Dr.Sidra Qaiser

Dr.Sidra Qaiser
Learning Objectives
 Students should be able to:
 Define resting membrane potential and how it is
 Relate Nernst Equilibrium potential for sodium,
potassium and chloride ion with resting membrane
 Describe the role of leak channels and sodium
potassium pump in the generation of resting membrane
 Students should be able to:
 Define action potential.
 Describe the phases of action potential.
 Explain the ionic basis of electrical events in
an action potential and types of channels
involved in it.
 Describe the properties of action potential (all or
none law, monophasic biphasic and compound
potentials, electro tonic potential) and variation in
action potential in different tissues like smooth,
skeletal and cardiac muscles.
 Illustrate difference between graded potential and
action potential with the few examples (motor end
plate potential, excitatory post synaptic potential,
inhibitory post synaptic potential).
 How action potential is propagated through
mylinated and unmylinated nerve fibers.
 What are the factors affecting the spread of
conduction of action potential.
 Animation
What is the difference in ionic
composition of ICF and ECF?
What is potential difference?
What is the charge on cell
What is membrane potential?
 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.
Basic Physics of Membrane
 Diffusion potential
 Equilibrium potwential
Nernst Equation
 Relation of diffusion potential to the concentration
difference…… resulting in Nernst (equilibrium)
 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 shows the polarity inside the cell.
For potassium
 If Ko = 5 mM and Ki = 140 mM
EK = -61 log(140/4)
EK = -61 log(35)
EK = -94 mV
For Sodium
 If Nao = 142 mM and Nai = 14 mM
EK = -61 log(14/142)
EK = -61 log(0.1)
EK = +61 mV
Role of multiple ions
Factors Affecting RMP
 3 factors
 Polarity of each ion
 Membrane permeability of the ions
 Concentrations of respective ions on both sides: (i=
inside), (o= outside)
 Vm=
- 86 mV
What is the role Na-K pump?
 Electrogenic pump
 Concentration gradient
 Contributes -4mV.
Action potential
These are rapid transient changes in the
membrane potential that spread rapidly
along the nerve fiber membrane .
Graded potentials
Graded potentials
Stages of Action potential
 Afterdepolarisation: The descending limb of action
potential dosenot reach to the baseline abbruptly, but
it shows a delay of few seconds.
 Decrease rate of K efflux.
 Afterhyperpolarisation: The descending limb of
action potential dips a little below the baseline of
 Continued K efflux.
Latent period
 After a stimulus is applied to a nerve, there is a latent
period before the start of the action potential. This
interval corresponds to the time it takes the impulse to
travel along the axon from the site of stimulation to
the recording electrodes. Its duration is proportionate
to the distance between the stimulating and recording
electrodes and inversely proportionate to the speed of
Propagation of Action potential
Unmyelinated nerve fiber
Myelinated nerve fiber
Effects of myelination
 Velocity
 Energy
 Sites
Effect of electrolytes
 Sodium:Decreasing the external Na+ concentration
reduces the size of the action potential but has little
effect on the resting membrane potential. The lack of
much effect on the resting membrane potential would
be predicted, since the permeability of the membrane to
Na+ at rest is relatively low.
 Potassium:Conversely, increasing the external K+
concentration decreases the resting membrane
 Calcium
 Negative ions:Intracellular proteins
Magnitude of stimulus
 Sub threshold stimulus
 Threshold stimulus
 Suprathreshold stimulus
Magnitude of stimulus
 It is possible to determine the minimal intensity of
stimulating current (threshold intensity) that,
acting for a given duration, will just produce an action
 Action potential fails to occur if the stimulus is
subthreshold in magnitude,produces graded
 Suprathreshold stimuli produce action potential
during relative refractory period.
Refractory period
"All-or-None" Law
 The action potential fails to occur if the stimulus is
subthreshold in magnitude, and it occurs with
constant amplitude and form regardless of the
strength of the stimulus if the stimulus is at or above
threshold intensity. The action potential is therefore
"all or none" in character and is said to obey the all-ornone law.
 Slowly rising currents fail to fire the nerve because the
nerve adapts to the applied stimulus, a process called
 Ionic basis:
Monophasic action potential
Biphasic action potential
Compound Action potential
Cardiac muscles
 Plateau greatly prolongs the period of
 This type of action potential with plateau is seen
in heart muscle fibers.
 Opening of fast channels causes the spike portion
of the action potential.
 The slow, prolonged opening of the slow calcium-
sodium channels mainly allows calcium ions to
enter the fiber.
 This is largely responsible for the plateau portion
of the action potential.
Smooth muscles
 Sensitive to stretch
 Slow wave potential
 Spike potential