It is over 60 years since Hodgkin and Huxley1 made the first direct recording of the electrical changes across the neuronal membrane that mediate the action potential. Using an electrode placed inside a squid giant axon they were able to measure a transmembrane potential of around 260 mV inside relative to outside, under resting conditions (this is called the resting membrane potential). The action potential is a transient (,1 millisecond) reversal in the polarity of this transmembrane potential which then moves from its point of initiation, down the axon, to the axon terminals. In a subsequent series of elegant experiments Hodgkin and Huxley, along with Bernard Katz, discovered that the action potential results from transient changes in the permeability of the axon membrane to sodium (Na+) and potassium (K+) ions. Importantly, Na+ and K+ cross the membrane through independent pathways that open in response to a change in membrane potential. As testimony to their pioneering work, the fundamental mechanisms described by Hodgkin, Huxley and Katz remain applicable to all excitable cells today. Indeed, the predictions they made about the molecular mechanisms that might underlie the changes in membrane permeability showed remarkable foresight. The molecular basis of the action potential lies in the presence of proteins called ion channels that form the permeation pathways across the neuronal membrane. Although the first electrophysiological recordings from individual ion channels were not made until the mid 1970s,2 Hodgkin and Huxley predicted many of the properties now known to be key components of their function: ion selectivity, the electrical basis of voltage-sensitivity and, importantly, a mechanism for quickly closing down the permeability pathways to ensure that the action potential only moves along the axon in one direction.

1. Action Potential
NEHA AGARWAL
155066
B.SC. HONS
DEI , AGRA

2. Communicate
 Neurons communicate by means of an electrical
signal called the Action Potential
 Action Potentials are based on movements of ions
between the outside and inside of the axon
 When an Action Potential occurs, a molecular
message is sent to neighboring neurons
 Action Potential is an All or Nothing Process
(like a gun firing)

3. HOW THIS POTENTIAL
IS GENERATED

4. To understand action potential, it
is important to understand how
the potential is maintained across
the membrane

5. Na+
Na+
Na+
Na+
Na+
Na+
Na+ =sodium
ions
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na
+
Na+
Cl-
Cl- Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl- = chloride ion
Cl- Cl-
Cl-
Cl-
K+K+
K+
K+
K+
2 K+
K+
K+
K+
K+
K+
K+
K+
K
+
K+
K+
K+
K+
K+ = potassium ion
K+
K
+
K
+
K+
K+ K+
K+K+
K+
Na+
3
Na+
A- = organic
A-
A-
A-
A
-
A- A- A-
A-
E.C.F.
I.C.F.
Potassium pump
Sodium pump
Na/k pump
Na+
POLARISATION

6. MEMBRANE (Resting)
POTENTIAL
 Potential across membrane is called as membrane
potential.
 Inside cell: conc. Of potassium ions and organic
compounds is more than outside the cell ( negatively
charged)
 Outside cell: concentration of chloride ions and sodium
ions is more than inside the cell (positively charged)
 Sodium open/leaky channels: flow of Na+ occurs in
and out of cell
 Potassium open/ leaky channels: flow of K+ occurs
in and out of cell
 Sodium-potassium pump: voltage gated channel;
allow the efflux of 3 Na+ ions for influx every 2 K+ ions;

7.  All the above channels and pump maintains
the resting potential (electro-chemical
gradient) across the membrane i.e. -70 mV

8. Na+
Na+
Na+
Na
+
Na+ Na+
Na+
Na+
Na+
Na+
Na+
Na+Na+
Na+
Na+
Na+
Cl-
Cl-
Cl-Cl-
Cl-
Cl-
Cl-
Cl-
Cl- Cl-
Cl-
Cl-
K+K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K
+
K+
K+
K+
K+
K+
K
+
K+
K+
K+ K+K+
K+
K+
Na+
A-
A-
A-
A-
A-
A- A-
A-
E.C.F
.
I.C.F.
K+ voltage gated
channels
Na+
Na+
Na+
Na+Na+ Na+
Na+
Na+
Na+
Na+
DEPOLARISATION
Na+ voltage gated channels

9. DEPOLARISATION
 As potential strike -55 mV (threshold potential) , Na+
voltage gated channels open, and allow the influx of
Na+ ions into the cell.
 Due to influx of sodium ions, the potential across the
membrane increases.
 More increase in potential, more influx of ions
 This leads to change in charge across membrane.
 Inside_ +vely charged
 Outside_ -vely charged
 As this potential reaches +40 mV(overshoot), the ,
Na+ voltage gated channels closes, and the , K+
voltage gated channels opens

10. REPOLARISATION
 As this potential reaches +40 mV, the , Na+ voltage gated
channels closes, and the , K+ voltage gated channels
opens. This leads to repolarisation
 This leads to efflux of potassium ions
 Thus the potential across the membrane decrease
 Inside_ -vely charged
 Outside_ +vely charged
 As the potential reaches -70mV, the potassium voltage-
gated channels closes.
 However, due to gradual closing of channel, the is some
amount of leaked ions, due to which the potential
decreases below -70mV. This leads to
HYPERPOLARISATION

11. Na+
Na+
Na+
Na+
Na+ Na+
Na+
Na+
Na+
Na+
Na
+
Na+Na+
Na+
Na+
Na+
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl- Cl-
Cl-
Cl- K+K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K
+
K+
K+
K+
K+K+
K
+
K+
K+
K+ K+
K+
K+
K+
Na
+
A-
A-
A-
A
-A-
A- A-
A-
E.C
.F.
I.C.F.
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na+
Na+
Na+
REPOLARISATION
K
+
K+
K+
K
+
K
+
K
+
K
+
K+ K+
K+
K+
K+
K
+
K+
K+

12. HYPERPOLARISATION AND
REFRACTORY PERIOD
 Due to gradual closing of channel, the is some
amount of leaked ions, due to which the potential
decreases below -70mV. This leads to
HYPERPOLARISATION
 the refractory period and the axon cannot fire again
until it returns to resting potential (negative polarized
state).
 Thus, the membrane undergoes the refractory period.
 In the refractory period ,the axon cannot fire again
until it returns to resting potential (negative
polarized state). It lasts for 3-5msec
 As the resting potential is restored via open
channels and Na+/K+ pump, new action potential
is fired.

13. • Each spike is followed by a refractory
period.
• An absolute refractory period - it is
impossible to evoke another action potential –
during spike and right after it (Na channels
are open and after that inactivated)
• A relative refractory period - a stronger than
usual stimulus is required to evoke an action
potential (hyperpolarization; part of Na
channels recovered)

16. All-or-None Principle
Throughout depolarisation,
the Na+ continues to rush
inside until the action
potential reaches its peak
and the sodium gates
close.
If the potential cross -55mV ,
then the action potential
will reach to its fate, via
repolarisation and
hyperpolarisation.
If the depolarisation is not
great enough to reach
threshold, then an action
potential and hence an
impulse are not produced.
This is called the All-or-
None Principle.

20. Action Potential Within a
Neuron

21. - without the depression (an energy comes from the cell) along
nerve or muscle fibers
- a wave (a spot) of electrical negativity on the surface (electrical
positivity on the internal site of membrane) due to openning
and closing of voltage gated ion channels
Propagation of action potential – local currents
refractoriness

24. 1. Threshold is reached
2. +Na ions enter beginning of
axon
3. this triggers the next Na
gates to open.
4. As they open & allow in Na+,
5. previous gates begin
pumping the Na+ out.
6. Before the action potential
has reached the end, the
beginning of the axon is back
at resting potential & ready for
another firing.

27. Graded potentials