45, 46 Touch and Pain

Question 1

describe the trigeminal touch pathway

Answer 1

• primary somatosensory neuron: connects receptor to principle sensory trigeminal nucleus (pons) via trigeminial nerve
• secondary somatosensory neuron: from pons to ventral-postero-medial (VPM) nucleus (thalamus) via ventral trigemino-thalamic tract (decussates)
• tertiary somatosensory neuron: from thalamus to S1 via internal capsule

Question 2

describe the primary somatosensory cortex (S1)

Answer 2

• in parietal lobe on postcentral gyrus, Brodmann’s areas 1, 2 and 3
• first stage of processing of touch info. in cerebral cortex

Question 3

Answer 3

Question 4

Answer 4

Question 5

describe what a lesion in the primary somatosensory cortex (S1) would affect

Answer 5

• causes contra-lateral sensory loss (touch, vibration, proprioception, pain and temp.) in a somatotropic pattern
• affects high cortical tactile functions

Question 6

describe the function and divisions of the internal capsule

Answer 6

• white matter structure/fiber bundle
• carries sensory and motor info. between thalamus and cortex
• division:
  
  • posterior limb: neurons carrying sensory info. to postcentral gyrus
  • anterior limb: fibers from frontal cortex to pons and thalamus to frontal lobe
  • genu (area where 1 and 2 meet): fibers from cortex to spine

Question 7

describe the blood supply of the internal capsule

Answer 7

blood supply = middle cerebral artery

• MCA stroke = contra-lateral sensory loss (touch, vibration, proprioception, pain) in somatotropic pattern

Question 8

describe the sensory pathway of the DCML

Answer 8

• primary somatosensory neuron: connects receptor to medulla via fasiculi gracilis (lower body) & cuneatus (upper body) (dorsal column)
• secondary somatosensory neuron: from medulla to thalamus via medial lemniscus; decussates at midline in caudal medulla
• tertiary somatosensory neuron: from thalamus to postcentral gyrus (cortex) via internal capsule

Question 9

describe phantom limb sensations

Answer 9

a likely explanation for phantom limb sensations is the reorganization of the cortical maps

• explained by rearrangement of cortical input to the area that formerly represented the hand
• touch pathways originating in the face are likely to form new circuits within the primary somatosensory cortex (S1) that connect them with cortical neurons that would have otherwise become “jobless” (lacking innervation from the missing limb)

Question 10

describe tabes dorsalis

Answer 10

• syphilitic myelopathy
  
  • significant demyelination of neural tracts in the dorsal root and dorsal column
• leads to ipsilateral paresthesia and/or reduced touch in a dermatomal pattern
• gait disturbances (from diminished proprioception)
• diminished patellar reflex
• involvement of DRG cells leads to short, shooting pain

Question 11

describe stocking glove symptoms

Answer 11

• metabolic disease e.g. diabetic peripheral neuropathy
• longest neurons have highest metabolic demand, fail earlier

Question 12

describe the spinothalamic pathway/anterolateral system (pain and temp. pathway)

Answer 12

• primary sensory neuron: from receptor to substantia gelatinosa via dorsal root
• secondary sensory neuron: from substantia gelatinosa to ventral postero-lateral (VPL) nucleus in the thalamus via spinothalamic tract (crosses midline)
• tertiary sensory neuron: from thalamus to S1 (cortex) via internal capsule

Question 13

contrast the DCML vs spinothalamic/anterolateral system pathway

Answer 13

Question 14

Answer 14

Question 15

describe first vs second pain

Answer 15

• first pain: immediate sharp pain
  
  • mediated by faster delta fibers
• second pain: delayed, longer-lasting, duller, throbbing
  
  • mediated by slower C fibers

Question 16

describe the different types of axons from the skin and the sensory receptors associated with them (A-alpha, A-beta, etc.)

Answer 16

Question 17

describe different types of referred pain

Answer 17

• heart attack = pain in upper chest wall and left arm
• appendicitis = pain around abdominal wall and navel
• pancreatitis = pain in left upper abdomen, extending to the back

Question 18

describe hyperalgesia and allodynia

Answer 18

• hyperalgesia: increased sensitivity to a painful stimulus
• allodynia: previously painless stimuli become painful
• due to sensitization of nociceptors
• lowers sensory neuron firing threshold
• chemical substances released within the area of insult
  
  • prostaglandins from damaged cells
  • leukotrienes from damaged cells
  • substace P from primary afferents

Question 19

describe the gate control circuitry

Answer 19

• large myelinated fibers carrying touch sensations activate inhibitory interneurons within the dorsal horn of the spinal cord, which then reduce the flow of nociceptive info. through the “gate” between nociceptors (first order neurons of the pain pathway) and their second order neurons
  
  • enkephalin (endogenous opiod peptide) is the neurotransmitter of the inhibitory interneurons acting on opiod receptors
  • these opiod receptors are found on the cell bodies of the primary afferent neurons, on their presynaptic ending and also on the postsynaptic membrane of the second order neurons (projection neurons)

Question 20

describe descending regulation of pain

Answer 20

• descending serotonergic (from periaqueductal grey, nucleus raphe magnus) or noradrenergic (locus ceruleus) fibers originating in the brainstem participate in the physiological modulation of pain
• these descending monoaminergic fibers run in the lateral and anterior funiculi of the spinal cord and terminate on the opiodergic interneurons in the dorsal horn, where they control the transmission of pain
  
  • the nuclei where the descending pain control fibers originate receive input from ascending ALS fibers carry nociceptive information

Question 21

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Question 22

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Question 23

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Question 24

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Question 25

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Question 26

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Question 27

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Question 28

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Question 29

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Question 30

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