“What’s in a name?”

Sorry, Shakespeare! The right answer is - quite a lot actually! In the case of Gap Junction, we’re not just talking about bridging a linguistic gap, we’re evoking a system of connection between cells.

A basic property of multicellular organisms, be they animals or plants, is that the component cells cooperate with each other to keep the organism alive and kicking. The more complex an organism, the more specialized its cells, and the more they need to communicate with other cells that perform the same or different functions.

Almost everyone knows of at least one instance where cells communicate: the synapse. Nerve cells or neurons are highly specialized cells that can send messages from one end of the body to the other faster than you can blink (and we won’t go into how many nerve cells are actually involved when you blink)! Who isn’t familiar with images from the movies or from television that show a nerve cell firing off a message, hissing and crackling all along its length like a naked electric cable? Apart from the fact that nerve cells are pretty well insulated, at least in more evolved species, that’s not a bad representation of what really happens.

What happens when this message arrives at the end of the nerve cell, though? Well, it gets passed on to the next cell by means of a synapse. There are different types of synapses. The more flashy ones are chemical synapses – the end of the nerve cell bulges out like a sort of club, and chemicals known as neurotransmitters gather in little balloons inside the club, waiting for the right message to come along. Once they get the signal, a complex series of events brings the balloons to the surface of the club where they burst, releasing the chemicals they contain into the space around the cell. Neighboring cells then pick up the chemicals from this space, and either act on the message or pass it along.

Sometimes the synapse isn’t chemical but electrical. When scientists first visualized living cells under the electron microscope, among the incredibly detailed structures they saw were patches where cells appeared to be connected to each other. In some cases, these cells were stuck very closely together indeed, with the walls between them almost seeming to thin out or disappear – these were named tight junctions. Other patches, however, seemed to leave a small space between the two cell walls – these were named gap junctions. Further, each gap junction seemed to consist of small particles on the cell wall of one cell that were lined up against similar particles on the wall of the adjacent cell. The particles were named connexons, and we know now, as a result of years of research, that each connexon is made up of six connexin molecules, and that when the conditions are right, the molecules twist to create a pore in the middle of the connexon, in a manner reminiscent of the opening of a camera aperture. Small substances, mostly electrically charged particles or ions, pass freely through this pore from once cell to another.

Unlike chemical synapses, gap junctions are not restricted to nerve cells. Other cells that need to communicate rapidly with each other, such as muscle cells that need to coordinate their contraction, also use gap junctions. Even the primitive sponge, a nominally multicellular animal, uses gap-junction-like connections between its cells. In other words, we’re talking about a system of communication that was invented six hundred million years ago. It must be effective indeed, if it’s still around!

Images (available shortly):

  1. Chemical synapse

  2. Electron micrograph of a gap junction

  3. Structure of a connexon

  4. Demonstration of cell-cell communication through gap junctions

References/links (available shortly)

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