All cells exhibit electrical properties. The inside of most cell membranes is negatively charged compared to the outside of the cell mem-brane, which is positively charged. This uneven distribution of charge means the cell membrane is polarized. In an unstimulated (or resting) cell, the uneven charge distribution is called the resting membrane potential. The outside of the cell membrane can be thought of as the positive pole of a battery and the inside as the negative pole. Thus, a small voltage difference, called a potential, can be measured across the resting cell membrane.
The resting membrane potential is generated by three main fac-tors: (1) a higher concentration of K+ immediately inside the cell membrane, (2) a higher concentration of Na+ immediately outside the cell membrane, and (3) greater permeability of the cell membrane to K+ than to Na+ (figure 8.7, step 1). Thus, the resting membrane potential results from differences in the concentration of ions across the membrane and the permeability characteristics of the membrane.
The difference in membrane permeability is due to the differ-ence in the number of open ion channels. Recall that ions cannot move freely across the cell membrane; instead, ions must flow through ion channels, which are proteins embed-ded in the cell membrane. Ions flow through channels due to the differences in their concentration across the membrane. There are two basic types of ion channels: leak channels and gated channels (see figure 3.5). Leak channels are always open. Thus, as the name suggests, ions can “leak” across the membrane down their concentration gradient. When a cell is at rest, the membrane potential is established by diffusion of ions through leak channels.
Because there are 50–100 times more K+ leak channels than Na+ leak channels, the resting membrane has much greater perme-ability to K+ than to Na+; therefore, the K+ leak channels have the greatest contribution to the resting membrane potential. Gatedchannels are closed until opened by specific signals. Chemically gated channels are opened by neurotransmitters or other chemi-cals, whereas voltage-gated channels are opened by a change in membrane potential. When opened, the gated channels can change the membrane potential and are thus responsible for the action potential, described next.
As mentioned earlier, there is a net negative charge inside and a net positive charge outside a resting cell. The primary source of negative charge inside the cell is a high concentration of negatively charged molecules, such as proteins, that cannot diffuse out of the cell because the cell membrane is impermeable to them (figure 8.7, step 2). Consequently, as positively charged K+ leaks out of the cell via the leak channels, the charge inside the cell membrane becomes even more negative. The negatively charged molecules inside the cell tend to attract the positive K+ back into the cell. The resting membrane potential is the point of equilibrium at which the tendency for K+ to move down its concentration gradient out of the cell is balanced by the negative charge within the cell, which tends to attract the K+ back into the cell (figure 8.7, step 2).
To compensate for the constant leakage of ions across the membrane, the sodium-potassium pump (Na+–K+ pump) is required to maintain the greater concentration of Na+ outside the cell mem-brane and K+inside. The pump actively transports K+ into the cell and Na+ out of the cell (figure 8.7, step 3). The importance of this pump is indicated by the astounding amount of energy it consumes. It is estimated that the sodium-potassium pump consumes 25% of all the ATP in a typical cell and 70% of the ATP in a neuron.