Pain and touch

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Pain and touch
2012-12-08 13:54:24
pain nhb touch sensory nociception pathway neuro

pain and touch lecture series
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  1. Which layer of skin are mechanoreceptors found in?
  2. Describe Pacinian corpuscles
    • Concentric layers of cellular membranes alternating with fluid filled spaces
    • Wide receptive field with concentrated central area
    • Sensitive to fast vibration (most sensitive ~300Hz, 10nm at 200Hz)
    • 350 in finger, 800 in palm
    • Wide distribution: muscles, periosteum, abdominal mesentery
    • Lamellae make it rapidly adapting
  3. Describe Meissner's corpuscles
    • 3-5mm diameter receptive fields - SMALL
    • low frequency vibration (most sensitive ~30Hz)
    • High sensitivity, low spatial resolution
  4. What is phase locking?
    A single AP for each phase of a sinusoidal wave: found in RA fibres
  5. Which is RA I and which is RA II? And SA I SA II?
    • RA 1 is Meissners, RA II is Pacinian
    • SA I is Merkel, SA II is Ruffini
  6. Analogy between mechanoreceptors and visual receptors
    • Meissner's like scotopic: enhanced sensitivity, poor spatial resolution
    • Merkel like photopic: high spatial resolution, decreased sensitivity
  7. Experiments to examine receptors and response to vibration
    • LA: raises threshold for Meissner's (closer to surface)
    • Can use pre-adaptation to desensitise for high (Pacinian)/low (Meissner's) frequencies
  8. Describe Merkel cells
    • small, localised receptive fields
    • linear response to indentation up to 1.5mm; can resolve detail of 0.5mm
    • sensitive to points, edges, curvature eg braille
  9. Describe Ruffini endings
    • Object motion, hand/finger position
    • Responsive to stretch (within spindles)
  10. Different values for two point limen
    40mm at shoulder, 2mm at fingers
  11. Relative amounts of cold and warm spots
    • 30x more cold spots
    • different concentrations in different places eg lip has 6x cold spots of sole
  12. explain spatial summation of warm spots
    more receptors than spots: 'warmth' requires the simultaneous activation of many receptors
  13. 4 channels involved in temperature detection
    • Trpv1: HOT (capsaicin, >45 degrees) C fibres
    • Trpv4: WARM (peak 40 degrees)
    • Trpm8: COLD (menthol, <25 degrees) - Adelta and C fibres
  14. Explain labelled line coding and give an example
    • activity in a nerve fibre is seen as whatever the nerve usually encodes, no matter what stimulates the nerve
    • eg paradoxical cold: heat >45 degrees on cold spot is seen as cold (just painful if diffuse)
  15. Sensory fibre types; blocked by?
    • Abeta: touch, proprioception
    • A delta: cold, stabbing pain blocked by hypoxia
    • C fibres: delayed, polymodal (thermal/mechanical/chemical) � warmth, itch, burning pain: blocked by LA
  16. Spinal cord divided into 2 by?
    Dorsal median sulcus, ventral median fissure
  17. How many ventral-dorsal 'divisions'? What are they called?
    • 9 Rexed's laminae
    • I-VI Dorsal Horn, VII Intermediate zone, VIII-IX Ventral Horn
  18. Describe the central pathway for touch and proprioception
    • Dorsal column-medial lemniscus.
    • Ascend in ipsilateral dorsal column (via dorsal horn) (Somatotopic): Gracile fascicle: below mid-thoracic; cuneate: above (nb no synapse)
    • to respective dorsal column nuclei at cervicomedullary junction
    • Sensory decussation at medulla
    • Ascend in medial lemniscus within medulla
    • To ventro-posterior nucleus of thalamus (main body to medial part (VPM), trigeminal system to lateral part (VPL))
    • 'Thalamic rod' projects to layer IV of SI (Brodmann's areas 3a, 3b, 1, 2)
    • SI projects to posterior parietal cortex (association) and SII (tactile object recognition)
  19. Describe the main central pathway for nociception
    • Spinothalamic tract
    • Axons from layers I and V-VII of dorsal horn.
