What ion is primarily responsible for maintaining the positive charge in the extracellular space of a resting neuron?

In a resting neuron, the primary ion that contributes to maintaining the positive charge in the extracellular space is sodium. This is due to the distribution of ions across the neuron's membrane. Sodium ions are more concentrated outside the neuron compared to the inside, which creates a concentration gradient. When a neuron is at rest, the membrane is more permeable to potassium, which can move out of the cell, but sodium remains largely outside due to the action of the sodium-potassium pump, which actively transports sodium out and potassium in.

This gradient is essential for the generation of action potentials. When the neuron is stimulated, sodium channels open, allowing sodium to rush into the neuron, leading to depolarization. However, in terms of resting potential, the extracellular space retains a higher concentration of sodium, thus contributing to its positive charge compared to the negatively charged interior. Understanding this distribution is crucial for comprehending how neurons communicate and process information.