Final answer:
The electrical force draws positively charged ions, such as sodium (Na+), into the cell when the membrane potential is negative. A negative inside voltage of -70 mV, established by the sodium-potassium pump, creates a net negative charge within the cell. Sodium enters the cell via voltage-gated channels during depolarization, and potassium exits during repolarization to restore the resting membrane potential.
Step-by-step explanation:
Whenever the membrane potential is negative, the electrical force will draw positively charged ions, or cations, into the cell. A neuron at rest, known as the resting membrane potential, is negatively charged with an inside voltage of approximately -70 millivolts (-70 mV) compared to the outside. This voltage is created by differences in the concentrations of ions inside and outside the cell, and the selective permeability of the cell membrane to these ions.
The sodium-potassium pump helps to establish this potential by removing three Na+ ions (sodium) from the cell and bringing in two K+ ions (potassium), using one molecule of ATP for each cycle. Since potassium ions have a higher concentration inside the cell and the membrane is more permeable to K+ ions, they tend to move out of the cell more than Na+ ions move in, leading to a net negative charge inside. An electrical gradient and a chemical gradient, together known as the electrochemical gradient, influence ion movement across the membrane.
During depolarization, a stimulus causes sodium channels to open, allowing Na+ ions, which are higher in concentration outside the cell, to rush into the cell, drawn by both the concentration gradient and the electrochemical gradient. This influx of Na+ makes the inside of the cell less negative and can eventually lead to a positive membrane potential. Subsequent repolarization restores the resting potential by moving K+ ions out of the cell, balancing the charge once more.