Mechanistically, exercise is entirely dependent on action potentials, while its incredible benefits may be characterized as the potential of action to enhance our lives. Ironically, perhaps, the luminous potential of action to promote our health is in a very fundamental way the exact opposite of the action potentials from which it derives.
I imagine that may be about as clear as the way through a Tough Mudder. No worries; I’ve got Windex.
An action potential refers to the mechanism that underlies the firing of our nerve cells, or neurons.
Famously, action potentials are “all or nothing.”
At rest, there is a slight electrical gradient maintained across the cell membrane of a neuron. That slight charge is energy dependent, requiring the constant work of ion channels that traverse the cell membrane, shuttling positively and negatively charged ions in opposite directions. When we talk about “resting energy expenditure,” or “basal metabolism,” these are the kinds of functions represented; our cells are always working even when we are not.
That electrical gradient is, quite literally, an “action potential,” because it primes the cell to take the one action it owns: depolarizing. When a stimulus reaches a neuron, if it is strong enough, it reverses the electrical charge at the site of contact. That reversal of charge, or depolarization, then courses along the length of that nerve cell, rather like a fast moving wave.
If the nerve cell in question is a sensory neuron, the result of that wave is that we feel or perceive something—a caress, a color, a shiver, or a symphony.
If it is a motor neuron, it ends at a muscle cell, which in turn is stimulated to contract. When a whole lot of muscle cells contract in unison, we have the familiar command over our moving body parts; such as my fingers, currently dancing over this keyboard.
We could, of course, go much deeper into the weeds, but that’s the relevant gist. What matters for today’s story is that the depolarization of every neuron, or muscle cell (myocyte) for that matter, is all or nothing. The stimulus reaching it is either enough to excite full depolarization, or it is not. There are no partial responses; there is no dose response curve.
Read More Exercise: How a Little Goes a Long Way