Fig. 1.

Generalized structures of invaginating synaptic processes, as described in the review trilogy (Petralia et al. 2015, 2016, and this review). a. a1 Standard synapses form directly along the surface of dendrite processes including shafts and spines (spines not shown). a2 Protrusions/projections of various sizes can extend into invaginations, and occur among axons, axon (presynaptic) terminals, dendrite processes including shafts and spines, and associated glial elements. Three examples are shown; reviewed in Petralia et al. (2015). a3 Postsynaptic spines can protrude/project into an invagination of the presynaptic terminal. These are reviewed in Petralia et al. (2016); examples are seen also in figures 6 and 7 of the current review. b1–3 As discussed specifically in the current review, some presynaptic terminals can be invaginated into a postsynaptic structure. The terminal may be fully (b1) or partially (indenting) (b2) invaginating into a postsynaptic invagination, or form an invaginating protrusion (b3). In these structures, the invaginating processes contain presynaptic active zones with concentrations of synaptic vesicles, and associated with postsynaptic specializations (typically a distinctive PSD; red arrow). Also, the axon/terminal orientation can be either mostly perpendicular (as shown) or mostly parallel (not shown; common for neuromuscular junctions) to the postsynaptic process. Often, the terminal has an intermediate orientation, extending in a third dimension at an angle from the plane of the section; for example, in ribbon synapses, the ribbon structure often extends as a ridge in an elongate invagination (see figures 4, 7). The postsynaptic structure can be a dendrite process, or muscle or gland cell (also in the case of the photoreceptor terminal invagination, the terminal membrane may be postsynaptic to horizontal cell processes). Examples illustrated in this review of the structure in figures 1b1, 1b2, and 1b3 are shown in figures 3, 5–9, 11, 12, figures 2, 3, 8–11, and figures 2–6, 11, respectively. See text and other legends for details. Note that all drawings in all figures are original and based on micrographs and drawings in the cited studies. In all drawings, the presynaptic terminals are colorless, postsynaptic processes (usually only the adjacent portion is shown) are blue, mitochondria are green, and Schwann/glial processes are pink (additional structures are yellow) (Color figure online)