Neurons in the lateral sacral cord of the cat project to periaqueductal grey, but not to thalamus.

Previous work of our laboratory has shown that neurons in the lateral sacral cord in cat project heavily to the periaqueductal grey (PAG), in all likelihood conveying information from bladder and genital organs. In humans this information usually does not reach consciousness, which raises the question of whether the lateral sacral cell group projects to the thalamus. After wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) injections into the sacral cord, anterogradely labelled fibers were found in the thalamus, specifically in the ventral anterior and ventral lateral nuclei, the medial and intralaminar nuclei, the lateral ventrobasal complex/ventroposterior lateral nucleus, and the nucleus centre median, lateral to the fasciculus retroflexus. Much denser projections were found to the central parts of the PAG, mainly to its dorsolateral and ventrolateral parts at caudal levels and lateral parts at intermediate levels. In a subsequent retrograde tracing study, injections were made in those parts of the thalamus that received sacral fibers, as found in the anterograde study. Labelled neurons were observed in the sacral cord, but not in the lateral sacral cell group. In contrast, a small control injection in the caudal PAG resulted in many labelled neurons in the lateral sacral cord. These results suggest that afferent information regarding micturition and sexual behaviour is relayed to the PAG, rather than to the thalamus.

C1-C3 spinal cord projections to periaqueductal gray and thalamus: a quantitative retrograde tracing study in cat.

By far, the strongest spinal cord projections to periaqueductal gray (PAG) and thalamus originate from the upper three cervical segments, but their precise organization and function are not known. In the present study in cat, tracer injections in PAG or in thalamus resulted in more than 2400 labeled cells, mainly contralaterally, in the first three cervical segments (C1-C3), in a 1:4 series of sections, excluding cells in the dorsal column and lateral cervical nuclei. These cells represent about 30% of all neurons in the entire spinal cord projecting to PAG and about 45% of all spinothalamic neurons. About half of the C1-C3 PAG and C1-C3 thalamic neurons were clustered laterally in the ventral horn (C(1-3vl)), bilaterally, with a slight ipsilateral preponderance. The highest numbers of C(1-3vl)-PAG and C(1-3vl)-thalamic cells were found in C1, with the greatest density rostrocaudally in the middle part of C1. A concept is put forward that C(1-3vl) cells relay information from all levels of the cord to PAG and/or thalamus, although the processing of specific information from upper neck muscles and tendons or facet joints might also play a role.