Topic 15: The Cutaneous Senses Flashcards
Somatosensory System
the system that includes the cutaneous senses (senses involving the skin), proprioception (the sense of position of the limbs), kinesthesis (sense of movement of the limbs), haptic perception and the vestibular system
Cutaneous Senses
the ability to perceive sensations, such as touch and pain, that are based on the stimulation of receptors in the skin
Kinesthesis
the sense that enables us to feel the motions and positions of limbs and body
Epidermis
the outer layer of the skin, including a layer of dead skin cells
Dermis
the layer of skin below the epidermis
Mechanoreceptors
receptor that responds to mechanical stimulation of the skin, such a pressure, stretching, or vibration
Merkel Receptor
a disk-shaped receptor in the skin associated with slowly adapting fibers and the perception of fine details
Meissner Corpuscle
a receptor in the skin, associated with RA1 mechanoreceptors
it has been proposed that the Meissner corpuscle is important for perceiving tactile slip and for controlling the force needed to group objects
Rapidly Adapting (RA1) Fiber
fiber in the cutaneous system that adapts rapidly to a stimulus and so responds briefly to tactile stimulation
Ruffini Cyclinder
a receptor structure in the skin associated with slowly adapting fibers
it has been proposed that the Ruffini cylinder is involved in perceiving “stretching”
SA2
a slowly adapting fiber in the cutaneous system that is associated with the Ruffini cylinder and is located deeper in the skin than the SA1 fiber
this fiber also responds continuously to a tactile stimulus
Pacinian Corpuscle
a receptor with a distinctive elliptical shape associated with RA2 mechanoreceptors
it transmits pressure to the nerve fiber inside it only at the beginning or end of a pressure stimulus and is responsible for our perception of vibration and fine textures when moving the fingers over a surface
RA2
fiber in the skin associated with Pacinian corpuscle receptors that is located deeper in the skin than RA1 fibers
Medial Lemniscal Pathway
a pathway in the spinal cord that transmits signals from the skin toward the thalamus
Spinothalamic Pathway
one of the nerve pathways in the spinal cord that conducts nerve impulses from the skin to the somatosensory area of the thalamus
Secondary Somatosensory Cortex (S2)
the area in the parietal lobe next to the primary somatosensory area (S1) that processes neural signal related to touch, temperature, and pain
Homunculus
Latin for “little man”
refers to the topographic map of the body in the somatosensory cortex
Tactile Acuity
the smallest details that can be detected on the skin
Two-Point Threshold
the smallest separation between two points on the skin that is perceived as two points
a measure of acuity on the skin
Surface Texture
the visual and tactile quality of physical surface created by peaks and valleys
Duplex Theory of Texture Perception
the idea that texture perception is determined by both spatial and temporal cues that are detected by two types of receptors
originally proposed by David Katz and now called the “duplex theory”
Spatial Cues
in tactile perception, information about the texture of a surface that is determined by the size, shape, and distribution of surface elements such as bumps and grooves
Temporal Cues
in tactile perception, information about the texture of a surface that is provided by the rate of vibrations that occur as we move our finger across the surface
Active Touch
touch in which the observer plays an active role in touching and exploring an object, usually with his or her hands
Passive Touch
a situation in which a person passively receives tactile stimulation
Haptic Perception
the perception of three-dimensional objects by touch
Exploratory Procedures (EPs)
people’s movements of their hands and fingers while they are identifying three-dimensional objects by touch
Interpersonal Touching
one person touching another person, a.k.a. social touch
CT Afferents
unmyelinated nerve fibers found in hairy skin, which have been shown to be involved in social touch
Microneurography
technique used to record neural signals that involves inserting a metal electrode with a very fine tip just under the skin
Social Touch Hypothesis
hypothesis that CT afferents and their central projection are responsible for social touch
Discriminative Functions of Touch
functions of the touch system such as sensing details, texture, vibration, and objects
Affective Function of Touch
the eliciting of emotions by touch
Inflammatory Pain
pain caused by damage to tissues, inflammation of joints, or tumor cells
this damage releases chemicals that create an “inflammatory soup” that activates nociceptors
Neuropathic Pain
pain caused by lesions or other damage to the nervous system
Nociceptive Pain
this type of pain, which serves as a warning of impending damage to the skin, is caused by activation of receptors in the skin called nociceptors
Nociceptors
a fiber that responds to stimuli that are damaging to the skin
Direct Pathway Model of Pain
