Electrical Properties of Nerve
Cells
24.1.13 +29.1.13
The resting membrane potential
Action Potential of Excitable Cells
Graded and Action Potential
 Cells which can respond to a stimulus are said to be
excitable (nerve and muscle cells)
 There is a threshold value for the intensity of a
stimulus which can generate an action potential
 Stimulus less than threshold value will generate a
“graded potential” which cannot be transferred over
long distances
 When stimulus is strong enough it will generate an
“action potential” that can be transferred over large
distance
Dendrites: receive information and undergo graded
potentials.
Neuron
Axons: undergo action potentials to deliver information,
Excitable Nerve cell
A single neuron typically consists of three basic parts
1. Dendrites
2. The cell body
3. The axon
• Dendrites and cell body are specialized to receive
information and produce graded potentials:
In most neurons, the plasma membrane of the dendrites and
cell body contain protein receptors that bind chemical
messengers/neurotransmitters from other neurons
Graded potentials are produced in response to stimulus (here
neurotransmitter released from other neurons)
• Axons are specialized to deliver information by generation of
action potentials
Axon hillock is the portion of the axon where action potentials
are triggered or initiated by the graded potential if it is of
sufficient magnitude
The action potentials are then conducted along the axon from
axon hillock to axon terminals
Electrical Signal Generation
• Action potentials can be initiated only in portions of
the membrane with abundant voltage gated Na+
channels
• Sites of a nerve cell specialized for graded potentials
such as dendrites and cell body do not undergo
action potentials because they have less voltage
gated Na+ channels
• Graded potentials generated in response to a
stimulus can spread to adjacent areas of the
membrane before dying out
Electrical Signal Generation
• The graded responses produced throughout the
dendrites or cell body is summed spatially and
temporally, and if the summed response is large
enough to pass the threshold by the time it reaches
axon hillock, an action potential will be generated at
axon hillock.
• The axon hillock has the lowest threshold in the
neuron because this region has a much higher
density of voltage gated Na+ channels than
anywhere else in the neurons
• Action potential originates at axon hillock
Graded Potential
• They are local changes in membrane potential
that occur in varying grades or degrees of
magnitude or strength in response to a
stimulus
• For example membrane potential could
change from -70mV to -60mV (a 10mV graded
potential) or from -70mV to -50mV (a 20mV
graded potential)
• Graded potentials are usually produced by a specific triggering
event that causes gated ion channels to open in a specialized
region of the excitable cell membrane
• The resultant ion movement produces the graded potential
which most commonly is a depolarization resulting from Na+
entry
The size of a
graded potential
(here, graded
depolarizations)
is proportionate
to the intensity
of the stimulus.
The duration of a
graded potential
(here, graded
depolarizations)
is proportionate
to the duration of
the triggering event
Graded
potential spread
by passive
current flow
Graded Potential die out over short distance
• As the current flows along the membrane, some of the
current leaks through open leak channels (mainly K+) in the
neighboring areas. As a result the membrane potential
progressively decreases with increasing distance from the
source point
Length constant of a membrane
• This spatial pattern is exponential and the
distance where the voltage changes to 37% of
its original value is the “ length constant”
• Larger the length constant, farther the graded
potential can travel along the membrane
Spatial or temporal summation
of graded potentials
• Graded responses can interact with each other and can be
spatially or temporally summed
• If two graded potentials occur at the same time in close
enough /same places, their effects add up. This is called
“spatial summation”
• If two graded potentials occur at the same place in
succession, their effects add up. This is called temporal
summation
• As an analogy, spatial summation is like using many shovels
to fill up a hole all at once. Temporal summation is like using
a single shovel to fill up a hole over time. Both methods work
to fill up the hole
Panel 1:
Panel 2:
Panel 3:
Panel 4:
Panel 5:
Two distinct, non-overlapping, graded depolarizations.
Two overlapping graded depolarizations demonstrate temporal
summation.
Distinct actions of stimulating neurons A and B demonstrate spatial
summation.
A and B are stimulated enough to cause a suprathreshold graded
depolarization, so an action potential results.
Neuron C causes a graded hyperpolarization; A and C effects add, cancel
each other out.
Graded Potential
• Graded potential causes potential change in
limited areas
• The graded potential spreads along the
membrane by changing the charge on the
membrane capacitance and by flowing
through opened channels
Remember:
1. Membrane potential changes due to change in
stored charge on membrane capacitor
2. Membrane conductance changes due to flow of
ions through gated channels during graded and
action potentials
Action Potential
 Depolarization:

When a stimulus opens the voltage gated channels, both
Na+ and K+ channels get opened, however Na+ channels open
faster and are responsible for the rising phase of action
potential. The opening of Na+ channels will depolarize the
membrane potential which will cause more Na+ channels to
open

The membrane then becomes overwhelmingly permeable
to Na+ ions. Sodium ions diffuse into the cell down a
concentration gradient. The entry of Na+ disturbs the resting
potential and causes the inside of the cell to become more
positive relative to the outside. Membrane potential shoots up
to +30 mV

Once open, however the Na+ channels spontaneously
close by inactivation gate and they cannot open again until the
membrane potential returns to resting membrane potential
Action Potential
 Repolarization:
Closing of Na+ channels causes the membrane potential to
return to its resting level. In addition K+ channels start to open
slowly and this facilitates the falling phase. Membrane becomes
more permeable to K+ now than to Na+
Potassium rushes outside, membrane potential drops back
down to -70mV. Lots of sodium inside, lots of potassium outside
(opposite of the resting state at start)
 Hyperpolarization:
Potassium channels doesn't close fast enough, so the
membrane potential actually drops below the resting potential
for a bit. The outside becomes comparatively more positive
relative to the interior than that at the resting stage
Action potential mechanism
The rapid opening of
voltage-gated Na+ channels
allows rapid entry of Na+,
moving membrane potential
closer to the sodium
equilibrium potential (+40 mv)
A cell is
“polarized”
because
its interior
is more
negative
than its
exterior.
Repolarization is movement back
toward the resting potential.
The slower opening of
voltage-gated K+ channels
allows K+ exit,
moving membrane potential
closer to the potassium
equilibrium potential (-90 mv)
The rapid opening of voltage-gated Na+ channels
explains the rapid-depolarization phase at the
beginning of the action potential.
The slower opening of voltage-gated K+ channels
explains the repolarization and hyperpolarization
phases that complete the action potential.
Important
Potentials
•
•
•
•
Resting membrane potential is -70mV
Depolarization peak is at +40mV
Hyperpolarization peak is at -90mV
Threshold potential is about -55mV
• +40mV is close to the Na+ equilibrium
potential
• -90mV is the K+ equilibrium potential