Neurons are the cells that make up our nervous system, and they’re made up of three main parts.
The dendrites, which are little branches off of the neuron that receive signals from other neurons, the soma, or cell body, which has all of the neuron’s main organelles like the nucleus, and the axon which is intermittently wrapped in fatty myelin.
Those dendrites receive signals from other neurons via neurotransmitters, which when they bind to receptors on the dendrite act as a chemical signal.
That binding opens ion channels that allow charged ions to flow in and out of the cell, converting the chemical signal into an electrical signal.
Since a single neuron can have a ton of dendrites receiving input, if the combined effect of multiple dendrites changes the overall charge of the cell enough, then it triggers an action potential, which is an electrical signal that races down the axon up to 100 meters per second, triggering the release of neurotransmitter on the other end and further relaying the signal.
So neurons use neurotransmitters as a signal to communicate with each other, but they use the action potential to propagate that signal within the cell.
Some of these neurons can be very long, especially ones that go from the spinal cord to the toes, so the movement of this electrical signal within the cell is super important!
But why does the cell have an electric charge in the first place? Well, it’s based on the different concentrations of ions on the inside versus the outside of the cell.
Generally speaking, there are more Na+ or sodium ions, Cl- or chloride ions, and Ca2+ or calcium ions on the outside, and more K+ or potassium ion and A- which we just use for negatively charged anions, on the inside of the cell.
Overall, the distribution of these ions gives the cell a net negative charge of close to -65 millivolts relative to the outside environment, and this is called the neuron’s resting membrane potential.
When a neurotransmitter binds to a receptor on the dendrite, a ligand-gated ion channel opens up to allow certain ions to flow in, depending on the channel.
Ligand-gated literally means that the gate responds to a ligand, which in this case is a neurotransmitter.
So if we take the example of a ligand-gated Na+ ion channel, which, when it opens, lets Na+ flow into the cell. The extra positive charge that flows in makes the cell less negative (since remember it’s usually -65mV), and therefore less “polar”, so that’s why gaining positive charge is called depolarization.
Neurotransmitters typically open various ligand-gated ion channels all at once, so ions like sodium and calcium might flow in, while other ions like potassium might flow out, which would actually mean some positive charge leaves the cell.