the idea that pain occurs when nociceptor receptors in the skin are stimulated and send their signals to the brain
this model does not account for the fact that pain can be affected by other factors in addition to stimulation of the skin
Phantom Limbs
a person’s continued perception of a limb, such as an arm or a leg, even though the limb has been amputated
Gate Control Model
Melzack and Wall’s idea that perception of pain is controlled by a neural circuit that takes into account the relative amount of activity in nociceptors, mechanoreceptors, and central signals
this model has been used to explain how pain can be influenced by factors other than stimulation of receptors in the skin
Transmission Cells
according to the gate control theory, the cell that receives (+) and (-) inputs from cells in the dorsal horn
T-cell activity determines the perception of pain
Placebo
a substance that a person believes will relive symptoms such as pain but that contains no chemicals that actually act on these symptoms
Placebo Effect
a relief from symptoms resulting from a substance that has no pharmacological effect
Nocebo Effect
a negative placebo effect characterized by a negative response to negative expectations
Multimodal Nature of Pain
the fact that the experience of pain has both sensory and emotional components
Sensory Component of Pain
pain perception described with terms such as throbbing, prickly, hot, or dull
Affective Component of Pain
the emotional experience associated with pain
for example, pain described as torturing, annoying, frightful, or sickening
Opioids
a chemical such as opium, heroin, and other molecules with related structures that reduce pain and induce feelings of euphoria
Naloxone
a substance that inhibits the activity of opiates
it is hypothesized that naloxone also inhibits the activity of endorphins and therefore can have an effect on pain perception
Endorphins
chemical that is naturally produced in the brain and that causes analgesia
Empathy
the ability to share and vicariously experience what someone else is feeling
Social Pain
pain caused by negative social situations, such as rejection
Physical-Social Pain Overlap Hypothesis
proposal that pain resulting from negative social experiences is processed by some of the same neural circuitry that processes physical pain
Hand Dystonia
a condition which causes the fingers to curl into the palm
Anosmia
loss of ability to smell due to injury or infection
What are cutaneous senses?
based on mechanoreceptors in the skin
touch/tactile perception, vibration, texture, shape, warmth, cold, itch, & pain
What is proprioception?
sensing position of the body and limbs
What is kinesthesis?
sensing movement of the body and limbs
What is haptic perception?
coordination of tactile and kinesthetic sensation
What is vestibular sense?
equilibrioception
balance/spatial orientation (semicircular canals and vestibular sacs)
What is the skin?
heaviest organ in human body: weighs 4 kg (8% of body mass)
on average, 1.8 m2 in area
thickness ranges from 0.5 mm (nose) to 4.0 mm (foot)
may be glabrous (non-hairy) or hairy
outer layer is epidermis, surface is corneum
inner layer is dermis
neural structures at the epi-/dermal boundary are believed to be receptors
What is specificity theory?
one mechanoreceptor type proposed for each basic sensation
complex sensation supposedly comprised of “touch blends”
What were the mechanoreceptor types proposed for each basic sensation in specificity theory?
warmth: Ruffini endings
cold: Krause end bulbs
touch: Meissner corpuscles
pain: free nerve endings
What is punctate (“pointlike”) sensitivity?
if there’s a receptor under the skin at a location, there must be sensitivity to the associated sensation
How was specificity theory determined?
stimulate skin, record sensation, determine which receptor type is responsible
microneurography: use electrode to find a nerve fiber (axon from receptor) triggered by the stimulus
histology: trace fiber back to the receptor
What are the problems with specificity theory?
slightly problematic for participants
self-dissection: no correlation between structures and sensations; retest impossible
skin transmits kinetic and thermal energy over a distance from the point of stimulation; many receptor types affected by any stimulus
cornea has only free nerve endings, but all four sensations
sensory spots changed over time (e.g., touch in the morning, cold in the evening)
thermal grill illusion: simultaneous presentation of separate warm + cold stimuli = painfully hot
What is the conclusion of specificity theory?
cannot determine which potential receptor structure associated with each sensation
What is pattern theory?
the pattern of neural impulses from a number of receptors corresponds to a particular sensation
assumed that receptors differ in structure, but not in function
the same receptor can yield two different kinds of sensations, depending on the intensity of activation
e.g., mild pressure –> touch; more intense pressure –> pain
but why have so many different kinds of receptors?
