A neuron becomes negatively charged inside relative to the outside primarily due to an unequal distribution of ions across its membrane at rest. This negative charge, known as the resting membrane potential, is crucial for the neuron's ability to transmit electrical signals.
The Role of Ion Distribution
The resting membrane potential is primarily established by the concentration gradient and permeability of key ions, namely sodium (Na+) and potassium (K+).
Ion Concentrations at Rest:
| Ion | Concentration Inside Neuron | Concentration Outside Neuron |
|---|---|---|
| Sodium (Na+) | Lower | Higher |
| Potassium (K+) | Higher | Lower |
Key Factors Leading to Negative Charge:
- Sodium-Potassium Pump: This protein actively pumps three Na+ ions out of the neuron for every two K+ ions pumped in. This action contributes to a net positive charge leaving the cell, making the inside more negative.
- Potassium Leak Channels: The neuronal membrane is more permeable to potassium ions than to sodium ions at rest. Potassium ions tend to leak out of the neuron down their concentration gradient. The loss of positive potassium ions contributes to the negative charge inside the neuron.
- Large Anions: The presence of large negatively charged proteins and other anions that are trapped inside the cell contributes to the negative charge.
The Result:
According to the reference, "the resting potential is negative due to the accumulation of more sodium ions outside the cell than potassium ions inside the cell". While this statement needs the clarification that it is the movement of ions across the membrane not just their accumulation, it is partially true since, the imbalance in ion concentrations and the selective permeability of the neuronal membrane to potassium ions, combined with the action of the sodium-potassium pump, result in a net negative charge inside the neuron, creating the resting membrane potential of approximately -70mV. This negative charge is essential for generating action potentials (nerve signals).
- The Inside of the Neuron: Develops a net negative charge compared to the outside.
- The Outside of the Neuron: Develops a net positive charge.
