Computerized ultrastructural analysis of the shape of the active synaptic zones in rat spinal cord

Active synaptic zones are cytoplasmic specializations that indicate where synaptic transmission occurs. We have used computerized three-dimensional reconstructions from serial ultrathin sections to define certain features of the geometry of these zones in mammalian spinal cord. Our main finding is that the active zones in the dorsal portion of the spinal cord can be placed in one of two categories with respect to curvature: (1) uncurved or slightly curved and (2) very curved. The very curved category is associated with simple axodendritic type synapses in which the axonal terminal arises from primary afferent fibers.