Cell Membrane Potentials

Our first topic in Veterinary Physiology 1 deals with Nerves and Synapses and contains several subtopics. This post will be discussing Cell Membrane Potentials. 

Ion Concentrations

Cells use many different ions for many different functions. Ions exist inside and outside the cell in the intracellular and extracellular fluids respectively in varying concentrations. Inside the cell, potassium ions (K+) exist in high concentration while other ions such as sodium (Na+), chloride (Cl-), and calcium (Ca 2+) exist in low concentrations. In the extracellular fluid, sodium, chloride and calcium ions exist in high concentrations while potassium is in low concentration. An easy way to remember this is to use the special K rule. With this rule, all the ions (Na+, Ca 2+, and Cl-) except K+ are at low concentrations inside the cell. The opposite applies when you consider the ion concentrations outside the cell.

The Effect of Electrical and Chemical Forces

Now, electrical and chemical forces affect the movement of these ions into and out of the cell. If a chemical gradient (a concentration gradient) for a particular ion occurs the ion will flow from the area where there is the highest concentration of that ion to the area where the concentration is lowest. For example, if sodium ion channels are opened in the cell membrane, sodium will flow into the cell. This is because there is a high concentration of sodium outside the cell and a low concentration inside the cell. 

Cells have what's called a membrane potential (Vm). A membrane potential is the difference in charge between the inside and outside of a cell at any point in time and is measured in mV. The charge of an ion can affect its direction of flow. If the membrane potential of a cell is negative and the ion is positive the ion will flow inside the cell. This is because opposite charges attract. If the Vm of a cell is negative and the ion is negative the ion will flow out of the cell. This is because like charges repel each other. 


There are some important concept that you need to understand. Firstly, an electrochemical gradient is the overall force on an ion due to a combination of electrical and chemical forces. Secondly, to determine the nett flow of a particular ion at a particular membrane potential we need to know the equilibrium potential. The equilibrium potential is the value of the membrane potential at which the chemical and electrical driving forces are equal in magnitude and opposite in direction, resulting on no nett movement of ions. Follow this link to a website which also explains equilibrium potentials and has some practise questions too.

Factors Determining Membrane Potential

Earlier, I described what a membrane potential is. There are two main factors which determine the membrane potential, solute concentration and membrane permeability. If a cell is permeable to more than one type of ion, the Vm will approach the equilibrium potential of the ion that has the highest permeability. 

Resting Membrane Potentials

The resting membrane potential (RMP) of a cell is the overall voltage across the cell membrane when it is not transmitting an electrical signal. The RMP is usually negative inside the cell relative to the outside. For most cells, the concentration gradient for K+ is slightly bigger than that for Na+. However, the cell membrane is much more permeable to potassium than it is to sodium. Potassium flows out the cell (efflux) because of its concentration gradient, leaving a nett negative charge on the inside of the cell. This results in an electrical gradient for both K+ and Na+ to flow into the cell (influx). At a particular value of Vm, the K+ efflux (due to its concentration gradient) will be roughly equal to the influx of Na+ (due to electrical and concentration gradient) and K+ influx (due to electrical gradient). This value of Vm is called the resting membrane potential. The membrane will be relatively stable at the RMP.     



The Sodium/Potassium Pump

Although the influx of Na+ and K+ and the efflux of K+ are roughly equal there is still a nett movement of Na+ and K+ at the Resting Membrane Potential. If this occurs for too long, the composition of the intracellular and extracellular fluids may change or the cell membrane potential may decrease. to prevent this the cell uses the Na/K ATPase pump to actively (requiring energy) move Na+ and K+ against their concentration gradients. In each cycle of the pump, 3 Na+ ions are pumped out of the cell while 2 K+ ions are pumped in. 

How the Na/K ATPase pump works