Chapter 13 – Touch (Somatosensation)

Key Terminology

• Kinesthesia: Perception of limb position & movement in space.

• Proprioception: Perception of body state mediated by kinesthetic + internal receptors.

• Somatosensation: Collective term for sensory signals from skin, muscles, tendons, joints, & internal receptors.

Neural Fibers of Somatosensory System

• All touch receptors connect to afferent nerve fibers (axon ± myelin).

  • A-alpha fibers

  • Diameter: 13!–!20\ \mu \text{m}

  • Conduction speed: 80!–!120\ \text{m/s}

  • Source: proprioceptors in muscles/tendons.

  • A-beta fibers

  • Diameter: 6!–!12\ \mu \text{m}; Speed: 35!–!75\ \text{m/s}

  • Carry mechanical (touch) info from skin mechanoreceptors.

  • A-delta fibers

  • Diameter: 1!–!5\ \mu \text{m}; Speed: 5!–!30\ \text{m/s}

  • Carry pain & temperature (sharp/fast pain).

  • C fibers (unmyelinated)

  • Diameter: 0.2!–!1.5\ \mu \text{m}; Speed: 0.5!–!2\ \text{m/s}

  • Carry dull pain, temperature, itch, pleasant touch.

Cutaneous Mechanoreceptors

• Located in epidermis & dermis; four classic types (“mechanoreceptors” = respond to mechanical force).

  • SA I (Merkel discs)

  • Slow adaptation, small RF.

  • Best for sustained pressure (<5\ \text{Hz}), coarse texture, pattern & form.

  • FA I (Meissner corpuscles)

  • Fast adaptation, small RF.

  • Sensitive to low-frequency vibration \sim5!–!50\ \text{Hz}; detect slip → grasp control.

  • SA II (Ruffini endings)

  • Slow adaptation, large RF.

  • Respond to skin stretch/ sustained downward pressure; encode finger position.

  • FA II (Pacinian corpuscles)

  • Fast adaptation, large RF.

  • Detect high-frequency vibration \sim50!–!700\ \text{Hz}; fine texture via vibration.

• Functional triad for each receptor: stimulus type, RF size, adaptation rate.

Deep Mechanoreceptors for Movement

• Kinesthetic receptors in muscles, joints, tendons.

  • Muscle spindle: senses muscle stretch/tension.

  • Tendon & joint receptors detect tension & extreme joint angles.

• Clinical note: Patient Ian Waterman lost kinesthesia (viral neuropathy) → must visually monitor limbs.

Thermoreception

• Thermoreceptors: warm & cold fibers that detect deviations from neutral skin temp (~34\,^{\circ}\text{C}).

• ThermoTRP channels (e.g., TRPV1 for heat/chili, TRPM8 for menthol/cool) transduce thermal/chemical signals.

Nociception & Pain

• Nociceptors = A-delta (fast, sharp) + C fibers (slow, dull).

• Two-stage pain experience:

  1. Quick sharp pain via A-delta.

  2. Throbbing via C fibers.

• Gate-Control Theory: Dorsal-horn interneurons modulate ascending pain.

• Endogenous opiates produce analgesia (e.g., soldier un-aware of wounds). External analogs: morphine, codeine, heroin.

• Pain sensitization:

  • Hyperalgesia (heightened response).

  • Neuropathic pain (nervous-system damage).

• Cognitive/affective pain areas:

  • S1/S2 = sensory-discriminative.

  • Anterior cingulate = unpleasantness.

  • Prefrontal cortex = expectation/executive (can amplify pain).

Pleasant Touch & CT Afferents

• Fifth discriminative component; mediated by unmyelinated C-tactile (CT) afferents in hairy skin.

• Optimal stimulus: slow caress (~3!–!10\ \text{cm/s}).

• Processed in orbitofrontal cortex (reward) not S1/S2.

• Psychophysics: Pleasantness peaks at CT-optimal speeds while perceived intensity increases linearly.

Spinal Processing & Ascending Pathways

• Fibers enter dorsal horn (laminar, somatotopic).

• Labeled lines intermix → complex coding.

• Two main tracts:

  1. Spinothalamic: pain & temperature (slow).

  2. Dorsal Column–Medial Lemniscal (DCML): touch, vibration, proprioception (fast).

Cortical Representation

• S1 (Post-central gyrus) subdivided into areas 1, 2, 3a, 3b; S2 lateral.

• Homunculus: magnified lips, hands, face; body image biased “top-heavy.”

