Somatosensation

2025-11-18

Rick Gilmore

Department of Psychology

Prelude

– Ross-Topic (2018)

– ThePoliceVEVO (2010)

Announcements

• Exam 3 on Tuesday, December 2, 2025

Today’s topics

• Warm up
• Somatosensation
• Pain

Warm up

Somatosensation

What is it?

• Sensations about the body (soma)

Types of somatosensation

• Internal (interoceptive)
  • How do I feel?
  • Where/how is my body positioned?
  • Proprioception (perception of the self)
• External (exteroceptive)
  • What’s in the world?
  • Where is it?

Internal senses

• Kinesthesia
  • Body position
  • Movement
• Vestibular sense
  • Head position (relative to gravity)
  • Head movement (rotation, translation)

Vesibulo-ocular response (VOR)

• Keeps eyes steady when head moves
• Fast: response < 10 ms
• Disturbed in vertigo

Vestibulo-ocular response (VOR): https://www.mcgill.ca/vestibular-gazecontrol-lab/files/vestibular-gazecontrol-lab/images/222_0.gif

Vestibulo-ocular response (VOR)

• Keeps eyes steady when head moves
• Can’t walk & text without it

– Castilla (2007)

External senses

• What’s out there and where is it located?

• Cutaneous (receptors in the skin) senses
  • Hot, cold
  • Pressure
  • Vibration
• Plus kinesthesia (why?)

Cutaneous receptors

• Receptors specialize in different info types, properties

Question

• Why are minty foods cool?
• Why are spicy foods hot?

Receptors detect chemicals + temperature

• Menthol/mint receptor (CMR1)
  • Also signals “cool” temperatures

Receptors detect chemicals + temperature

• Vanilloid Receptors (TrpV1/VR1, VRL1)
  • Respond to capsaicin (in peppers), allyl isothiocyanate (in mustard, wasabi)
  • Also signal “hot” temperatures

Axon size/speed trade-off

• Not all info travels at the same speed
• Thicker -> faster
• Myelinated -> faster

Axon size/speed trade-off

• Fast: Kinesthesia (from muscles and joints) the fastest
• Slow: Pain/itch, temperature

From skin to brain

• Cutaneous receptors ->
• Dorsal root ganglion in spinal cord ->
• Ventral posterior lateral (VPL) thalamus ->
• Primary somatosensory cortex (S1) ->
  • Post-central gyrus of parietal lobe

Dermatomes

• Patches of skin ennervated by a set of nerves

Functional segregation in spinal cord

• Separate pathways for different information types

• Dorsal column/medial leminiscal pathway
  • Touch, proprioception

Functional segregation in spinal cord

• Spinothalamic tract
  • Pain, temperature

Somatotopic maps

• In thalamus & cerebral cortex

Maps non-uniform

• Some areas of skin activate disproportionately sized areas of the cerebral cortex
  • hands, face, lips & tongue
• More sensors in skin…
• More axons to brain…
• Larger areas in brain

Parallel pathways

• S1 maintains functional segregation of input types

Interim summary

• Perceiving Where
  • Somatotopic maps – locations on skin
  • Kinesthesia – configuration of limbs
• Perceiving What
  • Patterns of smoothness, roughness, shape, temperature
• Functional segregation
  • Multiple receptors, parallel axon pathways

Phantom Limbs

– cogmonaut (2010)

Pain

Types of

• Nociceptive
• Neuropathic
• Nociplastic

Nociceptors

• Nociceptors (Latin nocere to harm or hurt) detect harmful or potentially harmful stimuli
  • chemical
  • mechanical
  • thermal

“Nociceptors” (n.d.)

Nociceptor locations

• External
  • Skin, cornea (eye), mucosa
• Internal
  • Muscles, joints, bladder, gut

Projections to CNS

• Fast (\(A\delta\)) and slow (\(C\)) transmission to CNS

Projections to CNS

• Via anterolateral system

Craig (2002) Figure 1

Key CNS nodes

• Periaqueductal grey (PAG) in midbrain
• Insular cortex (insula)
• Hypothalamus
• Amygdala

Figure 2 from (Craig, 2002)

Key CNS nodes

• Thalamus
  • Ventroposterior lateral nucleus
  • Ventroposterior medial nucleus
  • Ventromedial nucleus

Pain in the brain

Wager et al. (2013) Figure 1

Pain in the brain

…we used machine-learning analyses to identify a pattern of fMRI activity across brain regions — a neurologic signature — that was associated with heat-induced pain. The pattern included the thalamus, the posterior and anterior insulae, the secondary somatosensory cortex, the anterior cingulate cortex, the periaqueductal gray matter, and other areas…

– Wager et al. (2013)

Chemical factors

• Substance P
  • Released by peripheral sensory nerves
  • Neurotransmitter and neuromodulator in PNS & CNS
  • Promotes vasodilation and inflammation
  • Wikipedia contributors (2025a)

