There is normally a charge difference across the plasma membrane of a neuron. The outside of the membrane has a positive charge. The inside has a negative charge. Resting potential results from differences between sodium and potassium positively charged ions and negatively charged ions in the cytoplasm. Sodium ions are more concentrated outside the membrane, while potassium ions are more concentrated inside the membrane.
When a neuron receives signals, an abrupt, temporary reversal in the polarity is generated (an action potential). The inside becomes more positive. Any membrane that can produce action potentials is said to show membrane excitability. Voltage change causes voltage-gated channels in the membrane to open. As a result of ion flow through these channels, the inside of a neuron briefly becomes more positive than outside.
The resting potential tells about what happens when a neuron is at rest. An action potential occurs when a neuron sends information down an axon, away from the cell body