Ascending projections to the first two myelomeres

Afferent fibres to the first 2 myelomeres arising from more caudal spinal segments have been studied in rats by means of retrograde transport of wheat germ agglutinin-horse radish peroxidase (WGA-HRP). In 10 Wistar rats 30 nl of a 1% WGA-HRP solution were injected into the right dorsal horn of the first or second cervical segments. The results were similar for both levels. Labelled neurons were observed in all spinal myelomeres but their density was higher in cervical than in thoracic regions. The laminar distribution was, in decreasing order: V, VII, VIII, IV and X, and the location of labelled neurons in these laminae was predominantly medial. Both sides of the spinal cord contained labelled neurons, but these were more numerous on the ipsilateral side. The dorsal laminae did not show labelled neurons. The number of labelled cells was larger when the WGA-HRP deposit reached laminae V, VI and VII. These results suggest that nerve impulses arising in more caudal dermatomes may modulate the excitability of centrally transmitting neurons in the first 2 cervical myelomeres.