Neural Pathways and Reflexes

White Matter and Nerve Fibers

• White Matter: Consists of afferent and efferent nerve fiber tracts.
  • Myelinated: Covered in a fatty wrapping, hence white appearance.
• Afferent Fibers (Sensory):
  • Carry sensory information to the brain.
  • Represented as blue in charts.
• Efferent Fibers (Motor):
  • Carry motor commands from the brain to the body.

Naming of Nerve Pathways

• Named according to their origin and destination, providing clues about their function.

Neural Pathways

• Sensory Pathways: Generally consist of three neurons.
  • Example: Touch sensation involves three neurons from receptor to cerebral cortex.
• Motor Pathways: Typically consist of two neurons.
  • Neurons are involved in initiating motor actions like muscle contractions.

Decussation

• Crossing over of pathways:
  • Sensory Tracts: Often cross at the spinal cord level.
  • Motor Tracts: Frequently cross at the medulla oblongata level.

Specific Tracts

• Spinocerebellar Tract:
  • Related to muscle tension/stretch (unconscious level).
  • Pathway: Spine to cerebellum.
• Spinothalamic Tract:
  • Carries conscious sensation of pain, temperature, and touch.
  • Pathway: Spine to thalamus to postcentral gyrus (somatosensory cortex).
• Corticospinal/Pyramidal Tract:
  • Carries motor commands from cortex to spinal cord.
  • Crosses over at the medulla oblongata (decussation).

Motor Neurons

• Upper Motor Neurons (UMN):
  • From cortex to spinal cord.
• Lower Motor Neurons (LMN):
  • From spinal cord to muscles.
• Lesions:
  • UMN Lesion: Results in spastic paralysis and muscle spasms.
  • LMN Lesion: Results in flaccid paralysis.

Reflexes

• Definition: Involuntary, hard-wired responses to stimuli.
• Reflex Arc:
  • Receptor: Detects change (e.g., stretch, pain, heat).
  • Afferent Neuron: Carries the sensory impulse to the spinal cord.
  • Integration Center: Processes the information at the spinal cord level.
  • Efferent Neuron: Carries motor command to muscle to respond/react.