Pain and touch

Dermis

• 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

• 3-5mm diameter receptive fields - SMALL
• low frequency vibration (most sensitive ~30Hz)
• High sensitivity, low spatial resolution

A single AP for each phase of a sinusoidal wave: found in RA fibres

• RA 1 is Meissners, RA II is Pacinian
• SA I is Merkel, SA II is Ruffini

• Meissner's like scotopic: enhanced sensitivity, poor spatial resolution
• Merkel like photopic: high spatial resolution, decreased sensitivity

• LA: raises threshold for Meissner's (closer to surface)
• Can use pre-adaptation to desensitise for high (Pacinian)/low (Meissner's) frequencies

• 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

• Object motion, hand/finger position
• Responsive to stretch (within spindles)

40mm at shoulder, 2mm at fingers

• 30x more cold spots
• different concentrations in different places eg lip has 6x cold spots of sole

more receptors than spots: 'warmth' requires the simultaneous activation of many receptors

• Trpv1: HOT (capsaicin, >45 degrees) C fibres
• Trpv4: WARM (peak 40 degrees)
• Trpm8: COLD (menthol, <25 degrees) - Adelta and C fibres

• 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)

• 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

Dorsal median sulcus, ventral median fissure

• 9 Rexed's laminae
• I-VI Dorsal Horn, VII Intermediate zone, VIII-IX Ventral Horn

• 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)

• 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

• 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

• 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

• 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)

Nucleus in dorsal horn in thoracic spinal cord: pathway for lower limb proprioceptors to cerebellum

Trigeminal neuralgia aka tic doloureux

• (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

• Post-central gyrus, posterior of central sulcus
• Brodmann's areas 1, 2, 3

• 1: RA (skin)
• 2: pressure and joint position (deep tissue)
• 3a: muscle stretch receptors (deep tissue)
• 3b: SA and RA (skin)

• 3a - 3b - 1 - 2
• muscle spindle - SA - RA - pressure and joint position
• responses more complex moving more posteriorly - higher level of cortical analysis

• 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

• 300-600um wide
• preserve common location, single modality
• span all 6 layers

• Neuron in area 2 of SI: lots of firing from movement from ulnar wrist to radial fingers.
• Smallest response in opposite direction.

• 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

• Insula (autonomic component)
• Anterior cingulate cortex (emotional element)
• SI

• emotional
• firing when watching pain being administered, heightened when pain actually received

• 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'

• transcutaneous electrical nerve stimulation
• gate control theory: descending modulation of pain perception

• PAG (midbrain): integrates inputs from cortex/thalamus/hypothalamus
• raphe nuclei
• nuclei in rostral medulla

• Electrical stimulation - analgesia
• Naloxone injection into PAG abolishes morphine induced analgesia
• Bilateral transection of dorsolateral funiculus blocks stimulation and morphine based analgesia

• Mediated by endogenous opioid systems: abolished by IV infusion of naloxone
• Somatotopic organisation of PAG - spatially specific