Overview of Pain Pathways

Introduction to Pain

• Focus on ascending and descending pathways of pain.
• Discussion of the substantia gelatinosa.

Ascending Pathway of Pain

• Pain signals travel from injury site to the brain.
• Pain perception occurs in the brain, primarily in the somatosensory cortex.
• Key Areas of the Brain:
  • Cerebrum, Brainstem, Cerebellum
  • Somatosensory Cortex: Located posterior to the central sulcus.

Somatosensory Cortex

• Area where sensation, including pain, is perceived.
• Specific regions correlate to different body parts:
  • Left side of the somatosensory cortex: Represents the right side of the body (e.g., leg, hand, face).

Brainstem and Spinal Cord Connection

• Brainstem consists of:
  • Midbrain, Pons, Medulla.
• Spinal Cord: Focus on cervical spine area.
• Nerves:
  • Anterior root (motor) and posterior root (sensory).

Pain Signal Transmission

• Spinal Thalamic Tract: Ascending pathway consists of lateral and anterior tracts.
• Injury in the right hand leads to:
  • Damage to immune cells causes release of cytokines, including prostaglandins (PG).
  • Sensory nerve fibers respond to prostaglandins and send signals to the dorsal horn of the spinal cord.

Neuron Pathway Overview

1. First Order Neuron:
   • Synapses in the dorsal horn.
2. Second Order Neuron:
   • Crosses to the opposite side, enters spinal thalamic tract, ascends to thalamus.
3. Third Order Neuron:
   • Relays impulse to the specific area in the brain correlating with the injury.
   • Important to note: Sensation is perceived on the opposite side of the brain from the injury.
4. Substance P:
   • Released by the first order neuron to transmit signals.

Descending Pathway of Pain

• The descending pathway modulates and inhibits the ascending pathway.
• Key areas include:
  • Periaqueductal gray matter (midbrain)
  • Nucleus raphe magnus (medulla)
• Neurons from the periaqueductal gray matter synapse with serotonergic noradrenergic neurons.

Role of Serotonergic Noradrenergic Neurons

• Travel down to the dorsal horn of the spinal cord.
• Function:
  1. Inhibit communication between first order and second order neurons of the ascending pathway.
  2. Control pain signals.

Gate Control Theory of Pain

• The dorsal horn can be considered a gate that controls pain signals.
• Substantia Gelatinosa:
  • Important area in the dorsal horn for pain modulation.

Summary of Pain Transmission in the Dorsal Horn

1. First order neuron synapses with the second order neuron.
2. Substance P stimulates the second order neuron.
3. Descending Neurons (serotonin and noradrenaline) inhibit substance P release.
4. Endogenous Opioids (Enkephalins):
   • Released by interneurons in the substantia gelatinosa to inhibit pain signals.
   • Prevents presynaptic neuron from releasing substance P and inhibits postsynaptic neuron.

Conclusion

• Overview of the ascending and descending pathways of pain, highlighting the complexities of the pain perception and modulation.