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The spinothalamic tract, well known for its role in nociception, is the most frequently studied ascending pathway originating from the spinal cord. It is known that spinothalamic neurons are located in all segments of the spinal cord, but in most mammals the total number of spinothalamic neurons is not known. In three cats, after large wheat germ agglutinin-conjugated horseradish peroxidase injections involving all parts (one case) or almost all parts of the thalamus (two cases), the number of retrogradely labeled profiles was counted in a 1:4 series of sections of all spinal segments from C I to Coc2. After applying the correction factor of Abercrombie (Anat. Rec. 94 (1946) 239), it appears that a total of 12,000 cells in the spinal cord project to the thalamus. (C) 2004 Elsevier Ireland Ltd. All rights reserved.

We have reported that the lateral region of the caudal third of the midbrain periaqueductal grey (PAG), which mediates flight and hypertension, receives inputs from lamina I and IIo and the lateral cervical nucleus (LCN) of the upper cervical spinal cord (UCC); whereas the ventrolateral PAG region, which mediates hypotension, quiescence and immobility, is targeted by cells in laminae VII, VIII and X. In the UCC the cells of laminae VII and VIII receive a significant afferent input from the deep neck muscles, whereas cells of laminae I and IIo and the LCN receive a large input from cutaneous nociceptors. Thus we investigated the hypothesis that nociceptive activation of the deep neck muscles would activate the spinal-ventrolateral PAG projection, whereas cutaneous nociceptive stimulation would activate the spinal-lateral PAG projection, by examining the expression of Fos protein. We found that deep noxious stimulation led to Fos-positive cells predominantly in the ventrolateral PAG and superficial noxious stimulation led to Fos-positive cells predominantly in the lateral PAG. The results: (i) indicate that the UCC afferent regulation of the PAG arises from topographically separable and functionally dissociable populations of neurons and (ii) raise the possibility that the ventrolateral and lateral PAG play important but different roles in mediating the distinctive affective, emotional and autonomic responses evoked by pain arising from deep or superficial structures.

The periaqueductal gray (FAG) is involved in motor activities, such as movements of the neck, back and hind limbs, cardiovascular regulation, micturition, vocalization, and mating behavior, as well as in nociception control. To accomplish these functions the FAG uses information from other parts of the limbic system, from the lower brainstem, and from the spinal cord. To study the ascending projections from the spinal cord to the PAG, tracer was injected in different parts of the FAG, and the number of retrogradely labeled neurons were counted for each spinal segment. Results show that large segmental differences exist in the number of FAG projecting neurons throughout the length of the spinal cord and that different parts of the spinal cord project to specific areas in the FAG. (C) 1997 Elsevier Science Ireland Ltd.