Contralateral, midline, and commissural motoneurons of neck muscles: a retrograde HRP study in the cat.

Retrograde transport of HRP has been used to examine the distribution of motoneurons supplying the dorsal neck muscles biventer cervicis, and complexus in the cat. Retrogradely filled cells were present in the expected sites in lamina IX of the ventral horn, but were not confined to that area. Retrogradely labelled cells were also found more dorsally in the ventral horn and in the white matter medial to the ventral horn. Other filled cells were found in the commissural nuclei and in laminae VII and VIII contralateral to the nerve being dipped. Cells in the ipsilateral lamina IX ranged in size from 15 to 70 microns. Cells lying outside this region were smaller with the majority having mean equivalent diameters, of 40 microns or less.