The way that neurons send electrical impulses is incredible. It's really a bunch of ions diffusing in tiny fractions of a second and this is responsible for all activity in the brain (and the heartbeat as well - the heart is technically muscle tissue but it behaves an awful lot like nervous tissue).
So there are a bunch of sodium ions (Na+) and potassium ions (K+) in the intracellular and extracellular fluid of neurons. At rest, the cell is busy creating a concentration gradient by keeping the Sodium Potassium pump running strong. It pumps 3 Na+ ions out and brings 2 K+ ions in. Since more of the + charged ions are leaving than entering, the inside of the cell has a negative charge.
So, as I mentioned this is making a concentration gradient. You know how when you stir salt or sugar into water, it disperses evenly in the water (aka dissolves)? So solutes naturally try to spread themselves out evenly.
If there are a whole bunch of them on one side of a membrane and not many on the other, they want to cross the membrane to even things out. (Eeesh, I'm anthropomorphizing ions for heaven's sake!)
To visualize this, imagine being stuck in a small classroom with 60 students. The second the bell rings, everyone wants to rush out of the room into the less crowded hallway. That would be called "diffusion" or "moving down the concentration gradient."
So the Sodium Potassium pump is like purposely stuffing as many students into the small classroom as possible, so that when they do open the door at the right moment, students will come rushing through the door which creates energy! This is the same thing that the electron transport chain in Mitochondria does which creates the majority of your energy for living! Pretty amazing. (Click to watch a super cool video of the Mitochondria doing it's thing - by BioVisions.)
It's also the same concept behind hydro-power. People build a dam to hold back a bunch of water, then they create a small opening and let some of the water rush out which turns a turbine and generates electricity.
This is what is going on with a nervous impulse. Basically, the cell is resting at a negative voltage (-70 mV) because of all those sodium ions being pumped out, plus most of the stuff in a cell is negatively charged anyway. If it gets up to "threshold" voltage, then the voltage gated sodium channels open and positively charged sodium rushes into the cell, causing the voltage to go from -70 to +30. That's called depolarization.
At the top of that action potential, the voltage is at a point where no more sodium wants to come in, that channel closes and the potassium channel opens allowing K+ to rush out of the cell, which then brings the cell back to the negative voltage. This is called repolarization.
The K+ channels close kind of slowly, so that's why there's the little dip at the bottom, because the voltage drops below the resting point since it takes a little longer for the K+ channels to close, and then it goes back up to resting at -70 mV. When this electrical impulse reaches the end of the neuron's axon, it releases chemicals called neurotransmitters which a neighboring neuron can then pick up and start sending another electrical signal through that neuron, and on and on.
And THAT is how all your thinking and everything takes place. Amazing, huh?!!?
To learn more about neurons, go to this post:
|Click to learn more about neurons|
|Neuron Communication - click to learn more|
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