    • Ascend/descend in Lissaeur's tract
    • Synapse here (in substantia gelatinosa) then cross in ventral commisure
    • Ascend in contralateral anterolateral white matter
    • Mainly to ventroposterior (medial - Vmpo) and intralaminar nuclei of thalamus
    • Via internal capsule to SI, SII, prefrontal cortex, basal ganglia, anterior cingulate cortex
    • But also to hypothalamus, PAG, locus coerulus, raphe nucleus, rostroventral medulla
    • Especially for descending pathways
  20. Role of different central nociceptive pathways
    • Spinothalamic: accurate localisation
    • Spinoreticular: affective component (eg arousal): VII and VIII to reticular formation and thalamus. Some axons ipsilateral
    • Spinomesencephalic: I and V via anterolateral quadrant of spinal cord to mesencephalic (midbrain) reticular formation and PAG
  21. 4 spinal cord lesions and their effects
    • Brown-Sequard (hemisection): ipsilateral touch and proprioception, contralateral pain (damages spinothalamic and DC-ML)
    • Syringomyelia: fluid filled cavity in spinal cord (usually cervical). Disrupts decussating fibres - loss of pain and temperature ('cape like')
    • Posterior column syndrome (tabes dorsalis, caused by syphillis): bilateral absence of touch and proprioception
    • Complete transection: uh oh
  22. Describe route of trigeminal nerve
    • Enters brainstem in pons
    • PAIN AND TEMP afferents descend as spinal tract of V
    • Synapse in spinal nucleus of V in medulla
    • Afferents cross midline and ascend to thalamus (trigeminothalamic fibres) with the anterolateral fibres
    • SENSORY afferents synapse in chief sensory nucleus of V in pons (same level as incoming)
    • Ascend to mesencephalic nucleus of V (small nucleus in midbrain)
  23. What is Clarke's column?
    Nucleus in dorsal horn in thoracic spinal cord: pathway for lower limb proprioceptors to cerebellum
  24. Example of allodynia
    Trigeminal neuralgia aka tic doloureux
  25. Experimental evidence for Vmpo
    • (posterior part of ventral medial nucleus of thalamus) being responsible for pain/temp
    • neuronal activity inhibited by radiant warming of contralateral tongue
    • excited by cooling with wet ice cube
    • and increase in discharge rate with increases in painful heat on contralateral ulnar hand
  26. Where is SI?
    • Post-central gyrus, posterior of central sulcus
    • Brodmann's areas 1, 2, 3
  27. Roles of different Brodmann's areas in SI
    • 1: RA (skin)
    • 2: pressure and joint position (deep tissue)
    • 3a: muscle stretch receptors (deep tissue)
    • 3b: SA and RA (skin)
  28. Position of Brodmann's areas in SI, moving posteriorly. Consequence?
    • 3a - 3b - 1 - 2
    • muscle spindle - SA - RA - pressure and joint position
    • responses more complex moving more posteriorly - higher level of cortical analysis
  29. Layers in cortex
    • I
    • II, III: TO cortical regions (ipsi- SII, contra- SI, posterior parietal cortex, motor cortex)
    • IV: FROM thalamus
    • V: TO basal ganglia, brain stem, spinal cord
    • VI: TO thalamus
  30. Columns in SI
    • 300-600um wide
    • preserve common location, single modality
    • span all 6 layers
  31. Exp for direction-sensitive neurons
    • Neuron in area 2 of SI: lots of firing from movement from ulnar wrist to radial fingers.
    • Smallest response in opposite direction.
  32. Effect of experience/injury on cortex
    • Representations of body parts can grow/shrink in cortex
    • eg monkey maintaining contact with a rotating disc in order to get food: expansion of representation of digital aspects of digits 2 and 3 in area 3b
    • also string players and fingers of left hand
  33. Cortical areas involved in pain
    • Insula (autonomic component)
    • Anterior cingulate cortex (emotional element)
    • SI
  34. Exp for ACC role
    • emotional
    • firing when watching pain being administered, heightened when pain actually received
  35. Termination of nociceptive afferents in dorsal horn
    • lamina I: A delta and C
    • C: A beta from mechanoceptors, and other nociceptors - respond to innocuous stimuli at low intensity, noxious stimuli at high intensity - 'wide-dynamic range neurons'
  36. Basis for TENS
    • transcutaneous electrical nerve stimulation
    • gate control theory: descending modulation of pain perception
  37. Descending pain systems
    • PAG (midbrain): integrates inputs from cortex/thalamus/hypothalamus
    • raphe nuclei
    • nuclei in rostral medulla
  38. Analgesia exp associated with PAG
    • Electrical stimulation - analgesia
    • Naloxone injection into PAG abolishes morphine induced analgesia
    • Bilateral transection of dorsolateral funiculus blocks stimulation and morphine based analgesia
  39. Placebo response
    • Mediated by endogenous opioid systems: abolished by IV infusion of naloxone
    • Somatotopic organisation of PAG - spatially specific