Synapses in stratum radiatum of hippocampal area CA1 are distributed throughout the neuropil. Where an unmyelinated axonal bouton and a dendrite come into to contact there is the potential for a synapse. But only a few of these contacts are synaptic. Figure 1 shows the distribution of synapses within a volume of neuropil from stratum radiatum of the adult rat. There are 161 synapses in this 64 cubic micron volume. These are all chemical synapses, 94% of which are Type 1 (excitatory) synapses and 6% Type 2 (inhibitory) synapses.
This image was created by reconstructing the synapses in 3D from serially-sectioned tissue that was photographed on the electron microscope. As can be seen in the figure, synapses take on a variety of shapes. This can be more readily seen by viewing a sample of these synapses placed en face (Figure 2).
Most synapses cover a small area and have a compact, roughly convex shape, such as numbers 51, 59, and 81, above. These are referred to as macular synapses. Larger synapses are often exhibit "holes" in the middle. These holes are regions of cell membrane devoid of the specializations characteristic of the synapse, e.g. postsynaptic density, synaptic cleft, presynaptic active zone, etc. Synapses with holes, such as numbers 45, 46, 86, 90, 94, 96, and 100, are referred to as perforated synapses. Of the 161 synapses in this volume of neuropil, 148 are macular, while the remaining 13 are perforated.