Structural and functional post-embryonic development of a non-rectifying electrical synapse in the crayfish

Invaginating Presynaptic Terminals in Neuromuscular Junctions, Photoreceptor Terminals, and Other Synapses of Animals

Typically, presynaptic terminals form a synapse directly on the surface of postsynaptic processes such as dendrite shafts and spines. However, some presynaptic terminals invaginate-entirely or partially-into postsynaptic processes. We survey these invaginating presynaptic terminals in all animals and describe several examples from the central nervous system, including giant fiber systems in invertebrates, and cup-shaped spines, electroreceptor synapses, and some specialized auditory and vestibular nerve terminals in vertebrates. We then examine mechanoreceptors and photoreceptors, concentrating on the complex of pre- and postsynaptic processes found in basal invaginations of the cell. We discuss in detail the role of vertebrate invaginating horizontal cell processes in both chemical and electrical feedback mechanisms. We also discuss the common presence of indenting or invaginating terminals in neuromuscular junctions on muscles of most kinds of animals, and especially discuss those of Drosophila and vertebrates. Finally, we consider broad questions about the advantages of possessing invaginating presynaptic terminals and describe some effects of aging and disease, especially on neuromuscular junctions. We suggest that the invagination is a mechanism that can enhance both chemical and electrical interactions at the synapse.

Ultrastructure of electrical synapses: review

This article reviews studies providing information on the ultrastructure of electrical synapses. Although the review focuses on electron-microscopic investigations, its aim is to examine how the structure of an electrical synapse relates to its function. It begins by presenting a historical overview of the early studies which were responsible for the recognition of electrical synapses. The structure of gap junctions which are the morphological correlates of electrical synapses is illustrated and the ultrastructure and function of the two types of electrical synapse, rectifying and non-rectifying, described. Recent papers investigating the ultrastructure of electrical and mixed electrical-chemical synapses in invertebrates and vertebrates are reviewed. For earlier references, the reader is directed to previous reviews on the subject. Much new information, however, on the structure and formation of electrical synapses has been obtained from work on cultured neurons and from electron-microscopic, immunocytochemical, conformational and molecular studies. This article reviews those studies and in light of their findings, re-examines the relationships of the structure of electrical synapses with their function.