Anatomical and physiological studies of the gray matter surrounding the spinal cord central canal.

Recent histochemical evidence suggests that neurons in the gray matter surrounding the central canal may play a role in nociception. We attempted to evaluate this possibility by studying the response properties and ascending projections of these cells in the rat. In the first series of experiments, the ascending projections of neurons around the central canal were studied by the method of retrograde transport of horseradish peroxidase (HRP). Predominantly contralateral labeling of neurons around the central canal appeared after HRP injections into the paramedian medullary or pontine reticular formation in intact or cerebellectomized animals. Far fewer cells were labeled by injections into the lateral medulla and/or pons. A small number of cells was retrogradely labeled by HRP injections into the vermal and intermediate regions of the cerebellum or the periaqueductal gray matter. Injections into other brainstem areas outside of the reticular formation also failed to label large numbers of neurons around the centra canal. In a second set of experiments, we recorded extracellular unitary activity from the lumbar enlargement in spinalized, decerebrate, unanesthetized rats. Thirteen units were functionally characterized and histologically localized within 300 micrometers of the central canal. All of the units identified responded exclusively to noxious stimuli applied within highly circumscribed receptive fields. Thus, neurons around the central canal contribute strongly to long ascending spinal cord projections. Physiologically, neurons found within this region are reminiscent of the noxious-specific cells in the outer most layers of the dorsal horn.