The spinal terminals into the midbrain periaqueductal gray of the rat. A light and electron microscope study of the projections ascending via the ventro-lateral funiculus.

Light and electron microscope studies of the terminal fields of the spinal afferents ascending in the ventrolateral quadrant of the spinal cord to the periaqueductal gray matter (PAG) were carried out using silver degeneration techniques and anterograde WGA-HRP transport. At all mesencephalic levels the terminal degeneration and the labelling of the spinal fibres to the PAG were fairly dense, mainly concentrated in the outer part of the annular ring. At the electron microscope, the spinal nerve terminals were found ending almost exclusively in the neuropil, impinging on small dendritic profiles. The majority of these synaptic contacts were of the asymmetrical type and contained a prevalence of round vesicles. This study provides an accurate representation of the terminal domain of the spino-PAG afferents and gives the conclusive anatomical evidence that the fibres ascending in the ventro-lateral funiculus of the spinal cord do not simply pass through the PAG, but actually terminate in it, synapsing in the neuropil.