lateral geniculate nucleus

**Location and function:** The LGN is part of the posterior thalamus (metathalamus), along with the medial geniculate nucleus (MGN, auditory relay). It is the principal relay for conscious visual perception. Remember: "Lateral = Light, Medial = Music."

**Structure:** 6 layers, numbered 1–6 from ventral to dorsal. - Layers 1 and 2: Magnocellular (M cells). Large cells that detect motion, depth, and gross form. They are color-insensitive, with high temporal and low spatial resolution. They receive input from parasol (M-type) ganglion cells. - Layers 3–6: Parvocellular (P cells). Small cells that detect color, fine detail, and high spatial resolution. They receive input from midget (P-type) ganglion cells. - Koniocellular cells lie between the layers and carry blue-yellow color information. - Mnemonic: "Magno = Movement (bottom, layers 1–2); Parvo = Precision/color (layers 3–6)." - Laminar input (mnemonic "Crossed Rule 1-6: 1, 4, 6 = Contralateral; 2, 3, 5 = Ipsilateral"): Layers 1, 4, 6 receive input from the contralateral eye (nasal retina fibers, which cross at the chiasm). Layers 2, 3, 5 receive input from the ipsilateral eye (temporal retina fibers, which do not cross). Each layer receives input from only one eye.

**Visual pathway:** Retina → optic nerve → optic chiasm (nasal fibers cross) → optic tract → LGN (thalamus) → optic radiations (geniculocalcarine tract) → V1 (calcarine sulcus, striate cortex). - Meyer's loop: Inferior retinal fibers (superior visual field) pass through the temporal lobe to the lingual gyrus (below the calcarine sulcus). - Superior retinal fibers (inferior visual field) pass through the parietal lobe to the cuneus (above the calcarine sulcus). - Mnemonic: "PITS" = Parietal-Inferior visual field; Temporal-Superior visual field.

**Blood supply (high-yield):** Dual supply. - Anterior choroidal artery (branch of the ICA): anterolateral LGN, which serves the upper and lower visual field sectors. Occlusion causes a superior and inferior quadrantanopia (homonymous horizontal sectoranopia) with sparing of the horizontal meridian. - Lateral posterior choroidal artery (branch of the PCA): the hilum and central part of the LGN. Occlusion causes a homonymous horizontal wedge-shaped sectoranopia (the opposite pattern, with "sparing" of the upper and lower sectors). Together these produce the classic "quadruple sectoranopia" pattern. - The anterior choroidal artery also supplies the optic tract, the posterior limb of the internal capsule, and the globus pallidus.

**Clinical significance:** - Lesions of the LGN or optic tract cause a contralateral homonymous hemianopia, which is the same defect as lesions of the optic radiations. LGN lesions are rare and are usually vascular, as above. Optic tract lesions also produce an incongruous defect. - Congruity: The more posterior the lesion (toward the occipital cortex), the more congruous the visual field defects. Optic tract lesions are incongruous, and occipital lesions are highly congruous. - Macular sparing is classically a feature of PCA occlusion affecting the occipital cortex, not LGN lesions. This is because of dual supply from the MCA. - Pupillary light reflex: The afferent pupillary pathway bypasses the LGN. Fibers leave the optic tract before the LGN and synapse in the pretectal nucleus, so LGN or cortical lesions spare the light reflex. - Cortical feedback: Only about 10–20% of the synapses on LGN relay neurons come from the retina. Most come from V1 corticothalamic feedback, the brainstem reticular formation, and the thalamic reticular nucleus, which modulate attention and arousal. - Amblyopia: Monocular deprivation during the critical period causes shrinkage of the LGN layers that receive input from the deprived eye (Hubel and Wiesel). - Glaucoma and other optic neuropathies can cause transsynaptic degeneration and atrophy of LGN neurons.

**Neurotransmitter:** The retinogeniculate synapse is glutamatergic. LGN relay neurons release glutamate onto V1 layer 4C. The thalamic reticular nucleus provides GABAergic inhibition.

**Related thalamic nuclei:** VPL (body sensation), VPM (face sensation and taste), MGN (hearing), VL (motor, from the cerebellum and basal ganglia), VA (motor, from the basal ganglia), and the pulvinar (visual association).