ventral posterolateral nucleus

**Location and anatomy:** The VPL is part of the ventral tier of the lateral thalamic nuclei, lateral to the VPM (ventral posteromedial) nucleus and posterior to the VL (ventral lateral) nucleus. It is a specific relay nucleus, so it has well-defined input and output.

**Inputs (afferents):** - Medial lemniscus (dorsal column–medial lemniscus pathway): fine (discriminative) touch, vibration, proprioception. First-order neurons ascend in the fasciculus gracilis (lower body) and fasciculus cuneatus (upper body) and synapse in the nucleus gracilis and cuneatus. Second-order neurons decussate in the medulla as internal arcuate fibers, then ascend as the medial lemniscus to the VPL. - Spinothalamic tract (anterolateral system): pain, temperature, crude touch. Second-order neurons decussate in the anterior white commissure at the spinal level, then ascend to the VPL.

**Outputs (efferents):** Third-order neurons travel through the posterior limb of the internal capsule via thalamocortical fibers to the primary somatosensory cortex (S1; Brodmann areas 3, 1, 2) in the postcentral gyrus.

**Somatotopy:** The VPL is organized somatotopically. Legs are represented laterally and arms medially. This matches the sensory homunculus in S1, where the leg is medial and the face is lateral.

**VPL vs VPM:** - VPL: body (neck down) via the medial lemniscus and spinothalamic tract. - VPM: face via the trigeminal lemniscus (trigeminothalamic tract), plus taste via the solitary nucleus (gustatory pathway).

**Clinical significance:** - Thalamic stroke (Dejerine-Roussy syndrome, thalamic pain syndrome): usually from occlusion of the thalamogeniculate branches of the posterior cerebral artery (PCA), which supply the VPL/VPM. It causes contralateral hemisensory loss of all modalities, which is often followed weeks to months later by chronic, severe contralateral burning pain (central post-stroke pain), allodynia, and hyperpathia. - Lacunar infarct (pure sensory stroke): a small-vessel lesion in the VPL or posterior limb of the internal capsule causes isolated contralateral hemisensory loss without motor deficit. - Lesions above the decussation (the VPL or beyond) cause contralateral deficits for both sensory pathways. This is unlike spinal cord lesions, such as Brown-Séquard, where the pattern differs by pathway. - Memory aid: the PCA supplies the thalamus, and "VP" stands for "Ventral Posterior" (sensation). The VPL carries the Lower body and the VPM carries the Mouth/face.

**Related thalamic nuclei (high-yield relay summary):** - VPL: body sensation - VPM: face sensation and taste - LGN: vision - MGN: hearing - VL: motor (from the cerebellum and basal ganglia to the motor cortex) - VA: motor (from the basal ganglia) - Anterior nucleus: limbic system (mammillary bodies to the cingulate gyrus) - Pulvinar: higher-order visual and association processing

**Blood supply:** The thalamogeniculate arteries, which arise from the P2 segment of the PCA, supply the VPL and VPM. Remember "PCA = thalamus."