Lecture on White Matter of the Spinal Cord and Neural Pathways

Overview of Spinal Cord White Matter

• White Matter Divided into Funiculi
  • Three funiculi on each side of the spinal cord
  • Contains tracts, which are collections of axons
  • Tracts: Can be ascending (sensory input to CNS) or descending (motor output from brain)
  • Primarily myelinated fibers; some unmyelinated fibers present
  • Funiculi labeled as posterior, lateral, and anterior

Symmetry in the Spinal Cord

• Symmetry between the right and left sides of the body
• Symmetry with gray matter as well; involved in information reception and sending

Pathway of Sensory Input

• General Pathway: Input travels from peripheral nervous system (PNS) to central nervous system (CNS)
  • Typically involves three neurons: first, second, and third order neurons

First Order Neurons

• Conduct impulses from receptors (e.g., skin receptors) to CNS
• Part of the peripheral nervous system

Second Order Neurons

• Interneurons located in CNS
• Cell body often located in posterior horn of spinal cord or brainstem

Third Order Neurons

• Interneurons in the CNS
• Located in the thalamus, which sorts, edits, and relays information to the cerebral cortex

Example Pathway

• From receptor to first-order neuron, passing through posterior root into spinal cord
• First-order neuron synapses with second-order neuron (often in medulla)
• Second-order neuron crosses over (decussation) and synapses with third-order neuron in thalamus
• Thalamus relays information to somatosensory cortex

Motor Pathways

• Typically consist of upper and lower motor neurons
• Upper motor neurons originate in the cerebral cortex
• Pathway descends through brainstem (midbrain, pons, medulla)
• Synapse occurs with interneurons, and then with lower motor neurons

Lower Motor Neurons

• Cell bodies located in the anterior horn of spinal cord
• Conducts signals to skeletal muscles

Additional Neural Structures

• Basal nuclei influence pathways
• Input from cerebellum helps select and influence motor programs

Summary

• The discussed pathways are simplified models
• Coordination of pathways involves multiple brain structures
• Processes occur rapidly, enabling complex functions such as speech

Questions or clarifications were invited at the end of the lecture.