A Gap Junction is a direct connection between two separate cells. It allows molecules to pass between the two cells. The Gap Junction, also called a Nexus, is formed from six proteins called Connexins. The Connexins form a bridge with a sphincter than can open and close. The importance of the Gap Junction is that it allows transmission of electrical signals and molecules between cells, instead of being closed off by the cell membranes.
An example of Gap Junctions in action is the Heart. Instant communication is key; the Gap Junctions in the Heart's cellular tissue enable each cell to contract at the same time. Without the signals passing directly through the cell membranes, the cells would contract individually and likely cause an irregular heartbeat, which can lead to a heart attack. The speed and accuracy of the Gap Junction transmission is key to a healthy heartbeat. Cells naturally share a small amount of electricity during their normal function, but large-scale, coordinated sharing must occur across Gap Junctions to be effective.