The separation of charge is due to the membrane's differential permeability to sodium and potassium ions. The neuron and muscle cell membrane is very permeable to potassium ions and is relatively impermeable to sodium ions. Because of this, potassium ions are able to move back and forth across the plasma membrane along their diffusion gradient. However, sodium ions are not able to move back and forth across the membrane at will. Therefore, the sodium ions must remain outside the cell. This means that of the two ion species, there are both sodium and potassium ions outside the cell and mostly potassium ions inside the cell. This differential permeability of the membrane to these two ions creates the membrane potential.
Figure 1. Electrically Active Cell Membranes. Cells in our bodies have more sodium (Na+) on the outside of the cells and more potassium (K+) on the inside of the cells. Since these are ions (charged atoms) they need a protein channel to cross the membrane, and are unable to move across on their own. Therefore, more Na+ stays on the outside and more K+ stays on the inside unless a chemical or electrical change occurs at the membrane.
This membrane potential can be measured using a device similar to a battery meter. However, instead of measuring the potential in volts, millivolts are used. A millivolt is 1/1000 a volt. To measure the membrane potential, an electrode must be inserted into the cell and just outside the cell membrane. This is a tricky procedure that requires a good microscope, a micromanipulator, microelectrodes, and a millivoltmeter. The potential that exists across a typical membrane has a magnitude of 70 mV. Because the inside of the cell is less positive than the outside, the membrane potential is said to be -70mV. The symbol in front of the number indicates the relative charge on the inside of the cell. The inside of the cell is 70 mV less positive than the outside of the cell. This is a typical resting membrane potential for a neuron. The resting membrane potential describes the potential of a cell membrane that is not being stimulated.
Figure 2. The nervous system can be divided into regions that are responsible for sensation (sensory functions) and for the response (motor functions). Sensory input needs to be integrated with other sensations. Some regions of the nervous system are termed integration or association areas; these use the interneurons of the central nervous system to communicate. The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response.
