Spinal location of ascending lamina I axone in the macaque monkey

Ascending axons of lamina I neurons were identified at upper cervical and thoracic levels in 10 macaque monkeys after confined anterograde tracer injections in the superficial cervical or lumbar dorsal horn. Lamina I axons in the contralateral spinal white matter were of particular interest, because these include spinothalamic tract (STT) axons. These were clearly concentrated in the middle of the lateral funiculus, that is, in the location of the classically described lateral STT of humans. At the C1-2 level, they were more ventrally located. There was evidence of a general topography, in that lumbar axons were located superficially and cervical axons were located medially. There was some dispersion, in that a few axons were also found move dorsally and more ventrally (especially at the midthoracic level), and there was some variability between cases. In addition, ascending axons were more widely distributed in the ipsilateral white matter. These observations are consistent with prior anterograde findings in the cat. Together with clinical and other evidence, these observations indicate that lamina I neurons have an integral role in the effects of spinal lesions on pain, temperature, and itch sensations.