Brown-Sequard Question

[seminoma]

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Specifically regarding the spinothalamic tract lesion. Why does the contralateral impairment begin 1-2 segments below the lesion? I understand that the first nerve axon ascends 1-2 segments before synapsing with the second neuron, but if the lesion is in the spinothalamic tract (i.e. past the point of decussation) aren't all the axons running together already? I understand why it would be 1-2 segments below if the lesion occured before the decussation (though it would be ipsilateral in that case), but not why it's 1-2 segments below if the lesion is beyond the point of decussation.

 

[Inderjeet]

If u look in this picture, u'll see that the1st order neurons from dorsal root ganglion enter the spinal after ascending 1-2 segments, synapse with 2nd order neurons in posterior (dorsal) horn and decussate in the Ventral white commissure and ascend as lateral spinothalamic tract. Now in hemisection, lateral spinothalamic tract lesion will result in contralateral loss of pain n temperature 1-2 segments below lesion, and posterior horn lesion will result in anesthesia at the side of lesion.