Impulse transmission in a neuron depends on— the presence of membrane potential and channel proteins. Neurons have a small build-up of negative charge inside the cell membrane and a similar accumulation of positive charge on the outside, creating a membrane potential. The flow of ions across the membrane can change the membrane potential. This flow of ions— or current, generates electrical impulses — making the neurons electrically excitable. The direction of ion movement is largely determined by their electrochemical gradient. It is a combined effect of a concentration gradient, where the ions flow from an area of higher to lower concentration, and an electrical gradient, where the ions tend to flow towards an area of opposite charge. Because the cell membrane is impermeable to charged particles, they can only flow through specific proteins called ion channels. These channels can be non-gated, which opens and closes randomly, or gated, which requires a stimulus to open. The controlled opening and closing of these channels allow for a unidirectional flow of electrical impulses from one dendrite to the axon terminal in a neuron.