An electical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed by proteins known as gap junctions. Each junction consists of proteins called connexins. Six connexins combine to form one channel called a connexon through the membranes of the adjacent cells. When the voltage of one cell changes, ions may move through the channels from one cell to the next. This form of signaling is more rapid than that which occurs across chemical synapses, which are the predominant kind of junctions between neurons. At many electrical synapses, channels are able to close and able largely to prevent the flow of ions in one of the two directions. There is also evidence for "plasticity" at some of these synapses--that is, that the electrical connection they establish can strengthen or weaken as a result of activity. Electrical synapses are abundant in the retina and cerebral cortex of vertebrate animals.