Membrane Potential 6 / 5 /10

The cell membranes of all body cells in the resting condition are, polarized which means that they show an electrical potential difference, commonly used term for potential difference is only potential.

Membrane potential refers to a separation of charges across the membrane or a difference in the relative number of cations and anions in the ICF and ECF. Potential is measured is unit of volts but because the membrane potential is relatively low, the unit used is the millivolt (mV) (1 mV = 1/1,000 volt).

Excitable Tissues Tissues of the body can generate and propagate nerve impulses which are waves of electro-chemical activity along their membranes. These include nerve cells and muscle cells.

Membrane Potentials Caused by Diffusion Diffusion Potential caused by an ion concentration difference on the two sides of the membrane. Potassium Ion Potassium concentration is great inside a nerve fiber membrane but very low outside the membrane.

Because of the large potassium concentration gradient between inside and outside the membrane. There is a strong tendency for extra numbers of potassium ions to diffuse outward through the membrane as the membrane at this instant is freely permeable to the potassium ions but not to any other ion.

K-ions move outside i.e positive electrical charges go to the outside, thus creating electropositivity on the outside the membrane and electronegativity is created inside because of negative anions that remain behind that do not diffuse outward with the potassium.

Within a millisecond the potential difference between the inside and outside, called the diffusion potential, becomes great enough to block further net outward potassium diffusion. The potential difference is about 94 millivolts, with negativity inside the fiber membrane. (in the normal mammalian nerve fiber).

Sodium Ion : At this time There is high concentration of positively charged sodium ions on the outside as compared to inside the membrane. The membrane is highly permeable to the sodium ions but impermeable to all other ions.

Diffusion of the positively charged sodium ions to the inside creates a membrane potential that is negativity outside and positivity inside. Again, the membrane potential rises high enough within milliseconds to block further net diffusion of sodium ions to the inside.

The potential is about 61 millivolts positive inside the fiber (in the mammalian nerve fiber).

The excitable tissues have the ability to produce rapid, transient changes in their membrane potential when excited. These brief fluctuations in potential serve as electric signals.

Resting Membrane Potential The constant membrane potential present in the cells of excitable tissue when they are at rest i.e when they are not producing electric signals is termed as resting membrane potential.

Ions Involved In Memb. Potential The unequal disturbution of a few key ions between the ICF and ECF and their selective movement through the plasma membrane are responsible for the electrical properties of the membrane.

In the body, electric charge is carried by ions. The ions primarily responsible for the generation of the memb. potential are K-ion, Na-ion, Cl-ion. Other ions ( calcium, magnesium, phosphate, bicarbonate) do not make a direct contribution to the memb. potential.

Potassium ions are present in much higher concentration in the ICF whereas, Sodium ions are present in great conc. in the ECF. unit used for these ions is mmol/L.

Transport Of Na And K-ions

Various ions tend to diffuse from one side of the membrane to the other depending upon their Electrochemical gradients. The permeability of cell membrane to these ions which varies greatly.

Diffusion of K+ and Na+ ions can take place through the channels present in the cell membrane which are called K+ and Na+ ion leak channels. Na and K-ion inaddition, to the active carrier mechanism can passively cross the memb. through protein channels that are specific for them.

K-ion can cross more easily as the memb. has more channels open for it. Also at resting potential in a nerve cell membrane is 50-70% more permeable to K-ion than Na-ion.

There are still other processes by which these ions can move across the cell membrane these are: The voltage-gated Na+ and K+ channels. The electrogenic Na+ K+ ATPase pump.

The effects of an unequal concentration of ions on the two sides of semipermeable membrane can be analyzed as. Suppose the membrane is only permeable to K+ ions. K+ ions are present in higher concentration inside, therefore, they tend to diffuse along their concentration gradient into the extracellular fluid.

But soon an equilibrium is reached at which the efflux of K+ ions out of the cell stops. The membrane potential at which this equilibrium will exist is called the Equilibrium potential for K+.

Other names for equilibrium potential are Nernst or reversal or diffusion potentials. It is obvious that at the equilibrium potential for K+ ions, the cell membrane will become impermeable to K-ions.

The magnitude of the net forces tending to move each ion across the membrane is determined by the resting potential of the cell membrane and the equilibrium potential for that ion. Actually it is the difference between these two values.

The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the Nernst potential for that ion.

The magnitude of this Nernst potential is determined by the ratio of the concentrations of that specific ion on the two sides of the membrane.

End Of Todays Lecture!!!!