Why are action potentials in animals and plants all-or-none?
I haven't found anything that explains this, so I've written the below for use in lecturing undergrads. I'd welcome any comments, corrections, or links to other explanations.
The all-or-none nature of the action potential is how we can achieve long-range transmission without degradation of the signal.
-The signal is binary or digital; no action potential or yes an action potential; like 0 or 1, nothing in between.
-Imagine that it was instead analog, a graded signal instead of digital.
-The neuron might have a signal of, say,60 millivolts, travelling down the axon. You need to preserve the 60-mv msg all the way to the destination.
-So each bit of the neuron has to generate the mV that the previous bit generated.
-But the problem is you get errors, and the errors accumulate, just like in the Chinese whispers game.
-The second millimetre might be affected by random fluctuations and generate 61 mV. The third mm should then generate 61 mV but erroneous fluctuations might have it go to 63 mV. So, who knows what the message will be by the time it reaches the end of the axon.
-This man Claude Shannon had the insight that if you use digital, you can have perfect transmission, no errors.
-Instead of each bit of the neuron set up to try to reproduce the level in the previous bit of neuron,
it’s set up only to detect whether the membrane potential reaches higher than a threshold, and if the electrical potential is greater than that, it creates a predetermined signal.
-The threshold can have a value related to the size of random fluctuations, greater than the random fluctuations, in which case an action potential is never generated in error, and never erroneously not
-Shannon proved all this in his master’s thesis, the greatest master’s thesis in history. It led to all the digital electronics that we use today.
*If you're wondering about the mention of plants, many plants do indeed have action potentials: http://onlinelibrary.wiley.com/store/10.1111/j.1365-3040.2006.01614.x/asset/j.1365-3040.2006.01614.x.pdf;jsessionid=37878CEAC4D234280FB73E9BF27694CE.f01t04?v=1&t=i8o4o3x7&s=ebf6e66ce48f988202c04bb15a028a07b9c52bcf