The Skin Senses Touch Temperature Pain
Hairy Skin/Glabrous Skin
Pacinian Corpuscle
Complexi;es of the Skin Lots of “stuff” embedded in your skin What’s it all doing? Specificity theory: specialized receptors Johannes Müller (1801-‐1858) and doctrine of specific nerve energies • Maximilian von Frey (1852-‐1932) posited different types of receptors for pain, touch, warm, cold • Pa/ern theory: combo of nerve ac;vity that determines the perceptual experience • • • •
Complexi;es • Burgess & Perl mapped sensory spots on their own skin and then dissected to see what was underneath—li]le correla;on with receptors • Lele & Weddell s;mulated cornea (ouch!)— which has only free nerve endings—and found that people perceived pain, warmth, cold, and touch
From Skin to Brain Spinothalamic System Smaller nerve fibers Slower transmission Conveys temperature & sharp pain Crude touch informa;on
Lemniscal System Larger nerve fibers Faster transmission Conveys vibra;on & fine touch
Touch • • • •
Life without touch? Recep;ve fields (on-‐center/off-‐surround) Slowly adap;ng (SA) neurons Rapidly adap;ng (RA) neurons
Four Afferent Systems for Touch (Focus on func;ons of neurons and not receptors)
Afferent Type
SA1
SA2
RA1
RA2
Receptor Type
Merkel disk
Ruffini corpuscle
Meissner corpuscle
Pacinian corpuscle
Sensory func;on
Form & texture
Mo;on direc;on Hand shape
Mo;on detec;on Grip control
Vibra;ons
Effec;ve S;mulus
Edges, points, corners, curvature
Skin stretch
Skin mo;on
Vibra;on
Response to sustained pressure
Sustained, slow adapta;on
Sustained, slow adapta;on
None
None
Recep;ve field area
Very small
Fairly large
Fairly small
Rela;vely large
Sensi;vity
Moderate
Very low
High
Very High
Encoding Touch in the Brain
Sensory Homunculus
Passive vs. Ac;ve Touch • Passive Touch: object pressed against skin • Ac;ve Touch: explore object with skin
Passive Touch • Absolute thresholds • Two-‐point discrimina;on thresholds • Adapta;on to touch
Touch Absolute Thresholds
Optacon
Ac;ve Touch • Nature of hap;c explora;on • Visually impaired – Time of visual loss is important (late blind) – Braille alphabet
Interac;ons Between Vision and Touch • Norman et al. with bell peppers – Feel unseen pepper, then pick it out of 12 – Same-‐different judgments for pepper pairs be]er for see/see than feel/see
• Which sense “dominates?”: It depends • When would you trust touch over vision?
Rock & Harris
Look alone or feel alone
Different tests
Temperature • Body regulates temperature (98.6°), but to do so it must sense temperature • Proteins called transient receptor poten;al (TRP) channels create differences in free nerve endings • Six types of temperature receptors – Linkage to pain (A-‐ and C-‐fibers convey temperature) – Linkage to taste (same chemicals affect taste and TRP channels) – Sca]ered throughout body
Temperature
Temperature • Paradoxical heat (warm+cold = heat) • Difficulty in localizing temperature, especially when near “threshold” • Thermal adapta;on
Pain • “an unpleasant sensory and emo;onal experience associated with actual or poten;al ;ssue damage” (Merskey, 2008) • PERFINK (Krech) • Important protec;ve func;on – Not everyone perceives pain (SCN9A gene?) – Typically die young if you don’t perceive pain
Afferent Systems for Pain • Nociceptors (pain receptors)—free nerve endings • Small fibers A-‐fibers Several types (heat, pressure, noxious chemicals) Thinly myelinated Rapid response Sharp or pricking pain
C-‐fibers Several types (extreme heat, mechanical s;mula;on) Unmyelinated Slow response Extreme heat or burning pain
• Double pain • Large fibers Aβ (A-‐beta) inhibit pain
Gate Control Theory of Pain Melzack & Wall
Substan;a gela;nosa
Spinothalamic pathway: dorsal horn of spinal cord
Neuromatrix • Melzack proposed a configura;on of neurons that “represents” the en;re body • Three modules central to the neuromatrix – Sensory: somatosensory cortex – Affec;ve: anterior cingulate cortex, limbic system, amygdala – Cogni;ve: prefrontal cortex
Varying Pain Percep;on • Cultural differences (Bernard Tursky) • Out of Jeffry Mogil’s lab – Half see video inducing high empathy for actor – Half see video inducing low empathy for actor – All given painful heat s;mulus, while watching actor receiving the same s;mulus – Those in high empathy condi;on rated the pain as greater
• Sex differences (Wendy Sternberg)
Phantom Limbs • People experience a limb’s presence, even when it’s not there! • Neuromatrix? • Congenital (experience phantom limbs if absent from birth) • Located in unusual posi;ons (arm behind back) • Extra phantom limbs (arm from middle of chest)
Phantom Limbs • Even though congenital, experience is likely important (brain reorganiza;on?) • Can’t be fully explained by brain reorganiza;on, because phantom limbs occur soon aper surgery (insufficient ;me for rewiring)—connec;ons already exist? • Phantom limb pain (80% of amputees experience)
Referred Pain
Measuring Pain • Classical psychophysical methods – Pain threshold
• Signal Detec;on (separate sensory from criterion) • Magnitude es;ma;on
McGill Pain Ques;onnaire
Controlling Pain • Pharmacological: analgesic medica;on – Opium-‐based drugs (morphine) – Endogenous opiates (endorphins)
• Physical: counterirritants
– Acupuncture – Transcutaneous electrical nerve s;mula;on (TENS)
• Psychological
– Placebo (e.g., analgesic works be]er if “obvious”) – Hypnosis – Cogni;ve-‐behavioral approaches
