Brain BREAK! presents: Nerve Potentials.
Today, we look at the basics of graded potential versus action potential, and how they work.
Let’s say a mosquito lands on me and moves my skin around just a little bit, opening a mechanically-gated channel. A few neurons might pass this “hey, is that a mosquito?” message along. But let’s zoom in on one very vital neuron. We’ll call her Tracy.
Acetylcholine has been released into the synaptic cleft and along just one of Tracy’s little dendrites. They pick it up via chemically-gated channels (receptors). These also might be referred to as ligand-gated channels. Remember that neurotransmitters are chemicals, and thus, chemically-gated channels. The dendrite receives the message first.
Graded potential
Like streams into a river, the branches of the dendrites come together toward Tracy’s soma. After the chemically-gated channels have been activated, voltage-gated channels (Na+ and K+ ions, etc.) begin to transfer this mosquito signal toward the axon hillock. But here is why we call this graded potential: there are different grades (a range) of voltage differences that can be at play here. It’s a gray area.
The Trans-Membrane Potential (TMP), or resting potential, is described as 0 milliVolts outside the cell and about -75 mV inside the cell. In short, a TMP of -75. At this particular dendrite, let’s say the sodium-potassium conga line, triggered by the Acetylcholine, reaches -55 mV at the base of our dendrite. But it’s not to Tracy’s axon hillock yet. The voltage bounces along the perimeter of the soma: -55 mV, -56 mV, -58 mV… it loses strength along the way, like a phone signal.
Due to the All-or-Nothing Principle, Tracy’s axon hillock acts as a wall. The impulse stops before the first Schwann cell, “dying” before it can reach any other cell, much less my central nervous system. I don’t feel the mosquito. Why? The hillock looks for the summation (sum) of graded potentials reaching a polarity of -55 mV… at its doorstep. Without -55 mV, other voltage changes like -60, -65, etc. all look like nothing. I know it sounds pretty thoughtless of Tracy’s axon hillock – to ignore the dendrites like this – but let’s think about it. If every nerve transmitted information at the slightest voltage change, we would probably go insane from all the information. Like a recovering burn victim feels pain at every slight movement, a breeze might send our nervous system into overdrive. This -55 mV requirement, along with a few other mechanisms (like a refractory period) lets us pay attention to the stuff that matters without short-circuiting constantly.
So, while this mosquito’s teeny tiny legs might rile Tracy a little, they are not enough to warrant a signal to my central nervous system. BUT! If you are as allergic as I am, the story will change.
The mosquito’s spit creeps into my dermis. My white blood cells notice a foreign particle. The inflammation response starts. The blood vessels dilate. That chaos is communicated to the surrounding dendrites more and more. All the dendrites on Tracy are picking up Acetylcholine now, and the signal is picking up strength via summation. Like a wave! It’s still a graded potential, but instead of fading to obscurity, there is enough stimulus from all the combined dendrites to reach that -55 mV voltage shift at Tracy’s axon hillock. Instead of “nothing,” we now have “all,” and BOOM! We’ve upgraded to the action potential.
The action potential
The word “potential” seems contradictory here to me, so in my head, I just think ACTION! (I still kind of think of electricity as magic. Haven’t taken that class yet.) ACTION! is that leap of depolarization from -55 mV up to perhaps +30 mV, with those Na+ and K+ ions jumping in and out of the membrane. Depolarization in this case means becoming more positive, or toward zero. Where the mosquito’s legs were not even a blip, the voltage of an action potential looks like a beefy heartbeat on a monitor. Actually, a heartbeat on a monitor is an action potential, too, don’tcha know.
From here, the voltage-gated channels along Tracy’s axon are opened up, and that voltage is carried over Schwann cells by way of saltatory propagation (continuing by jumps). At the end of Tracy – the synaptic knobs – Acetylcholine is released, which travel to other dendrites, which join bigger nerve clusters, to major nerves, all the way to my CNS. Itch, itch, itch, ITCH, ITCH, ITCH! And I slap the back of my neck, but it’s too late. My frontal lobe goes on to interpret all of these impulses and I think, “Oh, great, and I forgot the anti-histamine lotion.” Gee, thanks, graded potential. Thanks a LOT.
(Sorry, Tracy. I do appreciate the work you do.)
So, to sum up Tracy’s axon hillock experience of all this.
-75 mV: just hanging out.
-60ish MV: those dendrites are always whining about something.
-55 mV: HOLY S&!% SOMETHING IS HAPPENING I HAVE TO TELL EVERYBODY.
This is why I never leave the house! -CNx