Variations in Membrane Potentials

The resting membrane potential and membrane excitabil­ity can be altered by changes in ion concentration or mem­brane permeability. For example, calcium ions decrease membrane permeability to sodium ions, and vice versa. If insufficient calcium ions are available, as in severe hypocal-cemia, the permeability to sodium increases. As a result, membrane excitability increases—sometimes causing spon­taneous muscle movements (tetany) to occur. Local anes­thetic agents (e.g., procaine, cocaine) act directly on neural membranes to decrease their permeability to sodium.

CLINICAL APPLICATION

The Nernst Equation for Calculating an Equilibrium Potential

The following equation, known as the Nernst equation, can be used to calculate the equilibrium potential (electromotive force [EMF] in millivolts [mV] of a univalent ion at body temperature of 37°C).

EMF (mV) = −61 × log₁₀ (ion concentration inside/ ion concentration outside)

For example, if the concentration of an ion inside the membrane is 100 mmol/L and the concentration out­side the membrane is 10 mmol/L, the EMF (mV) for that ion would be −61 × log₁₀ (100/10 [log₁₀ of 10 is 1]). Therefore, it would take 61 mV of charge inside the membrane to balance the diffusion potential created by the concentration difference across the membrane for the ion.

The EMF for potassium ions using a normal estimated intracellular concentration of 140 mmol/L and a normal extracellular concentration of 4 mmol/L is −94 mV:

−94 mV = −61 × log1⁰ (140 mmol inside/4 mmol outside)

This value assumes the membrane is permeable only to potassium. This value approximates the −70 to −90 mV resting membrane potential for nerve fibers measured in laboratory studies.

When a membrane is permeable to several different ions, the diffusion potential reflects the sum of the equilibrium potentials for each of the ions.

In summary, movement of materials across the cell's mem­brane is essential for survival of the cell. Diffusion is a process by which substances such as ions move from areas of greater concentration to areas of lesser concentration to attempt uniform distribution. Osmosis refers to the diffusion of water molecules through a semipermeable membrane along a con­centration gradient. In facilitated diffusion, molecules that can­not normally pass through the cell's membranes can do so with the assistance of a carrier molecule. Diffusion of water molecules by osmosis or ions by facilitated diffusion does not require an expenditure of energy by the cell and is therefore passive in nature. Another type of transport, called active trans­port, requires the cell to expend energy in moving ions against a concentration gradient. Two types of active transport exist, primary and secondary; both require carrier proteins. The Na+/K+ ATPase pump is the best-known type of active trans­port. It is estimated that up to one third of the energy expen­diture of the cell is used to maintain the Na+/K+ ATPase pump.

Endocytosis is a process by which cells engulf materials from the surrounding medium. Small particles are ingested by a process called pinocytosis and larger particles by phagocytosis. Some particles require bonding with a ligand, and the process is called receptor-mediated endocytosis. Exocytosis involves the removal of large particles from the cell and is essentially the reverse of endocytosis.

Ion channels are integral transmembrane proteins that span the width of the cell membrane and are normally com­posed of polypeptide or protein subunits that form a gating system. Many ions can diffuse through the cell membrane only if conformational changes occur in the membrane pro­teins that comprise the ion channel. Two basic groups of ion channels exist: leakage channels and gated channels.

Electrical potentials (negative on the inside and positive on the outside) exist across the membranes of most cells in the body. These electrical potentials result from the selective per­meability of the cell membrane to Na+ and K+; the presence of nondiffusible anions inside the cell membrane; and the activ­ity of the sodium-potassium membrane pump, which ex­trudes Na+ from inside the membrane and returns K+ to the inside. An equilibrium or diffusion potential is one in which no net movement of ions occurs because the diffusion and elec­trical forces are exactly balanced.