Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulated gate between cells. One gap junction channel is composed of two protein hexamers (or hemichannels) called connexons in vertebrates and innexons in invertebrates. The hemichannel pair connect across the intercellular space bridging the gap between two cells. Gap junctions are analogous to the plasmodesmata that join plant cells. Gap junctions occur in virtually all tissues of the body, with the exception of adult fully developed skeletal muscle and mobile cell types such as sperm or erythrocytes. Gap junctions are not found in simpler organisms such as sponges and slime molds. A gap junction may also be called a nexus or macula communicans. There has been some confusion with gap junctions being related to ephapses in the past. While an ephapse like a gap junction involves the transmission of electrical signals the two are different. Ephaptic coupling involves electrical signals external to the cells. Ephapses are often studied in the context of electrically induced potentials propogated amoung groups of nerve cell membranes even in the absence of gap junction communication with no discrete subcellular structures known. Unlike gap junctions no specific structure related to an ephapse has yet been described so the process of "ephaptic coupling" is often referred to rather than an ephapse. Structure In vertebrates, gap junction hemichannels are primarily homo- or hetero-hexamers of connexin proteins. Invertebrate gap junctions comprise proteins from the innexin family. Innexins have no significant sequence homology with connexins. Though differing in sequence to connexins, innexins are similar enough to connexins to state that innexins form gap junctions in vivo in the same way connexins do. The more recently characterized pannexin family, which was originally thought to form inter-cellular channels (with an amino acid sequence similar to innexins), in fact functions as a single-membrane channel that communicates with the extracellular environment, and has been shown to pass calcium and ATP. This has led to the idea that pannexins may never form intercellular junctions in the same way connexins and innexins do and so should not use the same hemi-channel/channel naming. Others have presented evidence based on genetic sequencing and overall functioning in tissues that pannexins should still be considered part of the gap junction family of proteins despite structural differences in