What is stimulus preference theory?
intermediate approach: each sensation arises from the firing of a number of receptor types, tuned to different aspects of a stimulus
What was the Lowenstein & Skalak (1996)?
dissected Pacinian corpuscles (PCs)
1-4 mm long; have 20-40 layers
corpuscle around nerve fiber acts to absorb sustained pressure
fiber itself only stimulated at onset or offset of stimulus (or by changing stimulus)
What recent evidence supports stimulus preference of other nerve fibers?
fibers are relatively specialized to certain stimulation, but may also respond (more weakly) to other stimuli
What is the spinothalamic pathway?
smaller nerve fibres synapse in the spinal cord
ascends via spinal cord and brainstem to the ventral posterolateral nucleus (VPN) of the thalamus
projects to the anterior cingulate cortex (involved in pain), insular cortex (involved in emotion), and primary somatosensory cortex
carries primarily temperature and pain information
What is the medial lemniscus pathway?
larger, longer nerve fibers synapse in the medulla
ascends via medial lemniscus to the thalamus
projects to the primary somatosensory cortex
conveys touch and proprioceptive information
What is the evoked potential method for measuring the somatosensory cortex?
electrode on scalp measures cortical activity
different skin loci stimulated until activity produced at electrode
very time-consuming
What is the stimulation method for measuring the somatosensory cortex?
operating to remove tumors
didn’t want to damage sensorimotor areas
stimulated cortex with low voltage, which produced sensations in awake patients
allowed mapping of sensory and motor cortexes
What is the somatotopic organization of the cortex?
adjacent points on the skin have adjoining cortical representations (and columnar organization similar to visual cortex)
What is the homunculus?
touch represented in somatosensory cortex as a body map, or homunculus (“little man”)
some parts of homunculus disproportionately large compared to corresponding body locus (e.g., lips)
these body areas are more sensitive, partly due to increased density of receptors
amount of cortical area can be changed via experience; even in adults; experience-dependent neural plasticity
there are multiple homunculi; each has a different function (e.g., discrimination of forms/shapes, textures, etc.)
What is touch/tactile perception?
touch is constant pressure; vibration is changing pressure
stimulus containing kinetic energy causes an abrupt change in skin tension
soft touch signals go to insular cortex, which processes emotion
plays a major role in development, social cooperation, and affiliation
may be evolutionary holdover from social grooming, important for maintaining social relationships
What was the Tiffany Field et al. (1986) study on touch perception?
premature babies massaged (vs. in incubator)
gained 47% more weight
more socially responsive, slept better
discharged 6 days earlier
What is public distance?
for public speaking (over 3.5 m, 12 ft)
What is social distance?
for interactions with stranger and acquaintances (1.2-3.5 m, 4-12 ft)
What is personal distance?
for interactions with friends and family (45-120 cm, 1.5-4 feet)
What is intimate distance?
for whispering, touching, or embracing intimate partners (15-45 cm, 6-18 inches)
What is the Cruso & Wetzel (1984) study on touch perception?
waitresses at two restaurants in Oxford, Mississippi enlisted to investigate the effects of touch on tipping
touch manipulations: no touch, placed hand on diner’s shoulder, touched diner’s palm when giving change
results: smallest tips, 18% greater, 37% greater
What is oxytocin?
hormones synthesized in the hypothalamus and stored in the pituitary gland
involved in contractions during labor and lactation
also released by touch and at orgasm in males and females; may cause endorphin release
What was the Turner et al. (1999) study on oxytocin?
oxytocin levels measured in blood samples of 26 women
relaxation massage: oxytocin levels rose slightly
recollection of a negative emotion: oxytocin fell slightly
recollection of a positive emotion: no effect
What are the ways of measuring touch threshold?
