1) The resting membrane potential of a mammalian nerve cell is -90mV, maintained by the sodium-potassium pump that pumps 3Na+ ions out of the cell for every 2K+ ions into the cell. 2) An action potential occurs when the membrane potential rapidly changes from negative to positive and back again. It is triggered when voltage-gated sodium channels open and allow sodium ions to rush into the cell, reversing the potential. 3) Voltage-gated potassium channels then open, allowing potassium ions to efflux and repolarize the membrane back to its resting potential. The sodium-potassium pump then restores the ion gradients across the membrane.

1. Department of Life Sciences
University of Calicut
Kerala, India 673 635

2. Electrical potential exist cross the membranes of all cells
Some cells are excitable
K+
concentration is greater inside the cell than outside
K+
moves out of the cell
Negative ions remain inside which prevent further movement of K+
to
the outside
Potential difference in large mammalian neurons is 94mv -negative
inside
Na +
ions are more outside than inside
Membrane is highly permeable to Na+
Na +
moves to the inside- inside become more positive
Na +
movement makes outside negative and inside positive
Now the potential difference is 61 mv with positive inside

3. Membrane Potentials
Resting membrane potential
in a mammalian nerve cell is
-90mv
Sodium –potassium pump –
Na+ to outside and K+ to
inside
Electronegative pump- more
positive charge pumped to
outside than to inside
3Na+ to outside for 2 K+ to
inside
Large concentration gradient
of K+ and Na+ inside and
outside

4. Electrochemical gradient
Na+ (outside cell) – 142mEq/L
Na+ (inside cell) –14mEq/L
K+ (outside cell) – 4mEq/L
K+ (inside cell) –140mEq/L

5. Channel proteins in rest
K+ and Na+ ions leak through channel proteins
Potassium-sodium leak channels
More K+ leak – 100 times than Na+

6. Channels and pumps

7. Action potential
Nerve signals are transmitted by action potentials
AP is a rapid change in the membrane potential
AP spreads rapidly along the nerve membrane
Sudden change from negative potential to positive
potential
Resting stage
Membrane remain
polarised
Postential at this
stage is -70mV

8. Depolarisation:
Membrane is very permeable to Na+ ions- large number of
Na+ moves into the cell
Charge inside become neutral
Potential rise in Positive direction – this is called
depolaristion
Some fibers, the
potential
overshoots and
reach positive
value

9. Repolarisation
The Na+ channel begin to
close
K+ channels open more
than normal
Rapid diffusion of K+ to the
exterior
Reestablish the normal
negative potential

10. Voltage gated channels
Voltage gated Na+
channels play a major role in
depolarisation and repolarisation during action potential
Voltage gated K+
channels also play major role in speeding
up the repolarisation
These are in addition to the Na+
-K+
pump and the Na+
-K+
leak channels

11. Voltage gated Na+ channels

12. Voltage gated Na+ channel –activation-
inactivation
AS voltage reach between -70 and -50- sudden
conformational change in activation gate
Gate opens
This is activated state of the gate
Na+
flow into the cell
 Na+
permeability increase 500-5000 fold
After a few 10,000ths of a second the inactivation gate closes
Na+
can not move into the cell
Repolarisation starts
 channels can open only after reaching resting potential
stage

13. Voltage gated K+
channels
During resting state the K+
channel remain closed
K+
can not pass out of the cell through the membrane
As membrane potential goes up from -90 – gate opens
slowly
K+
diffuse out
As opening of K+
is slow by the
Time they are open Na+ channels
Begin to close
This cause repolarisation

14. Action Potentials
An action potential occurs when there is a reversal of
the normal resting potential, going from negative to
positive. Also called depolarization.
Depolarization occurs when a stimulus causes the
voltage-gated Na+
channels to open, allowing Na+
to
rapidly influx down its concentration gradient.
The sudden in-rush of positive sodium ions reverses the
membrane potential for a few milliseconds.
Then the voltage-gated K+ channels open, allowing K+
to rapidly efflux due to its concentration gradient. This
brings the membrane back to negative inside and is
called repolarization.

16. Action Potentials
Even though the voltage has returned to
negative, the membrane is not at resting
potential because it now has too much Na+
inside
and not enough K+
ions.
The presence of high Na+
inside causes the
Na+
/K+
pumps to increase by a power of 3. The
faster pump rate quickly restores the membrane
back to its steady-state resting condition.

17. Sodium channels have 2
gates, a normal voltage
(activation) gate (which is
closed at rest) and an
inactivation gate (which
is open at rest). The rapid
opening of the voltage
gate lets Na+
rush in and
depolarizes the cell. This
is immediately followed
by closing of the
inactivation gate which
stops the Na+
influx. At
the same time the K+
gate
opens letting K+
efflux
(repolarization).
Widmaier, et al., 2006