Resting Potential | Why Do Your Brain Cells Have an Electric Charge Even at Rest?

The resting potential is the baseline electrical charge of a neuron when it is not actively sending a signal. This state is not truly "at rest" but is a dynamic equilibrium maintained by the neuron. The key players in creating this charge are ions—electrically charged atoms—specifically potassium (K+) and sodium (Na+). The neuron's cell membrane acts as a barrier, controlling the movement of these ions through specialized proteins called ion channels. In the resting state, the membrane is significantly more permeable to potassium ions than to sodium ions. This means K+ ions can flow out of the cell more freely, down their concentration gradient. As these positively charged potassium ions leave the neuron, the inside of the cell becomes negatively charged compared to the outside. This separation of charge across the membrane establishes the resting membrane potential, which is typically measured at approximately -70 millivolts (mV). This negative charge is crucial because it primes the neuron, making it ready to fire an electrical signal, known as an action potential, when stimulated.

The constant, slow leakage of ions across the membrane would eventually neutralize the resting potential if not for an essential mechanism: the sodium-potassium pump. This protein structure acts like a tiny, energy-dependent battery charger for the neuron. It actively transports ions against their natural concentration gradients, a process that requires energy in the form of adenosine triphosphate (ATP). For every cycle, the pump pushes three sodium ions (Na+) out of the neuron while bringing two potassium ions (K+) into it. This unequal exchange results in a net loss of positive charge from inside the cell, further contributing to the negative interior. The primary function of this pump is to maintain the steep concentration gradients of Na+ and K+, ensuring that there is always a high concentration of potassium inside the cell and a high concentration of sodium outside. This maintained gradient is the potential energy source that the neuron uses to generate electrical signals.

Disruption of the resting potential is fundamental to neuronal communication. When a neuron receives a stimulus, specific ion channels open, causing a rapid change in the membrane potential. If the stimulus is strong enough to raise the potential from -70mV to a threshold level (around -55mV), it triggers a massive, rapid influx of positive sodium ions. This event, called depolarization, momentarily flips the membrane potential to a positive value, generating an action potential. This action potential is the primary electrical signal used by the nervous system. After firing, the cell must quickly return to its resting potential—a process called repolarization—to be ready for the next signal. Therefore, a stable resting potential is not a static state but the essential foundation for a neuron's ability to fire and transmit information throughout the brain and body.

The negative value of the resting potential, approximately -70mV, is a direct result of the relative distribution of charged particles inside and outside the neuron. Two main factors establish this negativity. First, the cell membrane is far more permeable to potassium (K+) than to sodium (Na+) at rest. Because K+ is highly concentrated inside the cell, it flows outward, taking its positive charge with it and leaving the inside of the cell more negative. Second, the cell's interior contains large, negatively charged molecules, such as proteins and organic phosphates, which cannot cross the membrane. These trapped anions contribute significantly to the overall negative charge within the neuron. The sodium-potassium pump reinforces this negativity by pumping out more positive ions (3 Na+) than it brings in (2 K+). The final value of -70mV represents the equilibrium point where the outward chemical force on K+ is balanced by the inward electrical force pulling it back into the negatively charged cell.

The stability of the resting potential is critical for normal brain function. Any deviation can lead to improper neuronal firing, which is a hallmark of many neurological and psychiatric conditions. For instance, in some forms of epilepsy, mutations in genes that code for ion channels can destabilize the resting potential, making neurons hyperexcitable and prone to generating spontaneous, synchronized electrical bursts that result in seizures. In the context of anxiety, while the link is more complex, regulation of neuronal excitability is key. The brain's inhibitory systems, which often work by making the resting potential even more negative (hyperpolarization) to prevent neurons from firing, are crucial for managing anxious states. Imbalances in ion concentrations or malfunctions of the ion channels and pumps that maintain the resting potential can disrupt the delicate balance between neuronal excitation and inhibition. This disruption can contribute to the heightened state of arousal and aberrant signaling patterns observed in anxiety disorders and other conditions affecting the central nervous system.