absolute threshold: von Frey hair pushed into skin until hair bends; each bends with a different amount of force; use stronger hairs until absolute threshold established
difference threshold is called two-point threshold: smallest separation between two points on the skin that can be perceived
What is active touch?
actively handling an object aids identification
What is the J.J. Gibson (1962) study on active touch?
observers had to identify shape of six cookie cutters, while blindfolded (e.g. teardrop, star, triangle)
pushed onto palm (passive): 29% correct
actively handed: 95% correct
What are the different “exploratory procedures” that provide information on different stimulus features?
lateral motion and contour following: texture
pressure: hardness
static contact: temperature
unsupported holding: weight
enclosure: global shape, volume
enclosure and contour following: specific shape
What is thermoception?
Dallenbach (1927): found no correspondence between spots that responded to cool and warm; warmth and cold are separate
How is temperature encoded by two types of fibers?
warm fibers: 30-48 degrees
cold fibers: 20-45 degrees
thermal grill illusion simultaneously activates warm and cold fibers
What is paradoxical cold?
hot stimulus (45-50 degrees) produces sensation of cold; may be due to activation of cold fibers
What is thermal adaptation?
John Locke (1690): is local, not global
physiological zero: temperature at which receptors have become adapted; perception of temperature is relative at this point
only occurs completely within 29-37 degree range
takes ~20 min; depends on difference from normal temperature
What is pain perception and nociception?
an unpleasant sensory or emotional experience associated with actual or potential tissue damage
nociception is neural encoding of impending or actual tissue damage; i.e., noxious stimulation
once thought to be result of excess of intensity, but is more complex than this
evolutionarily adaptive: tissue damage –> intense pain –> lots of motivation
affected by non-physical factors, including cultural background
What is the specificity theory approach to pain perception?
nociceptors: receptors for noxious stimuli; peptide substance P released from C fibers
A delta fibers transmit sharp, pricking pain
C fibers transmit burning, throbbing pain
double pain: sharp pain, followed by dull pain
fibers enter spinal cord via dorsal horn, then to substantia gelatinosa
What are the structures involved in the gate-control theory of pain perception?
SG is substantia gelatinosa (in spinal cord)
T is pain transmission cell (in spinothalamic pathway)
L-fibers are large diameter axons from mechanoreceptors
S-fibers are small diameter axons from nociceptors
central control is top-down signals from the brain
What are the steps involved in the gate-control theory of pain perception?
activity in L fibers: closes gate
activates SG (-) –> T cell inhibited; no pain signal
activity in S fibers: opens gate
activates SG (+) –> T cell excited; pain signal sent
rubbing may reduce pain by activating A beta fibers
What is the placebo effect?
person led to believe that a substance will relieve symptoms, feels no pain
How may pain signals be attentuated?
may be due to endogenous opioids: natural analgesics in the brain; e.g., endorphins (pituitary) and enkephalin (CNS, intestines)
inhibit substance P
evidence: naloxone; narcotic antagonist (blocks opioids), wipes out effects of placebo and acupuncture, but not hypnosis
What is synesthesia?
involuntary physical experience of a cross-modal association, in which stimulation of one sensory modality reliably causes a perception in one or more different senses
not the same as using a metaphor to describe an experience
What is the experience of synesthesia like?
responses are generic and unelaborated (e.g., colors, simple shapes, textures, tastes)
one-way projections: a synesthete who sees colors when hearing spoken words does not necessarily hear words when viewing colors
idiosyncratic: different synesthetes have different experiences to same stimulus
What is the more connections theory of synesthesia?
synesthetes have genetic overabundance of neural connections
increased wiring is due to insufficient pruning
this results in a breakdown in the modularity of perception, so that one modality “samples” an event in another
evidence: synesthetes found to have greater neural connectivity
What is the less inhibition theory of synesthesia?
not due to abnormal connections
normally, excitation and inhibition in the brain are balanced
in synesthetes, there is less inhibition in feed-backward connections from high-level multisensory brain areas in single-sense areas
evidence: synesthesia may be induced in anyone by LSD or mescaline, which may activate existing pathways
What are the implications of synesthesia?
may reveal aspects of top-down processing, the binding problem, sensory integration and conscious experience
