12 - Somatosensory Pathways

Question 1

Medial Bundle

Answer 1

• DRG entry bundle
• Thick, myelinated axons
• Directly enter the dorsal column
  
  • Ascends to GT or CT
• Used by muscle spindles, GTOs, pacinian & miessner’s corpusles
  
  • Basically, anything not pain or temp

Question 2

Lateral Bundle

Answer 2

• DRG SC bundle
• Enters dorsal horn
  
  • Contacts second order neurons there
• Finely or non-myelinated fibers (AB, C)
  
  • Pain or temp

Question 3

DRG Bifurcation

Answer 3

• DRG axons in SC bifurcates and goes up or down the SC
• Gives off collaterals at multiple SC segments
  
  • Propriospinal interneurons
  • Projection neurons

Question 4

Propriospinal interneurons

Answer 4

• AKA local circuit neurons
• One option of DRG bifurcation in SC
• Reflexes/pattern generators (motor)

Question 5

Projection Neurons

Answer 5

• Tract
• One option of DRG bifurcation in the SC
• Project axons away from SC to higher centers

Question 6

3 major spinal reflexes

Answer 6

1. Myotatic Stretch Reflex
2. Golgi Tendon Reflex
3. Withdrawal Reflex

Question 7

Deep Tendon Reflex

Answer 7

• 1a Muscle spindle afferent is excited
• Enters the dorsal column via the medial DRG bundle (bc it’s not pain or temp)
• Directly excites LMN via monosynapse in SC
• Also excites an inhibitory interneuron that inhibits the antagonistic LMN in SC (polysynpatic)
• Causes muscle contraction
• Deficits:
  
  • Exaggerated reflex = UMN lesion
  • Diminished reflex = SC lesion (either efferent or afferent neurons)

Question 8

Golgi Tendon Reflex

Answer 8

• 1b afferent golgi tendon organ is excited
• Enters through dorsal column bc it’s not pain/temp
• Polysynaptic in both “arms”
  
  • Excites inhibitory interneuron that synapses on agonist LMN
  • Excites excitatory interneuron that synapses on antagonist LMN
• Protects against damage from overly contracted muscle
• Opposite of myotatic/deep tendon reflex

Question 9

Withdrawal Reflex

Answer 9

• A∂ afferent nociceptors (so, lateral bundle) bc it’s pain
• So enters through dorsal horn directly
• Has crossed extension component
• Polysynaptic input to agonist and antagonist LMNs but bilaterally
  
  • In leg that feels pain, reflex inhibits the extensor, excites flexor (withdrawal)
  • In other leg, reflex excites extensor, inhibits flexor (to prevent fall)

Question 10

Spinocerebellar Pathways and where they end

Answer 10

• Dorsal spinocerebrellar tract
  
  • Lower limb muscle proprioception
• Cuneocerebellar tract
  
  • Upper limb muscle proprioception
• Central spinocerebellar tract
  
  • Enter lower limb kinesthesia

All end in the ipsilateral cerebellum for nonconscious proprioception