An anterolaternal corodotomy relieving pain in right leg is effective because it interrupts the

C i.e. Left lateral spinothalmic tract The anterior/ ventral - spinothalmic tract carries crude touch and pressueQ from opposite half of body; whereas lateralspinothalmic tract carries pain and temperature from the opposite half of body. Therefore any lesion in lateral spinothalmic tract will result in loss of pain and temperature sensation contralaterally below the level of lesion; and lesion of anterior spinothalmic tract will 1/ t loss of crude touch & pressure sensation contralaterally.