Anterolateral System (ALS) / Spinothalamic Tract

1. Overview

• Also called Spinothalamic Tract.

• Two main divisions:

  • Anterior (ventral) spinothalamic tract → crude touch + pressure.

  • Lateral spinothalamic tract → pain + temperature.

• Modern view: sensations are not perfectly segregated → best considered as one functional system.

2. Functions

• Anterior tract: crude touch, superficial pressure.

• Lateral tract: pain + temperature (fast vs. slow pain).

• Together → ALS carries crude touch, pressure, pain, and temperature.

3. Receptors

a. Nociceptors (pain + temperature)

• Respond to tissue damage (mechanical, chemical, thermal).

• Two types of afferent fibers:

  • Aδ fibers → fast, sharp, pinprick pain.

    • Stimulated by mechanical injury + cold temps.

    • Myelinated → faster conduction.

  • C fibers → slow, dull, aching, burning pain.

    • Stimulated by hot temps, mechanical damage, and especially chemical mediators (protons, K⁺, bradykinin, histamine).

    • Unmyelinated → slower conduction.

b. Touch/Pressure Receptors

• Merkel’s disks – superficial pressure, fine touch.

• Peritrichial endings – around hair follicles (hair movement).

• Free nerve endings – crude touch, pressure.

4. Myelination & Conduction

• A fibers – heavily myelinated → fast.

  • Includes Aα, Aβ, Aδ (pain = Aδ).

• B fibers – moderately myelinated (autonomic).

• C fibers – unmyelinated → slow pain.

5. Entry into Spinal Cord

• First-order neuron: pseudo-unipolar cell body in dorsal root ganglion (DRG).

• Central process enters dorsal horn → can ascend/descend 1–3 levels in tract of Lissauer.

• Synapse locations (Rexed’s laminae):

  • C fibers → Lamina II (Substantia Gelatinosa) & III (Nucleus Proprius).

  • Aδ fibers → Lamina I (Marginal Zone) & V (Reticular Nucleus).

  • Crude touch fibers → Laminae III–V.

• Fibers cross in anterior white commissure → ascend contralateral side.

6. Ascending Pathways

a. Anterior Spinothalamic (ventral)

• Carries crude touch + pressure.

• 1st order: DRG neuron.

• 2nd order: dorsal horn neuron → cross midline → ascend in anterior column.

• Synapse in VPL + VPI nuclei of thalamus.

• Project to S1, S2 cortex (postcentral gyrus).

b. Lateral Spinothalamic

• Carries pain + temperature.

• Two subdivisions:

  • Neospinothalamic (Aδ fibers) → fast, sharp pain.

    • Goes mainly to VPL thalamus → S1, S2 cortex → conscious perception.

  • Paleospinothalamic (C fibers) → slow, dull pain.

    • ~85% terminate in reticular formation.

    • 15% project to intralaminar thalamic nuclei (CM, PF) → widespread cortex, including:

      • Cingulate gyrus + anterior insula → emotional/affective pain.

7. Collateral Pathways of ALS

ALS gives branches to other important centers:

• Spinoreticular tract → reticular formation (arousal to pain).

• Spinotectal tract → superior colliculus (eye/head orientation to stimulus).

• Spinomesencephalic tract → periaqueductal gray (pain modulation).

• Spinoparabrachial tract → parabrachial nucleus → amygdala (fear/emotional pain).

• Spinohypothalamic tract → hypothalamus (autonomic responses to pain).

8. Clinical Relevance

• Tract of Lissauer → lesion effect is 1–3 segments below lesion, contralateral side.

• Aδ lesions → loss of sharp, localized pain + cold sensation.

• C fiber lesions → loss of dull, aching, burning pain.

• Anterior spinothalamic lesion → loss of crude touch/pressure.

Summary Table

Pathway	Sensation	Fiber type	Lamina	Thalamic Nucleus	Cortex
Anterior Spinothalamic	Crude touch, pressure	Aβ, free endings	III–V	VPL/VPI	S1, S2
Neospinothalamic	Fast pain, cold	Aδ (myelinated)	I, V	VPL	S1, S2
Paleospinothalamic	Slow, dull pain, hot, chemicals	C (unmyelinated)	II, III	Intralaminar (CM, PF)	Widespread cortex (cingulate, insula, S1/S2)