Anterolateral System (Spinothalamic Tract)

Overview

• The anterolateral system (ALS), also known as the spinothalamic tract, is crucial for sensory input into the CNS.
• It is divided into two parts:
  • Anterior (Ventral) Spinothalamic Tract: Carries crude touch and pressure.
  • Lateral Spinothalamic Tract: Carries pain and temperature.

Functions

• Anterior Spinothalamic Tract: Traditionally believed to carry crude touch and superficial pressure.
• Lateral Spinothalamic Tract: Carries sensations of pain (fast and slow) and temperature.

Sensory Receptors

• Nociceptors: Detect pain and extreme temperatures.

  • Respond to tissue damage or extreme temperatures.
  • Types include:
    • A-delta fibers: Fast pain (pinprick), respond to mechanical stimuli and cold temps.
    • C fibers: Slow pain (burning or aching), respond to chemical stimuli, mechanical, and hot temps.

• Other receptors for touch and pressure:

  • Merkel's Discs: Superficial pressure/fine touch.
  • Peritracheal Nerve Endings: Surround hair follicles.
  • Free Nerve Endings: Respond to touch and pressure.

Fiber Types

• A-delta Fibers: Heavily myelinated, faster conduction.
• C Fibers: Lightly or non-myelinated, slower conduction.

Pathway into the Spinal Cord

Entry and Synapse

• A-delta Fibers: Synapse in Rexed Lamina I and V, cross via anterior commissure.
• C Fibers: Synapse in Rexed Lamina II and III, cross via anterior commissure.
• Crude Touch Fibers: Possibly synapse in Rexed Lamina III, IV, or V.

Tract of Lissauer

• Fibers ascend and descend 1-3 segments in the spinal cord before synapsing.
• Important for understanding sensory deficits in spinal lesions.

Ascending Pathways

• The fibers of the anterolateral system travel up the spinal cord to the brain.
• Ventral Spinothalamic Tract: Terminates primarily in the ventral posterior lateral and inferior nuclei of the thalamus.
• Lateral Spinothalamic Tract: Divided into:
  • Neospinothalamic Pathway: For A-delta fibers, synapse at specific thalamic nuclei.
  • Paleospinothalamic Pathway: For C fibers, primarily synapse in the reticular formation, and some reach nonspecific thalamic nuclei.

Central Processing

• C-fibers: Primarily synapse in the reticular formation, affecting alertness and arousal.
• Some fibers reach central thalamic nuclei (intralaminar), leading to widespread cortical activation.
• C-fibers are also involved in emotional pain perception through pathways to the cingulate gyrus and anterior insular cortex.

Ancillary Pathways

• Spinotectal Tract: Moves towards the superior colliculus to mediate reflexive movements in response to stimuli.
• Spinomesencephalic Tract: Connects to the parabrachial nucleus and the amygdala, influencing emotional responses to pain.
• Spinohypothalamic Tract: Relays autonomic responses to pain by connecting with the hypothalamus.

Clinical Significance

• Understanding of ascending and descending fiber pathways is crucial for diagnosing sensory deficits due to spinal cord lesions.
• The tract communicates pain and temperature sensations effectively to various parts of the brain, influencing both physical and emotional responses to pain.

Key Takeaways

• The anterolateral system is complex, integrating sensory information from the body to the brain.
• It is an essential pathway for detecting and processing pain, temperature, pressure, and crude touch.