Cells of origin of ascending and descending as well as branching fibers in the cervical spinal cord of the pigeon.

Ascending and descending projections of spinal neurons (cervical enlargement) were studied with the retrograde transport of horseradish peroxidase (HRP) and with the fluorescent tracers Fast blue and rhodamine isothiocyanate (single and double labeling). Ascending and descending projections arise from the same laminae of the spinal grey except for neurons in contralateral lamina I and avian cervical Clarke's column which have ascending fibers only. A significant number of cervical nucleus proprius neurons (lamina IV) descends to the lumbar enlargement. Neurons with branching fibers were rare (less than 10 per cent).