Chemical factors

• Prostaglandins
  • hormone-like effects, released in many places
  • Promotes vasodilation and inflammation
  • Wikipedia contributors (2025b)

Pain relief

• Paracetymol (acetaminophen)
  • mechanism not fully understood
  • may inhibit synthesis of prostaglandins via cyclooxygenase (COX) enzyme
  • may modulate endocannabinoid system
• Nonsteroidal anti-inflamatory drugs (NSAIDs): aspirin, ibuprofen
  • Also inhibit prostaglandins via COX

Pain relief: Opioids

• Activate endogenous opioid systems
• Multiple receptor types
  • \(\delta\), \(\kappa\), \(\mu\)

Figure 1 from Benarroch (2012)

Pain relief: Opioids

• PNS: peripheral sensory neurons
• CNS: amygdala, hypothalamus, PAG, spinal cord, cortex, medulla, pons,…
• brainstem opioid neurons provide descending inhibition of nociceptors

Figure 1 from Benarroch (2012)

Synthetic Capsaicin

• Binds to TrpV1/VR1 thermo/nociceptors
• Eventually causes decrease in TrpV1 response
• Alters how peripheral neuron responds to mechanical (rubbing) stimulation
• Borbiro, Badheka, & Rohacs (2015)

Why rubbing might help

Gate control theory

• Melzack & Wall (1965)
• Pain projection neurons receive nociceptive + non-nociceptive input
• Non-nociceptive input activates inhibitory interneuron

Wikipedia

Pain’s multiple components

Figure 1 from Papini, Fuchs, & Torres (2015)

Interim summary

• Specific receptors (nociceptors) for pain
• Highly interconnected CNS network
• Key nodes: PAG, amygdala, insula, nucleus accumbens, anterior cingulate
• Some pain relievers target synthesis of prostaglandins
• Endogenous opioid and cannabinoid receptors contribute
• Experienced pain combination of physical and psychological factors

Wrap up

Next time

• Action I
• Exam 3 review

Resources

About

This talk was produced using Quarto, using the RStudio Integrated Development Environment (IDE), version r Sys.getenv("RSTUDIO_VER").

The source files are in R and R Markdown, then rendered to HTML using the revealJS framework. The HTML slides are hosted in a GitHub repo and served by GitHub pages: https://psu-psychology.github.io/psych-260-2025-fall/

References

Benarroch, E. E. (2012). Endogenous opioid systems: Current concepts and clinical correlations. Neurology, 79, 807–814. https://doi.org/10.1212/WNL.0b013e3182662098

Borbiro, I., Badheka, D., & Rohacs, T. (2015). Activation of TRPV1 channels inhibits mechanosensitive piezo channel activity by depleting membrane phosphoinositides. Science Signaling, 8, ra15. https://doi.org/10.1126/scisignal.2005667

Castilla, V. (2007). Vestibulo-ocular reflex 13/25. YouTube. Retrieved from https://www.youtube.com/watch?v=j_R0LcPnZ_w

cogmonaut. (2010). Phantom limb video. YouTube. Retrieved from https://www.youtube.com/watch?v=1mHIv5ToMTM

Craig, A. D. (2002). How do you feel? Interoception: The sense of the physiological condition of the body. Nature Reviews. Neuroscience, 3, 655–666. https://doi.org/10.1038/nrn894

Melzack, R., & Wall, P. D. (1965). Pain mechanisms: A new theory. Science, 150, 971–979. https://doi.org/10.1126/science.150.3699.971

Nociceptors. (n.d.). In Nociception and spinal reflexes. Retrieved from https://uw.pressbooks.pub/spinal/chapter/chapter-1/

Papini, M. R., Fuchs, P. N., & Torres, C. (2015). Behavioral neuroscience of psychological pain. Neuroscience and Biobehavioral Reviews, 48, 53–69. https://doi.org/10.1016/j.neubiorev.2014.11.012

Ross-Topic, D. (2018). Touch me in the morning. YouTube. Retrieved from https://www.youtube.com/watch?v=nvZfQzlS9t8

ThePoliceVEVO. (2010). The police - king of pain (official music video). YouTube. Retrieved from https://www.youtube.com/watch?v=yuOPRfq-q6U&list=RDyuOPRfq-q6U&start_radio=1

Wager, T. D., Atlas, L. Y., Lindquist, M. A., Roy, M., Woo, C.-W., & Kross, E. (2013). An fMRI-based neurologic signature of physical pain. The New England Journal of Medicine, 368, 1388–1397. https://doi.org/10.1056/NEJMoa1204471

Wikipedia contributors. (2025a, October 2). Substance P. Retrieved from https://en.wikipedia.org/wiki/Substance_P

Wikipedia contributors. (2025b, October 4). Prostaglandin. Retrieved from https://en.wikipedia.org/wiki/Prostaglandin