Cells of origin of spinothalamic, spinotectal, spinoreticular and spinocerebellar pathways in the pigeon as studied by the retrograde transport of horseradish peroxidase.

The cells of origin of spinal neurons projecting to the thalamus, the midbrain, the reticular formation, and the cerebellum in the pigeon were studied with the method of retrograde transport of horseradish peroxidase (HRP). Only few spinal cells project up to the thalamus and to the tectum and their location is at the base of the dorsal horn (lamina V) and in the intermediate or ventral spinal grey matter (most contralateral). However, many cells in the dorsal column nuclei (including external cuneate nucleus) project up to these brain areas. Many spinal neurons project to the caudal brainstem and reticular formation. With medioventral injections of HRP labeled cells were found in lateral lamina I (bilateral) and laminae V-VIII (most contralateral) with a concentration in lateral lamina V/VI and lamina VIII. With dorsolateral brainstem injections there was a predominance of lamina I neurons, located bilaterally at the dorsolateral corner of the dorsal horn near Lissauer's tract. These results show that spinal cells, which in mammalian species project up to the thalamus, predominantly end in the caudal brainstem. Injections into the cerebellum disclosed that not only cells of Clarke's column but also sofar not known "spinal border cells", located dorsal to and in part intermingled with the motoneurons, are cells of origin of spinocerebellar tracts and that both groups of cells occur at the cervical and at the lumbar enlargement.