Pain and Temperature Pathways

Question 1

indirect spinothalamic pathway

Answer 1

• slow/crude pain
• diffuse pain
• Rs are assoc with viscera and is described as burning, deep, dull, aching poorly localized
• C type fibers–>nucleus proprius–>fasciculus proprius in the reticular formation–>centromedian nucleus in the hypothalamus

Question 2

primary neurons in the indirect spinothalamic pathway

Answer 2

• unmyelinated C fibers with a very slow conduction rate
• when they enter the SC, they bifurcate and ascend and descend a variable numbers of segments in the dorsolateral fasciculus of Lissaur
• primary fibers send collateral terminals to the nucleus proprius
  
  • some fibers from the C type fibers may also go to the substantia gelatinous and ascend in the direct spinothalamic system
• visceral or slow pain may be perceived as somatic or fast pain if enough C type fibers recruited which allows info to go to the direct spinothalamic pathway

Question 3

secondary neurons in the indirect spinothalamic pathway

Answer 3

• come from the nucleus proprius and course bilaterally in the anterior, lateral, and posterior regions of the fasciculus proprius
• most fibers terminate on interneurons which will form the neuronal patterns for the complex, stereotyped intersegmental reflexes characteristic of pain responses

Question 4

spinoreticular fibers

Answer 4

• slow pain info from the nucleus proprius may ascend to the thalamus as spinoreticular fibers
  
  • these fibers are part of a divergent, multi neuronal polysynaptic pathway and embedded in the anterior, lateral, and posterior regions of the fasciculus proprius
• ultimately terminate in the midline reticular formation of the brainstem, hypothalamus, and centromedian nucleus of the dorsal thalamus on both sides of the brain

Question 5

fasciculus proprius

Answer 5

-part of a diffuse neuronal net called the reticular formation which surrounds the gray matter of the SC and extends rostrally thru the core of the brainstem to the thalamus

Question 6

where is slow or visceral pain perceived?

Answer 6

thalamic level

Question 7

unilateral lesions of the spinoreticular fibers vs bilateral lesions

Answer 7

• do not result in significant sensory defects
• the indirect spinothalamic pathway is too bilateral and diffuse to be affected by unilateral lesions
• bilateral lesions such as SC transactions may eliminate crude pain sensations
  
  • if the transection is incomplete or at different levels, the spinoreticular fibers may find a route thru the intact portion of the fasciculus proprius which is the basis of persistent or intractable pain

Question 8

direct spinothalamic pathway

Answer 8

• fast pain
  1. primary neuron–located in the spinal ganglion
  
  • primary axon enters the SC, bifurcates, and ascends/descends 2 SC segments
    
    2. secondary neuron–located in a nucleus in the tip of the posterior horn of the SC
  • secondary axons: decussate within 2 SC levels of the incoming stimulus and ascend to a specific nucleus in the dorsal thalamus
    
    3. tertiary neuron–located in the dorsal thalamus
  • tertiary axons: project to the post central gyrus which is referred to as the primary somesthetic cortex

Question 9

primary neurons of the direct spinothalamic pathway

Answer 9

• in the spinal ganglia
• send central processes into the SC
• primary axon enters the SC, bifurcates, and ascends/descends 2 SC segments
• 2 types: A delta and C
  
  • the axons of both enter the SC thru the lateral division of the dorsal root, and bifurcate to ascend and descend +/- 2 segments in the dorsolateral fasciculus
  • these fibers terminate on secondary neurons in the substantia gelatinosa (A delta) and nucleus proprius (C)
• with a lesion here, we would get contralateral loss of pain and temp 2 sensory dermatomes away from the lesion

Question 10

secondary neurons of the direct spinothalamic pathway

Answer 10

• in the substantia gelatinosa
• send axons thru the anterior white commissure to form the lateral spinothalamic tract (LSTT)
  
  • secondary axons: decussate within 2 SC levels of the incoming stimulus and ascend to a specific nucleus in the dorsal thalamus

Question 11

tertiary neurons of the direct spinothalamic pathway

Answer 11

• in the VPL nucleus of the thalamus

- tertiary axons: project to the post central gyrus which is referred to as the primary somesthetic cortex

Question 12

reticular formation

Answer 12

• from the thalamus
• contain the fasciculus proprius
• role in consciousness/arousal
• original sensory/motor integration
• “battery of cortex”

Question 13

congenital absence of C type fibers

Answer 13

• allows the non-nociceptive fibers to close the gate

- person is insensitive to pain

Question 14

Herpes Zoster

Answer 14

• Herpes Zoster (shingles) infection may compromise the non-nociceptive A alpha/beta fibers, thereby allowing the nociceptive (C) fibers to open the gate
• person has an inc sensitivity to pain from the sensory dermatome of the affected N

Question 15

unilateral lesions of the lateral spinothalamic tract

Answer 15

• result in a contralateral loss of pain and temp sensation 2 sensory dermatomal segments below the level of the lesion
• problem with the direct spinothalamic pathway

Question 16

treatment of intractable pain

Answer 16

• LSTT may be transected for relief of intractable pain
  
  • the anterolateral quadrant of the cord is cut 2 segments above and on the opposite side of the area of pain
  • procedure is called the anterolateral cordotomy or tractotomy
• the denticulate ligaments serve as landmarks b/w the LSTT (anteriorly) and the corticospinal fibers (posteriorly)
• crude pain sensations usually remain intact or are only temporarily diminished

Question 17

unilateral lesions of the spinal lemniscus in the direct spinothalamic pathway

Answer 17

-result in contralateral hemianalgesia and thermal hemianesthesia

Question 18

prefrontal lobes and the prefrontal lobotomy

Answer 18

• prefrontal lobes play an important role in the emotional importance and response we have to pain
• pts with intractable pain used to have fibers from the prefrontal lobes surgically disconnected from the remaining hemispheres in a prefrontal lobotomy
  
  • the pt loses the anxiety and emotional component that is so often assoc with pain
  • pain is indifferent to pain but aware of pain

Question 19

syringomyelia

Answer 19

• gross cavitation and gliosis of the central canal occurring in the cervical regions of the SC
  
  • as the syrinx enlarges, the neurological deficits progressively worsen over a period of months or years
• may occur secondary to central cord syndrome
  
  • results in an abrupt onset of neurological deficits
• enlargement of the syrinx results in:
  
  1. destruction of the anterior white commissure with a bilateral loss of pain and temp sensations to upper extremities
  2. asymmetrical (unilateral or bilateral) destruction of the lateral corticospinal tracts results in spastic paralysis, hyperreflexia, hypertonia of lower extremity
  3. anterior horns may be destroyed unilaterally or bilaterally which results in LMN paralysis (flaccid paralysis, atrophy, areflexia, atonia) of the assoc upper limb Ms
  4. some part of posterior columns may be affected and result in ipsilateral anesthesia (proprioceptive and 2 point tactile sensations) below the level of the lesion