Spinal efferents and afferents of the periaqueductal gray Mouton, Leonora

The periaqueductal gray (PAG) plays an important role in analgesia as well as in motor activities, such as vocalization, cardiovascular changes and movements of the neck, back and hind limbs. Although the anatomical pathways for vocalization and cardiovascular control are rather well understood, this is not the case for the pathways controlling the neck, back and hind limb movements. This led us to study the direct projections from the PAG to the spinal cord in the cat. In a retrograde tracing study horseradish peroxidase (HRP) was injected into different spinal levels, which resulted in large HRP-labeled neurons in the lateral and ventrolateral PAG and the adjacent mesencephalic tegmentum. Even after an injection in the S2 spinal segment a few of these large neurons were found in the PAG. Wheat germ agglutinin-conjugated HRP injections in the ventrolateral and lateral PAG resulted in anterogradely labeled fibers descending through the ventromedial, ventral and lateral funiculi. These fibers terminated in lamina VIII and the medial part of lamina VII of the caudal cervical, thoracic, lumbar and sacral spinal cord. Interneurons in these laminae have been demonstrated to project to axial and proximal muscle motoneurons. The strongest PAG-spinal projections were to the upper cervical cord, where the fibers terminated in the lateral parts of the intermediate zone (laminae V, VII and the dorsal part of lamina VIII). These laminae contain the premotor interneurons of the neck muscles. This distribution pattern suggests that the PAG-spinal pathway is involved in the control of neck and back movements. Comparing the location of the PAG-spinal neurons with the results of these stimulation experiments leads to the supposition that the PAG spinal neurons play a role in the control of the axial musculature during threat display. Introduction The mesencephalic periaqueductal gray (PAG) can be considered as a part of the limbic system (Holstege, 1990). The PAG is best known for its relation to nociception control (Mayer et al., 1971; Liebeskind et al., 1973; Fardin et al., 1984; Oliveras and Besson, 1988; Levine et al., 1991), but physiological experiments in rat and cat have shown that stimulation in the PAG also produces motor activities, such as vocalization (Kanai and Wang, 1962; Jürgens and Pratt, 1979; Larson, 1985; Bandler et al., 1991; Jürgens and Chang-Lin, 1993), cardiovascular changes (Lindgren, 1955; Abrahams et al., 1960; Lovick, 1985a; Carrive and Bandler, 1991; Bandler et al., 1991), and movements of neck, back and limbs (Liebeskind et al., 1973; Fardin et al., 1984; Bandler and Carrive, 1988; Bandler et al., 1991). The question arises through which pathways the PAG controls these motor output systems. The most simple explanation would be that it projects directly to the motoneurons innervating the muscles involved. Another possibility is that the PAG projects to premotor interneurons in the brainstem or in the spinal cord. It has been demonstrated that the PAG does not influence motoneurons directly, but indirectly via interneurons in caudal brainstem and cervical cord. In respect to