Somatic sensory pathways

🟠 General Characteristics of Somatic Sensory Pathways

• 🖇 Any pathway is made of 3 Order Neurons (1st – 2nd – 3rd ON).
• 🖇 All are Ascending Tracts → going upward (opposite of Motor = Descending).
• 🖇 1st ON always originates from Dorsal Root Ganglion (DRG).
• 🖇 Cerebral Cortex = Manager (final perception: type + intensity + location).
• 🖇 Thalamus = Secretary (relay station before reaching cortex).
• 🚨 Decussation: most fibers cross to the opposite side → explains why left cortex lesion affects right body side.

🟣 Crude Touch Pathway (Ventral Spinothalamic Tract)

• 📌 Sensation: Crude Touch (basic awareness without localization/discrimination).
• 📌 1st ON: from DRG → ends in Lamina II, III, IV.
• 📌 2nd ON: starts in those laminae → crosses (Decussation) → goes to VPLN of Thalamus.
• 📌 3rd ON: from VPLN → via Thalamic Radiation → to Cerebral Cortex.

🟣 Pain & Temperature Pathway (Lateral Spinothalamic – Paleospinothalamic)

• 📌 Sensation: Slow Pain + Temperature.
• 📌 Fiber type: Type C fibers (Type 4).
• 📌 1st ON: from DRG → ends in Substantia Gelatinosa (Lamina II, III).
• 📌 2nd ON: starts from Lamina II, III → crosses to opposite side → ascends via Spinal Lemniscus.
• 📌 2nd ON termination: in a Specific Nucleus of Thalamus.
• 📌 3rd ON: from Specific Nucleus → Thalamic Radiation → to Cortex.
• 🚨 Extra relay compared to Crude Touch (Spinal Lemniscus + Specific Nucleus) → because pain requires stronger filtering to protect the body.

🟠 Key Points & Questions

• 🖇 Every sensory pathway = 3 Order Neurons.
• 🖇 1st ON always originates from DRG.
• 🖇 Opposite-side sensation due to Decussation.
• 🖇 Cortex vs. Thalamus: Cortex = Manager, Thalamus = Secretary.
• 🖇 Pathway differences:
• • Ventral (Crude Touch): 2nd ON ends in VPLN.
  • Lateral (Pain/Temperature): additional relay (Spinal Lemniscus) + ends in Specific Nucleus.