The Sensory Pathways

Almost everything intricate in clinical neurology — the dissociated sensory loss of a syrinx, the crossed deficits of Brown-Séquard, the contralateral hemianesthesia of a thalamic stroke — flows from one structural fact: somatic sensation travels to the cortex by two separate ascending systems that cross the midline at two different places. Each system is a three-neuron relay. The dorsal column–medial lemniscus pathway carries the "discriminative" senses and crosses high, in the medulla. The spinothalamic (anterolateral) system carries pain and temperature and crosses low, at the cord segment. Internalize where each pathway decussates and the confusing zoo of sensory syndromes resolves into a handful of predictable patterns.

The Common Plan: Three Neurons to Cortex

Both great somatosensory systems share the same skeleton. A first-order neuron carries the signal from the periphery into the central nervous system; a second-order neuron projects across the midline and ascends; a third-order neuron in the thalamus relays to the primary somatosensory cortex. What differs — and what matters clinically — is where the second-order axon crosses.

• First-order neuron: a pseudounipolar cell whose body sits in the dorsal root ganglion (for the face, in the trigeminal ganglion). Its peripheral process is the sensory receptor; its central process enters the cord (or pons).
• Second-order neuron: the cell whose axon decussates and ascends. Its location is the single most important variable — for the dorsal columns it lives in the caudal medulla, for the spinothalamic system it lives in the spinal dorsal horn.
• Third-order neuron: in the thalamus (VPL for the body, VPM for the face), projecting through the posterior limb of the internal capsule to the postcentral gyrus (S1).

Dorsal Column–Medial Lemniscus: Fine Touch, Vibration, Proprioception

This is the high-fidelity, high-resolution channel — it carries fine discriminative touch, vibration sense, and conscious proprioception (joint position sense). Its defining feature is that the first-order axon ascends a remarkable distance uncrossed before its system finally synapses and decussates in the medulla.

• First-order neuron (dorsal root ganglion): the central process enters the cord and turns upward in the ipsilateral dorsal columns without synapsing. The columns are somatotopically organized: fasciculus gracilis (medial) carries fibers from the legs and lower trunk; fasciculus cuneatus (lateral) carries fibers from the arms and upper trunk and appears only above roughly T6. A useful aid: the medial gracilis serves the "lower" body, the lateral cuneatus the "upper" body, with new fibers added laterally as the cord ascends.
• Second-order neuron (caudal medulla): the ascending fibers synapse in the nucleus gracilis and nucleus cuneatus of the lower medulla. The second-order axons then sweep ventromedially as the internal arcuate fibers and cross the midline — the sensory (lemniscal) decussation — to form the contralateral medial lemniscus, which ascends through the brainstem.
• Third-order neuron (thalamus): the medial lemniscus terminates in the ventral posterolateral (VPL) nucleus of the thalamus, which projects to the postcentral gyrus.

Clinical consequence: because these fibers do not cross until the medulla, a lesion of the dorsal columns within the cord produces loss of vibration and proprioception on the same (ipsilateral) side as the lesion.

Spinothalamic / Anterolateral System: Pain, Temperature, Crude Touch

This is the protective, lower-resolution channel — pain, temperature, and crude (non-discriminative) touch. Its defining feature is the mirror image of the dorsal columns: it crosses almost immediately, right at the spinal segment of entry.

• First-order neuron (dorsal root ganglion): the central process enters the cord and synapses promptly in the dorsal horn (the substantia gelatinosa and adjacent laminae), often after ascending or descending only a segment or two in the tract of Lissauer.
• Second-order neuron (dorsal horn): the axon crosses the midline within one to two segments through the anterior (ventral) white commissure, then turns upward in the contralateral lateral spinothalamic tract of the anterolateral cord.
• Third-order neuron (thalamus): the spinothalamic tract terminates in the VPL nucleus (alongside the medial lemniscus, plus projections to intralaminar nuclei for the affective dimension of pain), relaying to S1.

Clinical consequence: because these fibers have already crossed in the cord, a spinothalamic lesion within the cord produces loss of pain and temperature on the opposite (contralateral) side, typically beginning a segment or two below the level of the lesion.

The Crux: Two Pathways, Two Crossing Points

Strip everything else away and one contrast does the diagnostic work: the dorsal columns cross HIGH (in the medulla); the spinothalamic tract crosses LOW (at the cord segment). Within the spinal cord, pain and temperature have already switched sides, while vibration and proprioception have not. A single cord lesion therefore strikes these two modalities on opposite sides of the body — the anatomical engine behind dissociated sensory loss.

• Brown-Séquard syndrome (cord hemisection): below the lesion there is ipsilateral loss of vibration and proprioception (dorsal columns, not yet crossed) together with ipsilateral weakness (corticospinal tract, also not yet crossed), and contralateral loss of pain and temperature (spinothalamic, already crossed).
• Syringomyelia (central cord cavity): an expanding central canal first interrupts the decussating spinothalamic fibers in the anterior white commissure, producing a "cape-like," suspended, bilateral loss of pain and temperature across the shoulders and arms, while vibration and proprioception in the dorsal columns are spared until late.

The Face: The Trigeminal System

Sensation from the face follows the same three-neuron logic but enters at the pons rather than the cord, with its first-order cell bodies in the trigeminal (Gasserian) ganglion. The two modality streams split at the brainstem:

• Fine touch from the face synapses in the main (principal/chief) sensory nucleus of the trigeminal nerve in the mid-pons.
• Pain and temperature from the face descend in the spinal trigeminal tract to synapse in the spinal trigeminal nucleus, which extends down into the upper cervical cord — the functional analogue of the dorsal horn for the face.
• Second-order axons cross and ascend as the trigeminothalamic tract to the ventral posteromedial (VPM) nucleus of the thalamus, then to the face area of S1.

Pearl: because the spinal trigeminal nucleus sits in the lateral medulla, a lateral medullary (Wallenberg) infarct classically produces the crossed pattern of ipsilateral facial pain/temperature loss (spinal trigeminal nucleus) with contralateral body pain/temperature loss (the already-crossed spinothalamic tract) — another vivid demonstration of two pathways crossing at two levels.

Cortical Processing and the Sensory Homunculus

Both systems converge on the primary somatosensory cortex (S1) in the postcentral gyrus, organized as the somatotopic sensory homunculus: the leg drapes over the medial surface (parasagittal, in the territory of the anterior cerebral artery), while the face and hand occupy the lateral convexity (middle cerebral artery territory). The body is represented with grossly distorted proportions, with disproportionately large areas devoted to the hand, lips, and tongue — the regions of densest receptor innervation and finest discrimination.

• Primary modalities (pinprick, temperature, vibration, joint position) depend on the intact pathway up to and including S1.
• Discriminative / "cortical" sensation — stereognosis, graphesthesia, two-point discrimination, and tactile localization — requires S1 plus the parietal association cortex, which integrates raw sensory data into meaningful percepts. These tests are only interpretable when the primary modalities are intact.
• Localizing principle: if primary sensation is normal but discriminative sensation fails (astereognosis, agraphesthesia, sensory extinction), the lesion is cortical/subcortical parietal, contralateral to the deficit.