fMRI “electrocute my boyfriend” studies had greater activation of what area?

Answer 49

Anterior cingulate cortex