Neurons at rest have a net negative charge inside relative to the outside. This resting membrane potential is approximately −70 millivolts (mV), meaning the inside of the neuron is about 70 mV more negative than the outside. The negative charge at rest is primarily due to the uneven distribution of ions, especially potassium (K+) ions, which are more concentrated inside the cell, and sodium (Na+) ions, which are more concentrated outside. The neuron membrane is more permeable to potassium ions, allowing them to move out more easily, causing the inside to be negatively charged. Additionally, large negatively charged organic anions inside the cell contribute to this negativity. When neurons become active, they do not simply become positive overall; instead, they undergo a rapid change in membrane potential called an action potential. During an action potential, voltage-gated ion channels open to allow sodium ions (Na+) to flow rapidly into the cell, temporarily reversing the charge difference so that the inside becomes positively charged relative to the outside. This temporary positive charge is an electrical signal that neurons use to communicate. The membrane potential then quickly returns to its resting negative state after the action potential passes. In summary, neurons are negatively charged at rest due to ion distribution and membrane permeability. They become positively charged internally only transiently during activity through the influx of sodium ions, enabling the electrical signaling crucial for neuronal communication.