Anterolateral System (Spinothalamic Tract)

Overview

• Focus: detailed exploration of the anterolateral system (ALS) or spinothalamic tract
• Key components: anterior (ventral) and lateral spinothalamic tracts
• Function: transmission of specific sensory information (pain, temperature, crude touch, and pressure)

Divisions of the Spinothalamic Tract

Anterior Spinothalamic Tract

• Carries sensations of crude touch and superficial pressure
• Crude touch: non-discriminative touch

Lateral Spinothalamic Tract

• Transmits pain and temperature
  • Pain: fast (pinprick) pain and slow (aching) pain
  • Temperature: sensations of hot and cold

Sensory Receptors

• Nociceptors: respond to pain and extreme temperatures
  • Types: A-delta fibers (fast pain) and C fibers (slow pain)
• Mechanical Receptors: respond to mechanical stimuli (e.g., pressure)
• Thermal Receptors: respond to temperature changes
• Chemical Stimuli: released during tissue damage (e.g., protons, potassium, bradykinin, histamines)
• Other receptors: Merkel's disks (fine touch and superficial pressure), peritrichial nerve endings (around hair follicles), free nerve endings

Pathways

Fast Pain (A-delta fibers)

• Respond to mechanical stimuli and extreme cold
• Path: Peripheral process → central process → dorsal horn (Rexed lamina I and V) → cross via anterior white commissure → lateral spinothalamic tract

Slow Pain (C fibers)

• Respond to mechanical stimuli, extreme heat, and chemical stimuli
• Path: Peripheral process → central process → dorsal horn (Rexed lamina II and III) → cross via anterior white commissure → lateral spinothalamic tract

Crude Touch and Pressure

• Path: Peripheral process → central process → dorsal horn → cross via anterior white commissure → ventral spinothalamic tract

Ascending and Descending Fibers (Lissauer's Tract)

• Fibers can ascend or descend 1–3 spinal segments before synapsing in the dorsal horn
• Important for understanding the clinical significance of spinal cord lesions

Pathway to the Brain

• Crude Touch & Pressure: Ventral spinothalamic tract → Ventral Posterior Lateral Nucleus (VPL) → primary (S1) and secondary (S2) somatosensory cortex
• Fast Pain (A-delta): Lateral spinothalamic tract (Neo spinothalamic pathway) → VPL and VPI nucleus
• Slow Pain (C fibers): 85% to reticular formation (Paleo spinothalamic pathway), some to non-specific intralaminar nuclei in thalamus (Centro median nucleus and parafasciculus)

Additional Pathways

• Spinotectal Tract: Superior colliculus → orient head and eyes toward stimulus
• Spino-mesencephalic Tract: Parabrachial nucleus → amygdala (emotional aspects of pain), periaqueductal gray matter (pain modulation)
• Spinal Hypothalamic Tract: Hypothalamus (autonomic responses to pain)
• Spino-reticular Tract: Reticular formation (alerting and arousal)

Clinical Relevance

• Lesions: Understanding the ascending/descending components critical for localizing spinal lesions
• Pain Modulation: Periaqueductal gray matter's role in controlling pain through descending pathways
• Contribution to autonomic and emotional responses through communication with the hypothalamus, amygdala, and various nuclei

Summary

• The anterolateral system is complex, involving multiple tracts and pathways for transmitting various sensory modalities to the brain.
• Understanding the individual components and their roles aids in comprehending clinical symptoms related to spinal cord and sensory pathway lesions.