Somatosensation in the Central Nervous System

Core Concepts

Entry: dorsal roots and the dorsal horn

Somatosensory neurons have their cell bodies in the dorsal root ganglia (DRG) just outside the spinal cord. Their peripheral processes innervate the skin; their central processes enter the spinal cord through the dorsal roots. Inside the cord, different fiber classes take different routes: large-diameter fibers carrying touch/proprioception ascend in the dorsal columns; small-diameter pain/temperature fibers synapse in the dorsal horn (a region with layered organization — laminae — including the substantia gelatinosa). The face is served by the trigeminal nerve, whose cell bodies sit in the trigeminal ganglion.

Pathway 1: Dorsal column–medial lemniscus (touch, vibration, proprioception)

1. First-order fibers enter the cord and ascend ipsilaterally (same side) in the dorsal columns: the gracile fasciculus carries information from the lower body and the cuneate fasciculus from the upper body (commonly taught division around the midthoracic level).
2. They synapse in the medulla — the gracile and cuneate nuclei.
3. Second-order neurons cross the midline (decussate) in the medulla as internal arcuate fibers and ascend as the medial lemniscus.
4. They synapse in the ventral posterior lateral (VPL) nucleus of the thalamus.
5. Third-order neurons project to the primary somatosensory cortex.

Key fact: the DCML pathway crosses in the medulla, so a lesion above that point affects the contralateral (opposite) side of the body.

Pathway 2: Spinothalamic (pain, temperature, crude touch)

1. First-order fibers synapse in the dorsal horn of the spinal cord at the level of entry.
2. Second-order neurons cross the midline within the spinal cord (via the anterior white commissure, usually within a segment or two of entry) and ascend contralaterally in the anterolateral column — hence the name anterolateral pathway.
3. They synapse in the VPL of the thalamus (pain and temperature also project to other thalamic nuclei and brainstem structures).
4. Third-order neurons project to cortex (S1 and other regions, including insular and cingulate areas involved in the emotional aspects of pain).

Key fact: the spinothalamic pathway crosses in the spinal cord, so a spinal lesion affects pain/temperature on the opposite side below the lesion.

The face: the trigeminal pathway

Facial somatosensation enters through the trigeminal nerve (CN V) to the trigeminal ganglion. Touch information goes to the principal sensory nucleus of the trigeminal complex; pain and temperature go to the spinal trigeminal nucleus. Second-order fibers decussate and ascend to the ventral posterior medial (VPM) nucleus of the thalamus, then to the face region of S1. The rule is the same as for the body: each pathway crosses, so the cortex on one side represents the face on the other.

Why crossover location matters: the clinical signature

The different crossover points create a double dissociation that is the classic way to test pathway knowledge:

• A lesion in the medulla or above (e.g., a medullary stroke) → contralateral loss of both fine touch/vibration and pain/temperature, since both pathways have crossed by then.
• A hemisection of the spinal cord (Brown-Séquard syndrome, commonly taught) → ipsilateral loss of fine touch/vibration/proprioception below the lesion (DCML had not yet crossed) plus contralateral loss of pain/temperature below the lesion (spinothalamic crossed at entry) — touch and pain deficits on opposite sides of the body.

This pattern is a textbook illustration of the two pathways and a frequent exam scenario.

Thalamus and cortex: VPL/VPM to S1

The VPL receives the body and the VPM receives the face; both project to S1, which spans Brodmann areas 3a, 3b, 1, and 2 and contains multiple, parallel body maps. Commonly taught division of labor: 3b is the principal cutaneous map (the "core" of touch processing), 3a receives proprioceptive input, 1 processes texture, and 2 processes shape and size. S1 is organized somatotopically: adjacent body regions map to adjacent cortex, producing the famous Penfield homunculus — a distorted figure in which the hand, lips, and tongue occupy huge territories because they are densely innervated and behaviorally important. Within a cortical column, neurons tend to share receptive fields and modality, a columnar organization seen throughout sensory cortex.

Beyond S1: S2 and the posterior parietal cortex

Secondary somatosensory cortex (S2), in the parietal operculum, receives input from S1 and has bilateral receptive fields — it integrates touch from both sides and is involved in tactile object recognition and attention to touch. The posterior parietal cortex integrates somatosensation with vision and spatial information; damage here can produce astereognosis (inability to recognize objects by touch despite intact basic sensation) and neglect (failure to attend to one side of space). These higher areas show that "touch" is not finished at S1 — recognition and spatial context are built by association cortex.

Plasticity: maps that move

Somatosensory maps are not fixed. With practice or altered input, cortical representations reorganize: experienced Braille readers show expanded finger representations, and after limb amputation the deafferented cortical territory can be "invaded" by neighboring body regions — a mechanism thought to underlie phantom limb sensations. Lateral inhibition (a neuron's activity suppressing its neighbors) sharpens the borders of tactile stimuli at every relay, improving two-point discrimination.

Common Confusions