Neurons in lamina I of cat lumbar spinal cord were identified as having projections to the brain by antidromic activation at the rostral cervical spinal cord. Many of these neurons were also found to project to the midbrain (nucleus cuneiformis and lateral periaqueductal gray; n=89), thalamus (both medial and lateral nuclei; n=8), or both the midbrain and thalamus (n=8). The majority of lamina I projection neurons (85% overall and 92% of identified spinomesencephalic and/or spinothalamic neurons) responded exclusively to noxious stimulation of their peripheral receptive fields. Central conduction velocities of lamina I neurons was slow; 0.9 to 26 m/s. Intracellular staining revealed that lamina I projection neurons were of diverse morphological types, but were generally confined to lamina I and the overlying white matter and were oriented in the rostrocaudal direction. Morphological characteristics did not appear to correspond to physiological categories. Immunocytochemical staining of serotonin fibers and varicosities showed that lamina I projection neurons received either a high or low density of serotonergic contacts on dendritic shafts. The degree of innervation did not correspond to response category or identified terminations. Retrograde labeling of spinomesencephalic neurons showed high concentrations of cells in laminae I and V. Bilateral lesions of the dorsolateral funiculi at low thoracic levels resulted in the absence of labeled lamina I cells below the lesion. Local injection of colchicine to block transport of the retrograde marker allowed visualization of fibers, bilaterally in the dorsolateral funiculi and in the ipsilateral anterolateral quadrant.
