Lecture Notes: Anterolateral System and Spinothalamic Tract

Overview

• Anterolateral System (ALS)
  • Also known as the spinothalamic tract
  • Divided into two components:
    1. Anterior/Ventral Spinothalamic Tract: Crude touch and pressure
    2. Lateral Spinothalamic Tract: Pain and temperature

Function and Divisions

• Historical Beliefs:
  • Anterior (Ventral): Carries crude touch, superficial pressure
  • Lateral: Carries pain (fast and slow) and temperature sensations
• Current Understanding:
  • These pathways have mixed and intertwined functions, carrying all types of sensations mentioned above.

Receptors Involved

• Nociceptors: Receptors picking up tissue damage and extreme temperatures
  • Types of stimuli:
    • Tissue Damage: Chemical burns, mechanical damage
    • Extreme Temperatures: Cold or hot conditions
• Types of Fibers:
  • Aδ (Delta) Fibers: Fast pain, pinprick pain, activated by mechanical stimuli and cold temperatures
  • C Fibers: Slow pain, burning/aching pain, activated by hot temperatures, mechanical stimuli, and chemical factors (e.g., protons, potassium, bradykinin, histamines)

Pathways and Synaptic Connections

• Aδ Fibers:
  • Synapse in Rexed Lamina I and V
  • Cross via anterior white commissure, ascend
• C Fibers:
  • Synapse in Rexed Lamina II and III
  • Cross via anterior commissure, ascend
• Crude Touch/Pressure Pathway:
  • Synapse in various lamina (II, III, IV, V)
  • Cross via anterior white commissure, ascend through the ventral white column
• Tract of Lissauer:
  • Ascending and descending before synapsing
  • Important for indicating lesion impact two segments below and on the contralateral side

Ascending Pathways

• Ventral Spinothalamic Tract (Crude Touch/Pressure):
  • First-order neuron in dorsal root ganglion
  • Crosses at the anterior commissure, ascends
  • Targets the Ventral Posterior Lateral (VPL) and Ventral Posterior Inferior (VPI) Nucleus in thalamus
• Lateral Spinothalamic Tract (Pain/Temperature):
  • Aδ fibers: Assumed to be part of the Neo Spinothalamic Pathway
  • C fibers: Part of the Paleo Spinothalamic Pathway
  • Major portion synapse in reticular formation (85%)
  • Small portion: Ascend to thalamus, synapse in Intralaminar Nuclei (e.g., centro median nucleus, parafasciculus)
  • C fibers signal emotional pain aspects via the cingulate gyrus and anterior insular cortex

Additional Components

• Spinal Tectal Tract: Movements towards stimuli
• Spinal Mesencephalic Tract: Connects with para brachial nucleus (pons) to amygdala (emotional/fear aspects)
• Spinal Hypothalamic Tract: Autonomic responses to pain
• Spinal Reticular Tract: Alerts and arouses the CNS

Clinical Relevance

• Lesion at T6 Spinal Cord Segment:
  • Affected area two segments below on contralateral side
• Reticular Formation:
  • Key for awake/arousal states
  • Constant achy pain (e.g., arthritis) keeps reticular formation active, preventing sleep
• Contralateral and Ipsilateral Pathways:
  • Some fibers remain ipsilateral, impacting the reticular formation and intra laminar nuclei

Summary

• Anterolateral System is crucial for transmitting tactile and pain information.
• Pathways and receptors are mixed, integrating for both crude touch, pressure, pain, and temperature recognition.
• Lesion studies are pivotal in understanding the cross-sectional implications and synaptic connections.