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It contains centrally projecting axons carrying discriminative pain and temperature information (location, intensity and quality), which enter the spinal column ascend or descend one or two spinal segments in this tract before penetrating the grey matter of the dorsal horn where they synapse on second-order neurons. The axons of these second-order neurons cross the midline and ascend in the anterolateral quadrant of the contralateral half of the spinal cord, where they join the spinothalamic tract. The second-order neurons ultimately synapse on neurons in the ventral posterior lateral nucleus (VPL) of the thalamus.
It consists of fine fibers which do not receive their myelin sheaths until toward the close of fetal life.
In addition it contains great numbers of fine non-myelinated fibers derived mostly from the dorsal roots but partly endogenous in origin.
These fibers are intimately related to the substantia gelatinosa which is probably the terminal nucleus.
The non-myelinated fibers ascend or descend for short distances not exceeding one or two segments, but most of them enter the substantia gelatinosa at or near the level of their origin.