[Verse 1] One million axons, bundled fine and tight, Thirty to forty percent, born in M1's light, The rest come whispering from premotor halls, SMA, cingulate, parietal calls, A parliament of regions voting on each move, Casting their ballots before a muscle moves. [Chorus] Corticospinal, descends tout bas, From the cortex to the muscle, regarde ça, Un million d'axones en voyage, mon ami, Corona, capsule, pyramide — c'est ici, Descends, descends, la route est longue, Jusqu'à la moelle où la force appartient. [Verse 2] Corona radiata fans out like a crown, Funnels through the posterior limb, squeezing down, Through the cerebral peduncles, sleek and narrow, Into the medulla, the pyramids like an arrow, Each tract a traveler following the only road, Carrying the cortex's heavy, urgent load. [Chorus] Corticospinal, descends tout bas, From the cortex to the muscle, regarde ça, Un million d'axones en voyage, mon ami, Corona, capsule, pyramide — c'est ici, Descends, descends, la route est longue, Jusqu'à la moelle où la force appartient. [Verse 3] At the pyramidal crossing, eighty-five to ninety bow, Swapping sides forever, don't ask them how, Lateral tract is born where the fibers decussate — A changing of the guard at the medulla's gate, The small percent that's left stays loyal, stays the same, A vestige, ventral, uncrossed, still answering its name. [Bridge] But hands need something extra, something primates alone own, Corticomotoneuronal cells synapse straight onto the motor neuron's throne, Skipping interneurons, direct as a telephone call, Found only in primates, strongest of all for the fingers' sprawl, Precision on the phalanges, those fine bones in your hand, C'est la dextérité — maintenant tu comprends. [Verse 4] Sever this tract and the body tells a tale, Weakness creeps in, muscles turn frail, Spasticity tightens, reflexes jump too fast, Hyperreflexia ringing an alarm that won't go past, Stroke the sole and watch the big toe twist, The Babinski answers — a sign you can't dismiss. [Verse 5] Lateral tract rules the limbs, the fingers, the fine art, Ventral tract stays close to home, controlling trunk and heart, Axial muscles, posture's steady frame, Bilateral innervation keeps the core the same, Somatotopy maps the homunculus, precise and true, Legs medial, arms lateral — the body's point of view. [Chorus] Corticospinal, descends tout bas, From the cortex to the muscle, regarde ça, Un million d'axones en voyage, mon ami, Corona, capsule, pyramide — c'est ici, Descends, descends, la route est longue, Jusqu'à la moelle où la force appartient. [Outro] Un million d'axones, du cortex à la main, Corticospinal — tu comprends maintenant, c'est certain, Descends, descends, mon ami neurone, La route du mouvement, c'est notre chanson.
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