Membrane Potentials and
Action Potentials
Nerve Potentials
Nernst Equation




Relation of diffusion potential to the
concentration difference…… resulting in
Nernst (equilibrium) potential
For any univalent ion at body temperature of
37° C
EMF (mV)= +/-61log (Conc.inside/Conc.outside)
Calculate for K+ and Na+



K= -61log(140/4)
Na= -61log(14/142)
Sign is –ve for +ve ion and vice versa
Multiple Channels
Multiple ions and diffusion potential

3 factors




Polarity of each ion
Membrane permeability of the ions
Concentrations of respective ions on both
sides: (i= inside), (o= outside)
Goldman-Hodgkin-Katz equation
EMF (mV)= -61.log
CNa i.PNa+CK i.PK+CCl o.PCl
CNa o.PNa+CK o.PK+CCl i.PCl
Measuring membrane potential
Nerve

A nerve cell (neuron) has two major
functions:


Propagation of an action potential (nerve
impulse, signal) along its axon.
Transmission of this signal from one neuron
to another neuron or to an effector cell to elicit
a response.
Resting membrane potential of
nerves
Na-K pump (active
transport)
Ratio with units in mEq/L
 Na:
inside(14)/outside(142)
= 0.1
 K:
inside(140)/outside(4)=
35.0

Resting Membrane Potential of a nerve

Contribution of





K (-94mV)
Na (61mV)
Na and K (-86 mV)
Na and K and Na-K
pump (-90 mV)
Goldman equaiton
says it is -90 mV
Nerve action Potential


Action potential
Stages




Resting
Depolarisation
Repolarization
Voltage gates and pump
Action Potential and Voltage gated
channels
Voltage Clamp
Changes in Na-K conductance and their ratio
Positive
afterpotential
 Hyperpolarization
 Positive feedback
opens Na
channels
 Threshold

Role of ions


Impermeant ions
Calcium ions




Ca pump
Ca – Na Channels (slow and fast)
Role of Ca in Na permeability
↓ Ca → ↑Na excitibility
Initiation of Action Potential


Positive feedback
Threshold

Leads to nerve or
muscle impulse
Opening
of Na+ channels generates local current circuit
Propagation:
that depolarizes adjacent membrane, opening more Na+
channels…
Rest
Stimulated
(local depolarization)
Propagation
(current spread)
Propagation of Action Potential



Mechanism
Direction
All or none principle

Extracellularly recorded APs
-
Most text books show intracellularly
recorded action potentials
 such recording are usually not made in
clinical practice


extracellular recordings are made
a so-called ‘bi-polar’ action potential is
seen
b
Stimulus
artefact
NOTE:
-
2 mV
a
e
c
d
+

Why does the action potential look
like this?
+
a
+++++ -------------------- +++++++++++
-
+
b
---+++++++ -------+++---------+++++++
-
+
c
------ +++++++----+++++ ---------++++
- +
d
------------ +++++++
++++++++++ ---------
-
+
e
----------------++++
+++++++++++++----direction
Re-esablishment of normal ionic gradients



Role of Na-K ATPase pump
ATP use
Plateau in some action potentials


Fast channels
Slow channels
Resting and action potentials
•
There are some terms that need to be understood & remembered:
– excitability
– depolarization
– hyperpolarization
– overshoot
• means positive to 0
mV
– repolarization
overshoot
0 mV
repolarization
• towards resting
potential
– threshold (for action
potential generation)
-90 mV
threshold
depolarization
hyperpolarization
excitability
+
resting
potential
Rhthmicity- Repititive
Discharge
Refractory Periods
Threshold
mV
0
-40
-80
0
1
Absolute
2
3
Relative
4
5 msec
ARP - due to voltage inactivation of Na channels
Refractory periods limit maximum frequency of APs
ACTION POTENTIAL
GRAPH of Action Potential
Direction of spread and role of
refractory period
Monophasic and Biphasic Action
Potential
Monophasic
and Biphasic
Graded and Action Potential
Propagation of
Action
Potential
Action Potential and Twitch
All or Nothing Principle

Depolarization either travels across entire
membrane or does not travel at all during
action potential
Re-esablishment of normal ionic
gradients
• Role of Na-K ATPase
pump
• ATP use
• Plateau in some
action potentials
– Fast channels
– Slow channels
Innamal Aamalu
Biniyyaat
Thank you