#9 The Nervous System, Part2 - Action! Potential!

When a neuron is stimulated enough, it fires an electrical impulse that zips down its axon to its neighboring neurons.

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

That buzz, that nerve impulse, is called the action potential.

It's one of the most fundamental aspects of anatomy and physiology, and really life in general.

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<Your body is a Sack of Batteries>

We keep 'em separated to build potential.

A battery just sitting on its own has both a positive and negative end, and the potential to release energy.

Each neuron in your body is like its own little battery with its own separated charges.

It just needs an event to trigger the action that brings those charges together.

Voltage:
is the measure of potential energy generated by separated charges

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

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.

Resistance:
whatever's getting in the way of the current

Currents indicate the flow of positively or negatively charged ions across the resistance of your cells' membranes.

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<How Electricity Works Inside your Nervous System>

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.

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.

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<Sodium-Potassium Pump>

This little protein straddles the membrane of the neuron, and there are tons of them all along the axon.

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

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<Types of Ion Channels>

Most are volatge-gated channels, which open at certain membrane potentials, and close at others.

But some others are ligand gated channels - they only open up when a specific neurotransmitter, like serotonin, or a hormone latches on to it.

This movement of ions is the key to all electrical events in neurons.

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<Graded Potential vs. Action Potential>

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.

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.

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<Depolarization>

-55mv: all or none phenomenon

A brief depolarization caused by changes in currents.

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<Repolarization>

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.

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<Hyperpolarization>

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.

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<Refactory Period>

It can't respond to any other stimulus, no matter how strong. This is called the refractory period, and its' there rto help prevent signals from traveling in both directions down the axon at once.

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.

Action potentials also vary by speed, or conduction velocity.

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.