OPTION 2

•            A neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (−70 mV, note that this number varies by neuron type and by species).

•            This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium.

•            Because ions cannot simply cross the membrane at will, there are different concentrations of several ions inside and outside the cell.

 

 

Key Concept:

 

The resting membrane potential is a result of different concentrations inside and outside the cell. The difference in the number of positively charged potassium ions (K+) inside and outside the cell dominates the resting membrane potential.

If K+ can cross via channels, it will begin to move down its concentration gradient and out of the cell. The movement of K+ ions down their concentration gradient creates a charge imbalance across the membrane.

The charge imbalance opposes the flow of K+ down the concentration gradient.