Cations are positive ions, anion are negative ions. There are also some negatively charged protein molecules. It is important to remember that nerve cells are surrounded by a membrane perforated by proteins that allows some ions to pass through while it blocks the passage of other ions. This type of membrane is called semi-permeable. At rest, the inside of the neuronal membrane is slightly negative relative to the outside.
Cellular membrane have highly apolar inner volumes.
They do so through ionic channels. They vary by the specificity for certain ions, or certain charges, and by the mechanism of gating. Information in neurons is carried by electrical activity. Since there are a lot more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Note that, because the membrane potential moves from negative towards zero, i.e. from a more polarized state to a less polarized state, this process consists of a membrane depolarization .