Gap junction

Gap junction channels link two cells together via electrical synapses. In an electrical synapse, either neurone can excite the other and is therefore known as bidirectional. The speed of conduction across an electrical synapse is much quicker than that across a chemical synapse as the cytoplasm is continuous between the cells. The presynaptic cell also does not have to reach a threshold potential in order to trigger an action potential in the postsynaptic cell, instead a depolarisation is triggered in the postsynaptic cell that is proportional to the electrical current from the presynaptic cell. 

Gap channels are composed of two hemichannels, where one hemichannel is situated in each opposed cell. Many thousand gap junction channels can make up an electrical synapse. 

Gap junctions are formed from protein monomers known as 'connexins', six connexins form one 'hemichannel' (half of the gap junction) on one cell and another six connexins on a neighbouring cell form the other half of the channel (each 'hemichannel' is known as a connexon)[1][2]. Each connexon interacts with another connexon forming a full channel which contains a pore running through the middle[2]. They enable the passage of molecules across adjacent cells in order to enable intercellular communication to allow entire tissues to function efficiently[1][3]. Gap junctions do not allow all molecules to pass through however, for example larger molecules such as proteins are not able to move from cell to cell simply because they are too large[1][2]. 

References

1. Alberts B & Johnson A. Lewis J. et al. Molecular Biology of the Cell. 4th edition, New York: Garland Science; 2002

2. J.M. Berg, J.L. Tymoczko, L Stryer and G.J. Gatto, Jr. Biochemistry. 8th edition. New York: W.H. Freeman; 2015

3. Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 7th edition. New York: W. H. Freeman; 2013