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!!!!