The ultrastructure of the periaqueductal gray matter of the cat.

The neurons of the periaqueductal gray matter (PAG) in cat are generally spindle shaped, small and medium size cells. They have a very high nucleo-cytoplasmic ratio, frequent complex nuclear inclusions and very few axo-somatic contacts. The larger neurons have spine-like extrusions. The neuropil is very extensive, made up mainly by small unmyelinated axons and small dendrites. Also the synaptic boutons are very numerous, and sometimes have particular modalities of contact and arrangement (axo-axonic and dendro-dendritic synapses, 'boutons en passant', and glomerulus-like structures). No significant differences have been observed in the dimensions and shape of the vesicles present in the axo-somatic synapses compared with those present in the neuropil synapses. In both cases pleomorphic vesicles predominate. The particular complexity and abundance of the synaptic contacts in the neuropil suggests that the neuropil is the main site where the afferent inputs to the PAG are modulated and integrated.