Pain Physiology

Master answer

Withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli.
The upper centres are not involved.

Afferent

• Nociceptors detect the painful stimulus
• Conveyed to the dorsal horns of spinal cord by Aδ (fast) fibres and C (slow) fibres
  • Through the cauda equina
• Synapse at the same spinal level, or 1-2 levels above via Lissauer’s tract

Reflex arc

• (fast response to painful stimulus)
• (outside of conscious control, but can be modulated by the cerebral cortex)
• Interneurons:
  • Travel to the ipsilateral anterior horn, synapse with motor neurons
  • Cross the midline to the contralateral anterior horn
• Efferent
  • Motor neurons in the ipsilateral limb cause contraction of flexors, and relaxation of extensors
  • Contralateral limb motor neurons cause relaxation of flexors, and contraction of extensors
• Causes withdrawal of painful limb, and shift In centre of gravity to the opposite limb

Conscious pathways

(slow response to painful stimulus)

Interneurons:

• Cross the midline via the anterior white commissure
• Ascend in the spinothalamic tract to the thalamus
• Synapse in the thalamus
• Travel to the post-central gyrus of the cerebral cortex

Efferent:

• Originate in the motor cortex (pre-central gyrus)
• Project to medulla
• 90% decussate at pyramids to innervate limbs
  • Travel down the lateral corticospinal tract
• 10% continue down ipsilateral anterior corticospinal tract to innervate axial muscles
  • Decussate at the target spinal level
• Synapse at the anterior horn to lower motor neurons

Mooney / Sakurai 2016

Answer by Mooney, Sakurai