Spinothalamic projections in the pigeon.

The spinothalamic projection in an avian species, the pigeon, was studied both with anterograde and retrograde means. Anterograde transport of wheatgerm agglutinin conjugated horseradish peroxidase (WGA-HRP) was used in order to determine the termination of the spinothalamic tract in the thalamus. Application to the lumbar enlargement of the spinal cord resulted in a dense terminal field in a thalamic nucleus now known as n. dorsointermedius ventralis anterior (DIVA). Less dense labeling was found in the thalamic nuclei n. intercalatus thalami (ICT), n. subrotundus (SRt) and possibly stratum cellulare externum and internum (SCE/SCI). After application of WGA-HRP to the cervical enlargement there was no labeling in the above-mentioned nuclei and only one distinctly labeled terminal in n. dorsolateralis posterior (DLP). Under electrophysiological control the fluorescent tracer Fast blue was applied to the DIVA. A considerable number of retrogradely labeled neurons was found in the lumbar enlargement only (contralateral intermediate grey). These results show that there is a substantial direct spinothalamic projection from the hindlimbs (legs) but not from the forelimbs (wings) in pigeons.