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DLMS and ALS Pathways of the Spinal Cord

Overview

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Pathway Diagrams

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1. DLMS — Dorsal Column–Medial Lemniscal System

Receptors and First-Order Neurons

• Subserve fine/discriminative touch, vibration, proprioception, weight perception, and two-point discrimination
• Receptors: Meissner's corpuscles, Merkel discs, Pacinian corpuscles, Ruffini endings, muscle spindles, Golgi tendon organs
• First-order neurons have cell bodies in the dorsal root ganglion (DRG); large-diameter, heavily myelinated Aα/β axons

Spinal Cord (First-Order Ascent — Ipsilateral)

• Axons enter the spinal cord via the medial bundle of the dorsal root and pass directly into the dorsal funiculus (posterior white column) without synapsing in the dorsal horn
• Somatotopic arrangement in the dorsal columns:
  • Fasciculus gracilis (Goll's column) — medial position; carries fibers from the lower body (sacral, lumbar, lower 6 thoracic levels)
  • Fasciculus cuneatus (Burdach's column) — lateral position; carries fibers from the upper body (upper 6 thoracic + cervical levels)
• Fibers ascend ipsilaterally all the way to the medulla

Brainstem — First Synapse (Second-Order Neurons)

• Fasciculus gracilis → Nucleus gracilis (lower medulla)
• Fasciculus cuneatus → Nucleus cuneatus (lower medulla)
• Second-order axons from these nuclei decussate (internal arcuate fibers / sensory decussation) in the lower medulla → form the medial lemniscus
• The medial lemniscus ascends through the medulla, pons, and midbrain (somatotopically: dorsal = neck, ventral = foot)
• Also joined by trigeminal lemniscus fibers (head/face sensation)

Thalamus (Third-Order Neurons)

• Terminates in the Ventral Posterior Lateral (VPL) nucleus of the thalamus (contralateral)
  • VPL = limbs and trunk
  • VPM = face (via trigeminal lemniscus)
  • VPS (ventral posterior superior) = proprioception → projects to area 3a

Cortex (Final Projection)

• VPL/VPM → Primary somatosensory cortex (S-I), postcentral gyrus (Brodmann areas 3a, 3b, 1, 2)
• Somatotopic organization: legs on top, head at foot of gyrus (sensory homunculus)
• Area 3b receives cutaneous touch; area 3a receives proprioceptive input
• S-I projects to sensory association cortex (parietal) and secondary somatosensory cortex (S-II, in the wall of the lateral sulcus)

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2. ALS — Anterolateral System

Receptors and First-Order Neurons

• Subserve pain, temperature, itch, and crude touch
• Free nerve endings in skin/viscera
• First-order neurons: cell bodies in DRG; small-diameter fibers:
  • Aδ (thinly myelinated) — sharp/fast pain, cool temperature → Rexed laminae I, IIo, V
  • C fibers (unmyelinated) — slow/burning pain, warmth, itch → Rexed lamina II (substantia gelatinosa)

Entry and Lissauer's Tract

• Fibers enter via the lateral bundle of the dorsal root
• Bifurcate into short ascending and descending branches in Lissauer's tract (dorsolateral fasciculus)
  • Aδ: ascend 3–4 segments before synapsing
  • C fibers: ascend ~1 segment before synapsing
• Synapse on dorsal horn neurons (second-order neurons)

Spinal Cord — Decussation (Immediate, at Cord Level)

• Second-order axons immediately cross the midline via the anterior white commissure (at the same spinal level or 1–2 segments above entry)
• Ascend in the contralateral ventrolateral funiculus as the spinothalamic tract
• Somatotopic arrangement in the anterolateral column: sacral fibers lie most superficially/laterally; cervical fibers lie most medially (clinically important — intrinsic cord lesions can spare sacral sensation)

Subdivisions of the ALS

Brainstem Ascent

• Fibers ascend through the medulla, pons, and midbrain without synapsing (lateral spinothalamic), running in the anterolateral quadrant
• They come to lie alongside but separate from the medial lemniscus in the upper brainstem
• Spinoreticular fibers synapse in the reticular formation of the brainstem

Thalamus (Third-Order Neurons)

• Lateral spinothalamic → VPL nucleus (discriminative pain)
• Spinoreticular → centrolateral (intralaminar) nuclei → frontal lobe, limbic system
• Also projects to the posterior complex of the thalamus

Cortex (Final Projection)

• VPL → Primary somatosensory cortex (S-I) — discriminative/localizing component of pain
• Intralaminar nuclei → Cingulate gyrus, insular cortex, frontal lobe, amygdala — motivational-affective component of pain
• This dual system explains why pain has both a sensory-discriminative dimension (where it is, how intense) and an emotional-affective dimension (suffering, unpleasantness)

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Key Differences Summary

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Clinical Correlation: Brown-Séquard Syndrome

• Ipsilateral to the lesion: Loss of fine touch, vibration, proprioception (DLMS fibers haven't crossed yet)
• Contralateral to the lesion: Loss of pain and temperature (ALS fibers already crossed in the cord)
• Ipsilateral: Motor loss (UMN signs) below the lesion

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• Ganong's Review of Medical Physiology, 26th Ed., pp. 177–179
• Neuroscience: Exploring the Brain, 5th Ed., pp. 1207–1210
• Localization in Clinical Neurology, 8th Ed., pp. 41–45, 244–245
• Kandel: Principles of Neural Science, 6th Ed., pp. 497–499