medial geniculate nucleus

**Location/anatomy:** The MGN is part of the metathalamus (along with the lateral geniculate nucleus), on the posterior-inferior aspect of the thalamus, lateral to the superior colliculus/midbrain.

**Auditory pathway (high-yield):** Cochlea (hair cells) -> spiral ganglion -> cochlear nerve (CN VIII) -> cochlear nuclei (dorsal and ventral; medulla/pontomedullary junction) -> superior olivary complex (pons; first site of bilateral input, sound localization) -> lateral lemniscus -> inferior colliculus (midbrain) -> brachium of the inferior colliculus -> MGN of the thalamus -> acoustic (auditory) radiation through the sublenticular part of the internal capsule -> primary auditory cortex (transverse temporal gyri of Heschl, BA 41/42, in the superior temporal gyrus). Mnemonic: "E COLI" - Eighth nerve, Cochlear nuclei, Olivary nucleus (superior), Lateral lemniscus, Inferior colliculus, MGN.

**Organization:** Tonotopic organization is maintained from the cochlea through to the cortex. The ventral division is the main lemniscal (core) relay to A1. The dorsal and medial divisions are non-lemniscal and have more diffuse, multisensory connections.

**Bilateral representation:** Because of the extensive bilateral crossing at the level of the superior olive and above, a unilateral lesion central to the cochlear nuclei (for example, of the MGN or the auditory cortex) usually causes only mild bilateral hearing loss, with difficulty localizing sound. It does not cause unilateral deafness. Unilateral deafness suggests a lesion of the cochlea, CN VIII, or the cochlear nuclei (peripheral to the superior olive).

**Comparison with the LGN:** The LGN is the visual relay (retina -> optic tract -> LGN -> optic radiations -> V1). Both are thalamic relay nuclei. Remember: Medial = Music (hearing), Lateral = Light (vision). Both receive input from midbrain structures (inferior and superior colliculus, respectively). Other thalamic relay nuclei: VPL (body sensation via the spinothalamic tract and dorsal columns/medial lemniscus), VPM (face sensation and taste), VL (motor, from the cerebellum/basal ganglia), VA (motor, from the basal ganglia), and the anterior nucleus (limbic, Papez circuit).

**Blood supply:** The MGN is supplied by branches of the posterior cerebral artery (thalamogeniculate branches) and the posterior choroidal arteries.

**Clinical correlations:** - Bilateral lesions of the auditory pathway above the cochlear nuclei (including the MGN or auditory cortex) can cause central hearing loss or cortical deafness. Auditory agnosia (inability to recognize sounds despite intact hearing) and word deafness are also described. - Stroke in the posterior circulation (PCA territory/thalamic) can affect the MGN. - Central auditory processing disorders and tinnitus have been linked to altered MGN activity. - The MGN also receives descending corticothalamic feedback and participates in attention and sound-filtering circuits.

**Boards pearls:** (1) Know the order of the auditory pathway relays. (2) Know that a lesion above the superior olive produces bilateral, mild deficits. (3) Do not confuse the MGN (hearing) with the LGN (vision). (4) The inferior colliculus is the midbrain relay for hearing, and the superior colliculus mediates the visual/saccade (tectal) reflexes.