What is a signaling pathway?

The word pathway has become a standard term in the biologist’s lexicon to describe the routes through which information flows in cells, for example from a particular receptor that gets activated by its ligand, all the way to the nucleus where the propagated signal might ultimately result in the expression of a particular gene. The depictions of these signaling pathways in biology textbooks and review articles do little to dispel this idea of biological signals passing through hard-wired circuits much like the kind that we design in our electronic devices, yet these “pathways” don’t really exist in the sense in which they’re being described. A cell signaling pathway resembles a lightning strike much more than it does an electronic circuit in your TV for example. In the same way that the path of a lightning strike can be described only after the strike has occurred, as the route taken by the electrical discharge from the charged cloud to earth, a cell signaling “pathway” can only be described (assuming that we could follow its progress) after the information has propagated through it. Furthermore, since the biological signal is propagated by transient molecular interactions in the cell, at no point is there ever a continuous circuit of the kind depicted in the textbooks. A better analogy for biological signaling might be the spread of an item of gossip through a crowded room full of people at a party. Some people in the room considered to be inside the circle of trust will be potential recipients of the gossip but once the word starts to get out, there could be many possible routes by which any one of them could receive it. Some gregarious types will be more likely to spread the gossip, perhaps even to multiple people at once, coalescing them momentarily into a huddled group. By contrast, some of the more discrete souls who hear it might even consider waiting until much later to pass it on, and then only very selectively and one-to-one. Human nature and the the various affinities of the personalities in the room for one another ensure that those who do receive the gossip will be much more likely to pass it on rather selectively, perhaps to people with whom they have a bond of friendship or trust. And all the while that this gossip is being spread around, there will be many in the room whom for whatever reason are not part of the circle of trust and who will spend the entire evening mingling with the crowd without even catching a whisper of what is being passed around.

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