Spinothalamic tract (anterolateral system)

Also known as: anterolateral system, ALS pathway, lateral spinothalamic tract, anterior spinothalamic tract, pain pathway

The ascending pathway for pain, temperature, crude touch, pressure and itch. Its second-order fibres cross in the anterior white commissure within 1–2 segments of entry and ascend contralaterally to the thalamus.

Overview

First-order neurons are small DRG cells with thinly myelinated Aδ fibres (fast, sharp pain and cold) and unmyelinated C fibres (slow, burning pain, warmth and itch), arising from free nerve endings. On entering the cord their central branches run up or down 1–2 segments in the dorsolateral fasciculus (Lissauer tract). They then synapse in the dorsal horn, mainly in laminae I, II and V, using glutamate and substance P.

Second-order axons cross the midline in the anterior white commissure, ascending as they cross over about 1–2 segments. They form the contralateral lateral spinothalamic tract (pain and temperature) and anterior spinothalamic tract (crude touch and pressure), which together are the anterolateral system. In the brainstem the fibres run as the spinal lemniscus, lateral to the medial lemniscus, to the VPL nucleus of the thalamus.

Third-order neurons project to the primary somatosensory cortex for localisation of pain, and to the insula and anterior cingulate cortex for its affective component. Collateral pathways run in parallel: spinoreticular (arousal), spinomesencephalic (to the periaqueductal grey, driving descending pain inhibition) and spinohypothalamic.

Because crossing occurs near the level of entry, a cord lesion causes contralateral pain and temperature loss beginning 1–2 segments below the lesion.

Parts & subdivisions

Lateral spinothalamic tract

Carries pain and temperature in the lateral funiculus. It is laminated with sacral fibres lateral and cervical fibres medial, which explains sacral sparing in central cord lesions.

Anterior spinothalamic tract

Carries crude (non-discriminative) touch and pressure in the anterior funiculus. Crude touch is also carried by the dorsal columns, so it is rarely lost completely.

Spinoreticular and spinomesencephalic fibres

Collaterals to the reticular formation (arousal, autonomic responses) and to the periaqueductal grey and superior colliculus (pain modulation, orienting).

Anatomy

Course

Free nerve ending → DRG (1st neuron) → Lissauer tract → dorsal horn laminae I, II, V (2nd neuron) → crosses in the anterior white commissure → contralateral anterolateral funiculus → brainstem (spinal lemniscus) → thalamic VPL (3rd neuron) → somatosensory cortex, insula and cingulate.

Function

Conveys pain (nociception): fast pain via Aδ fibres, slow pain via C fibres.

Conveys temperature sensation (warm and cold).

Conveys crude touch, pressure, itch and tickle.

Drives arousal and the emotional and autonomic responses to pain via its collaterals.

Physiology

Pain transmission is modulated in the dorsal horn. By gate control, Aβ input activates inhibitory interneurons in the substantia gelatinosa. Descending inhibition runs from the periaqueductal grey to the nucleus raphe magnus (serotonin) and locus coeruleus (noradrenaline) and on to dorsal horn enkephalinergic interneurons. Opioids act presynaptically, reducing Ca²⁺ entry, and postsynaptically, opening K⁺ channels. Central sensitisation (wind-up via NMDA receptors) underlies hyperalgesia and allodynia.

Clinical significance

Syringomyelia

A fluid-filled cavity (syrinx) expanding around the central canal, often associated with Chiari I malformation. It first destroys the decussating fibres in the anterior white commissure, causing bilateral, cape-like loss of pain and temperature with preserved touch and proprioception (dissociated sensory loss). Painless burns of the hands are typical, and later the anterior horns are involved (hand wasting).

Brown-Séquard and cordotomy

Hemisection causes contralateral loss of pain and temperature starting 1–2 segments below the lesion. Anterolateral cordotomy deliberately cuts this tract to relieve intractable cancer pain on the opposite side.

Lateral medullary (Wallenberg) syndrome

A PICA or vertebral artery infarct involving the spinothalamic tract (contralateral body) and the spinal trigeminal nucleus and tract (ipsilateral face). The result is a crossed pain and temperature loss.