Lecture Notes: Spinal Cord and Sensory Pathways

Overview of the Spinal Cord Regions

• Regions: Sacral, Lumbar, Thoracic, and Cervical
• White Matter: Emphasized in the dorsal columns
• Dorsal Root Ganglion: Houses sensory neuron cell bodies from proprioceptors or exteroceptors

Sensory Pathways

Lower Extremities

• Enter the spinal cord and descend in the dorsal columns
• Form a bundle known as Fasciculus Gracilis
• Collateral branches synapse in dorsal horns for reflex activities

Trunk

• Sensory axons enter thoracic segments
• Ascend in dorsal columns laterally to sacral and lumbar segments

Upper Extremities and Neck

• Enter at cervical levels
• Form part of the lateral section of dorsal columns with neurons from the upper trunk
• Known as Fasciculus Cuneatus

Spinal Cord-Medulla Junction

• Decussation: Sensory decussation occurs caudal to motor decussation
• Fibers of fasciculus gracilis and fasciculus cuneatus synapse with second-order neurons

Medulla to Thalamus

• Axons in Nucleus Gracilis and Nucleus Cuneatus cross midline as internal arcuate fibers
• Form Medial Lemniscus Tract
• Somatotopic organization shifts: medial-lateral to ventral-dorsal

Changes in Medial Lemniscus

• Orientation changes as it ascends
• Dorsal fibers become medial
• Pattern flips compared to the spinal cord

Midbrain to Thalamus

• Medial lemniscus passes through rostral midbrain
• Enters the Ventral Posterior Lateral Nucleus of dorsal thalamus
• Third-order neurons project to primary somatosensory cortex

Somatosensory Cortex

• Maintains somatotopy for tactile and proprioceptive information

Conclusion

• Importance of pathways in processing sensory information
• Role of primary somatosensory cortex in conscious perception of touch and proprioception

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