A qualitative and quantitative analysis of the distributions of cells in the spinal cord and spinomedullary junction projecting to the thalamus of the rat.

Horseradish peroxidase retrograde transport has been used to locate and make a quantitative study of neurons of the rat spinal cord which project to the thalamus. Across the grey, labelled cells were found in the head and neck of the dorsal horn (in three locations, but not the substantia gelatinosa); in the region of the internal basilar nucleus of Ramón y Cajal; in the intermediate grey zone; in the lateral cervical nucleus and funiculus; and in the ventral horn. The latter contains the largest population of neurons projecting ipsilaterally. The most significant finding concerns the craniocaudal distribution of the cells. More than 50% of the thalamically projecting neurons are confined to the upper four cervical segments, where every population is represented. Some populations are continuous with thalamically projecting populations in the lower medulla. The cervical enlargement contains less than 5% of spinothalamic cells. The lumbar enlargement contains 33%, most of which are in the region of the internal basilar nucleus. This population is also marked in the upper cervical segments. Thus, in the rat, the origin of the spinothalamic tract is distributed along the cord in a very uneven manner. This may indicate that sensory information delivered to any one segment of the spinal cord by a primary afferent is not always relayed direct to the thalamus by a local second order neuron; that different parts of the body are not represented to the same extent in the spinothalamic system; or that these two factors combine to produce the observed distribution.