BM

Touch & Somatosensory Processing — Study Guide

1) Big Picture Map (what talks to what)

  • Receptors in skin → Peripheral nerve (Aβ fibers) → Dorsal root ganglion (DRG) → Spinal cord (dorsal columns) → Medulla (gracile/cuneate nuclei) → Cross (decussate) → Medial lemniscus → Thalamus (VPL) → Primary somatosensory cortex (S1: 3b→1→2) → Association areas (S2, posterior parietal) for perception, integration, and use in action.

  • Thalamus serves as the major relay to cortex; cerebellum uses proprioceptive input for coordination and error correction; brainstem contains key relays and tracts.

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2) The Receptors (what each is best at)

  • Merkel discs (SA1): edges, form, fine texture (slowly adapting).

  • Meissner corpuscles (RA1): light flutter & grip control (rapidly adapting).

  • Pacinian corpuscles (RA2): vibration, tool use feedback (rapid).

  • Ruffini endings (SA2): skin stretch, hand shape (slow).

  • Hair follicle receptors: hair movement.

  • Proprioceptors (muscle spindles, GTOs, joint receptors) travel largely with touch fibers in the same dorsal-column system to cortex and via spinocerebellar tracts to cerebellum.

Key fiber: Aβ (large, myelinated) → fast, precise touch & proprioception.

3) Two Ascending Systems (which carries what)

  • Dorsal Column–Medial Lemniscus (DCML)
    Carries: discriminative touch, pressure, vibration, conscious proprioception (fast, precise).

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  • Anterolateral/Spinothalamic System
    Carries: pain, temperature, some crude touch (slower Aδ/C). (Useful contrast for exam items.)

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  • Spinocerebellar tracts
    Carries unconscious proprioception to cerebellum for real-time movement correction (no conscious perception).

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4) The Exact DCML Pathway (touch): Step-by-Step

  1. Stimulus → Receptor transduction
    Skin deformation opens mechanically gated channels in the appropriate receptor → receptor potential → action potentials in Aβ fiber.

  2. Peripheral nerve → DRG
    The first-order neuron soma sits in the dorsal root ganglion; its central process enters the spinal cord dorsal root.

  3. Ascend ipsilaterally in dorsal columns

    • Below T6 (legs & lower trunk): Fasciculus gracilis (medial).

    • At/above T6 (upper trunk, arm, neck): Fasciculus cuneatus (lateral).
      (Still uncrossed at this point.)

  4. Synapse in medulla

    • Lower body afferents: Nucleus gracilis.

    • Upper body afferents: Nucleus cuneatus.

  5. Decussation (crossing) in medulla
    Second-order axons arc across as internal arcuate fibers, then form the medial lemniscus on the contralateral side.

  6. Ascend via medial lemniscus → Thalamus (VPL)
    The ventral posterolateral (VPL) nucleus of the thalamus relays body somatosensation to cortex. (Thalamus = major relay station.)

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  7. Thalamus → Primary Somatosensory Cortex (S1)

    • Area 3b: primary cutaneous input (“touch cortex”).

    • Area 1: motion, texture features.

    • Area 2: size/shape, proprioceptive integration.
      Somatotopic homunculus: legs medial → face lateral.

  8. Beyond S1 → S2 & Posterior Parietal (Areas 5,7)
    Integration → object recognition by touch (stereognosis), body schema, planning.
    Cerebellum uses spinocerebellar inputs for smooth, accurate movement and error correction (efference copy comparison).

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5) Quick DCML vs Anterolateral Comparisons (exam gold)

  • Modality:
    DCML = touch/pressure/vibration/proprioception; Anterolateral = pain/temp/crude touch.

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  • Fiber types:
    DCML = Aβ (large, fast); Anterolateral = Aδ/C (smaller, slower).

  • Where they cross:
    DCML crosses in medulla; Anterolateral crosses near entry in spinal cord (within 1–2 segments).

  • Clinical:
    Hemisection (Brown-Séquard) → ipsilateral loss of fine touch/proprioception (DCML) below lesion; contralateral loss of pain/temp (spinothalamic) starting a couple segments below.

6) Roles of Thalamus, Cerebellum, Brainstem (what to say if asked)

  • Thalamus (VPL): relay/integration to cortex; contributes to attention and perception of pain across nuclei; “gateway” for conscious touch.

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  • Cerebellum: compares intended movement (efference copy) with actual sensory feedback to detect/correct movement error; essential for smooth & accurate actions, coordination, timing, posture.

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  • Brainstem: houses medulla relay (gracile/cuneate), the medial lemniscus, and integrative networks important for sensorimotor control.

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7) Must-Know Terms & Points

  • Dermatomes: skin territory per spinal nerve; helpful for localizing lesions.

  • Two-point discrimination: best in fingertips (high receptor density, small receptive fields).

  • Adaptation: RA (Pacinian/Meissner) vs SA (Merkel/Ruffini).

  • Somatotopy: preserved from cord → thalamus → cortex.

  • Lesion logic:

    • Cord (one side): ipsilateral DCML loss + contralateral pain/temp.

    • Medial medulla lesion: hits medial lemniscus → contralateral touch/proprio loss.

    • Thalamic (VPL): contralateral hemibody sensory loss.

8) Ultra-Compact “Script” to Recite Before the Exam

“Touch deforms a mechanoreceptor → Aβ afferent fires → DRG → enters dorsal horn → ascends ipsilaterally in dorsal columns (gracilis/cuneatus) → synapse in nucleus gracilis/cuneatus (medulla) → decussate as internal arcuate fibers → ascend as medial lemniscus → VPL thalamus → S1 (3b→1→2) → S2/posterior parietal for integration; cerebellum gets proprio via spinocerebellar for online error correction.”

9) Lightning Self-Test (answer from memory)

  1. Which pathway carries vibration and fine touch to cortex? Where does it cross?

  2. Which thalamic nucleus relays body touch?

  3. Name two rapidly adapting vs two slowly adapting receptors.

  4. A right T10 hemisection causes what sensory losses for touch vs pain/temp?

  5. Which structure uses spinocerebellar input for movement accuracy?

(Check yourself: DCML; crosses in medulla; VPL; RA=Meissner/Pacinian, SA=Merkel/Ruffini; Right DCML loss ipsilaterally below T10, left pain/temp loss; Cerebellum.)

10) From Your Slide Deck (tie-ins you can cite on exam)

  • Dorsal column: transmits proprioception, touch & pressure (that’s DCML).

  • Anterolateral system: pain & temperature (contrast with DCML).

  • Spinocerebellar tract: primary pathway for proprioceptive info to cerebellum (unconscious).

  • Thalamus: relay to cortex; attention & pain perception roles noted.

  • Cerebellum: smooth & accurate movements, error detection/correction using an efference copy.

  • Brainstem components provide integrative sensorimotor functions.

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If you want, I can turn this into printable one-pager notes or make flashcards next. And feel free to drop the other PowerPoints—you don’t have to wait; I’ll fold them into a full exam study pack.

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