• Plasticity examples:

  • Tool use elongates arm representation.

  • Post-surgical limb lengthening resets map in \approx6 months.

  • Blindfold 5 days → V1 recruited for Braille (reversible).

  • Phantom limb sensations = cortical crosstalk; face stim → hand percepts.

  • Transplanted / prosthetic hands can gain cortical representation; biomimetic feedback evokes naturalistic sensations.

Tactile Sensitivity & Acuity

• Von Frey method: nylon monofilaments calibrated by diameter/force.

• Sensitivity hierarchy: Face > arms/fingers > trunk > legs/feet.

• Smallest detectable raised dot ≈ 10\ \text{nm} = 10^{-8}\ \text{m} (FA I).

• Vibration thresholds vary with frequency; FA II best at \sim250\ \text{Hz} (lowest threshold).

• Two-point threshold: minimal distance to perceive 2 touches; lowest on fingertips/face (≈2\ \text{mm}).

• Temporal acuity: two tactile pulses distinguishable at \approx5\ \text{ms} separation (better than vision, worse than audition).

• Age: sighted adults lose acuity; congenitally blind maintain high acuity (implications for late-onset blindness & Braille).

• Star-nosed mole: 25 k mechanoreceptors, 11th ray = tactile fovea; cortex contains multiple star maps.

Haptic Perception (Active Touch)

Perception for Action

• Use proprioception + cutaneous cues to grasp, manipulate, maintain posture.

Action for Perception: Exploratory Procedures (EPs)

• Lateral motion → texture.

• Pressure → hardness.

• Static contact → temperature.

• Unsupported holding → weight.

• Enclosure / contour following → global & exact shape.

• Different EPs optimize information acquisition; FA/SA fibers contribute differentially (e.g., FA II for fine textures via vibration “timbre”).

The “What” System of Touch

• Haptic recognition relies on material > geometric properties (opposite of vision).

• 2-D line drawings recognizable visually but not haptically.

Haptic Search & Preattentive Features

• Certain properties “pop-out” regardless of distractor set size: rough-vs-smooth, hard-vs-soft, cool-vs-warm, edges-vs-flat.

• Orientation (horizontal vs vertical) does not pop-out.

• Response-time slopes differ for target-present vs target-absent as in visual search.

Pattern Perception & Braille

• Touch acts like blurred vision; confusion matrices match when visual stimuli are low-pass filtered to fingertip acuity.

• Tactile agnosia: parietal lesions → object-recognition loss by touch (hemispheric specificity case study).

The “Where” System

• Locating objects egocentrically; haptic egocenter lies near knuckle of middle finger / between shoulders (debated).

• Frame-of-reference transformations crucial when searching alarm snooze button in dark.

Tactile Spatial Attention

• Spatial cueing (arrow predicts vibration) yields facilitation (valid) & cost (invalid) akin to visual covert attention.

• Cross-modal cueing: expecting touch but receiving vision/audition leads to largest costs → narrow tactile attentional channel.

Social & Developmental Touch

• Incidental touch biases social judgments (e.g., weight = importance).

• Rat maternal licking/grooming → lifelong gene expression (epigenetic) & reduced anxiety; pups adopt style of foster mother.

Multisensory Integration

• Vision vs touch dominance depends on question asked: density (vision) vs roughness (touch).

• Maximum-likelihood integration demonstrated with virtual raised surfaces (Ernst & Banks, 2002): perception weighted by modality reliability; yields compromise heights.

Ethical, Clinical & Practical Implications

• Importance of pain perception for survival (case “Miss C”).

• Design of prosthetics with sensory feedback improves embodiment & control.

• Understanding CT pleasant touch informs therapies for social disorders.

• Age/vision interactions suggest early Braille training & tactile enrichment for low-vision populations.

Key Numerical & Formulaic References

• Conduction velocity ranges summarized above.

• Optimal CT stroking speed \approx3!–!10\ \text{cm/s}.

• Two-point threshold fingertip \approx2\ \text{mm}; face similar; calf >4\ \text{cm}.

• Temporal discrimination thresholds: touch 5\ \text{ms}, vision 25\ \text{ms}, audition 0.01\ \text{ms}.

• Detectable raised dot height 10\ \text{nm} = 10^{-8}\ \text{m}.

These bullet-point notes consolidate every major and minor concept, example, figure insight, and quantitative detail from Chapter 13 “Touch,” forming a comprehensive stand-alone study resource.