I keep hearing that there's always electrical activity taking place inside the human brain. Our heart and various other organs function because it receives electrical signals called impulse. But, what's the "voltage source" for the brain? How does it receive signals? Does it generate on it's own? If so, how?
IMHO it is question of biology or at least chemistry, surely not physics. All energy in every living object is generated from ATP. There are few possible sources of ATP, usually inside humans it is generated during glucose oxidation.
ATF turned into electricity by K and Na pumping. Using ATP as a source of energy neurons move positively charge ions outside (or inside, I don't remember) and become a kind of small capacitors. You may start exploring details here. Though, I'd recommend textbook on microbiology.
These voltages are generally the result of small channels in cell membranes that swap charged ions back and forth such that an electrical potential is created. This can happen with Na+, Ca2+, and others. The basic idea is that if you bring +1 of charge in one direction and push +2 in the other direction, you've generated a charge.
That static charge then depolarizes neighboring channels, causing them to make the same swap, and so on, in a chain-reaction.
It's a little more complex than that :-) but that'll get you started.
It's more thorough than I think you realize. Your own thoughts wouldn't be possible if not for the electrical characteristics of your tens of billions of neurons. The reason for the confusion is that biological "electronics" is chemical in nature. And even forgetting about electricity/electrons, biological systems use all sorts of clever signal processing, both digital and analogue, all over the place by way of chemicals/proteins.
And I wonder what kinds of signalling you might find even within, say, a single protein molecule.
I recommend going to your local library and seeing if they have this course series. (See lectures 38-43).