The Nervous System, Part 2 - Action! Potential!: Crash Course A&P #9

It doesn't matter if it's a spider on your knee or an elephant, a paper cut or stab wound, the strength of that action potential is always the same. What does change is the

-- frequency of the buzz. A weak stimulus tends to trigger less frequent action potentials. And that includes if the stimulus is coming from you, like your brain telling your muscles to perform some task.

If only a few channels open, and only a bit of sodium enters the cell, that causes just a little change in the membrane potential in a localized part of the cell.

This is called a graded potential

that nerve impulse, is called the

action potential.

A resting neuron is like a battery just sitting in that sack that is you. When it's just sitting there, it's more

negative on the inside of the cell, relative to the extracellular space around it. This difference is known as the neuron's resting membrane potential, and it sits at around -70 millivolts.

And because the voltages in this process are always pretty much the same -- the initial threshold around

-55 mV, and the peak at depolarization at +40 mV -- your neurons only communicate in a single, monotone buzz.

Outside of a resting neuron, there's a bunch of positive sodium ions floating around, just lingering outside the membrane. Inside, the neuron holds potassium ions that are positive as well, but they're mingled with bigger, negatively-charged proteins.

And since there are more sodium ions outside than there are potassium ions inside, the cell's interior has an overall negative charge.

But they've only got one signal that they can send, and it only transmits at one uniform strength and speed.

And your brain can translate these signals, reading them like binary code, organizing them by location, sensation, magnitude, and importance,

When a neuron has a negative membrane potential like this, it is said to

Be polarized

And your best bet for making a signal send down the axon is to depolarize that resting neuron --

I mean, cause a big enough change in its membrane potential that it'll trigger the voltage-gated channels to open.

As soon as all that's underway (Voltage in axon), the process of repolarization kicks in. This time the voltage-gated potassium ion channels open up, letting those potassium ions flow out, in an attempt to rebalance the charges.

If anything, it goes too far at first, and the membrane briefly goes through hyperpolarization: Its voltage drops to -75 or so mV, before all of the gates close and the sodium-potassium pumps take over and bring things back to their resting level.

Resistance is just whatever's getting in the way of the current.

Something with a high resistance is an insulator, like plastic, and something with a low resistance is a conductor, like metal.

But in order to send long-distance signals all the way along an axon, you need a bigger change -- one big enough to trigger those voltage-gated channels.

That is an action potential

The sodium-Potassium pump. This little protein straddles the membrane of the neuron, and there are tons of them all along the axon. For every two potassium ions it pumps into the cell, it pumps out three sodium ions.

This creates a difference in the concentration of sodium and potassium, and a difference in charges -- making it more positive outside the neuron.

when the gates do open, ions quickly diffuse across that membrane down their electrochemical gradient, evening out the concentrations, and running away from other positively charged ions

This movement of ions is the key to all electrical events in neurons, and thus is the force behind every. single. thing. we think, do, and feel.

neurons send ALL the impulses responsible for every one of your

actions, thoughts, and emotions.

Each of your neurons has lots of voltage-gated sodium channels. So when a few in one area open, that local current is strong enough to

change the voltage around them. And that triggers their neighbors, which triggers the voltage around them, and so on down the line.

When a neuron is stimulated enough, it fires an

electrical impulse that zips down its axon to its neighboring neurons.

Because this is an all-or-nothing phenomenon. If the stimulus is too weak, and the change doesn't hit that level, it's like a

false alarm -- the neuron just returns to its resting state.

And the factor that affects a neuron's transmission speed the most, is whether there's a myelin sheath on its axon. Axons coated in insulating myelin conduct impulses

faster than non-myelinated ones, partly because, instead of just triggering one channel at a time in a chain reaction, a current can effectively leap from one gap in the myelin to the next. These little gaps are the delightfully named Nodes of Ranvier, and this kind of propagation is known as saltatory conduction, from the Latin word for "leaping."

And because opposite charges attract, we need barriers, or membranes, to

keep positive and negative charges separate until we're ready to use the energy that their attraction creates.

unlike graded potentials, which are small and localized, an action potential

kicks off a biological chain reaction, which sends that electrical signal down the axon

At that threshold, the voltage-gated sodium channels open, and there are tons of these, so all of the positive sodium ions rush in, making the cell

massively depolarized -- so much so that it actually goes positive.

In a cell, we refer to this difference in charge as the

membrane potential. The bigger the difference between the positive and negative areas, the higher the voltage, and the larger the potential.

your body as a whole is electrically neutral, with equal amounts of

positive and negative charges floating around. But certain areas are more positively or negatively charged than others.

Action potentials also vary by speed, or conduction velocity. They're fastest in pathways that govern things like

reflexes, for example, but they're slower in places like your glands, guts, and blood vessels.

When part of an axon is in the middle of all this, and its ion channels are open, it can't respond to any other stimulus, no matter how strong. This is called the

refractory period, and it's there to help prevent signals from traveling in both directions down the axon at once.

Voltage, for example, is the measure of potential energy generated by

separated charges. It's measured in volts, but in the case of your body, we use millivolts because it's a pretty small amount.

And just like there's voltage in your body, there's also current --

the flow of electricity from one point to another. The amount of charge in a current is related both to its voltage and its resistance.

currents indicate the flow of positively or negatively charged ions across

the resistance of your cells' membranes.

Thankfully, the sodium-potassium pump isn't the only way in or out of the cell -- the membrane is also riddled with ion channels, large proteins that can provide safe passage across the membrane, when their respective gates are open.

voltage-gated channels, which open at certain membrane potentials, and close at others. For example, sodium channels in your neurons like to open around -55 mV. But some others are ligand gated channels -- they only open up when a specific neurotransmitter, like serotonin, or a hormone latches on to it. And then we also have mechanically gated channels, which open in response to physically stretching the membrane.