Comprehensive Overview of Membrane and Action Potentials in Neurons

Membrane
Potentials and
Action Potentials

Basic Physics of
Membrane
Potentials

Membrane potentials
Caused by Ion concentration
• Differences across a selectively
permeable membrane
• Normal mammalian nerve fiber,
potential difference is about 94 mv
with negativity inside the fiber
membrane

Nernst Equation
The diffusion potential level across a membrane that
exactly oppose the net diffusion of a particular ion
through the membrane
Potential inside the membrane

Goldman Equation
Depends on 3 factors
1. Polarity of the electrical charge of each ion
2. Permeability of the membrane to each ion
3. Concentrations of the respective ions on the inside
and outside of the membrane

Resting
Membrane
Potential of
Neurons

Origin of the Normal Resting
Membrane Potential
1. Contribution of the K+
Diffusion Potential
2. Contribution of Na+
Diffusion through the nerve
potential
3. Contribution of the Na+
-K+
Pump

Membrane Potential
Diffusion Potential
• High ratio of K+
(35:1)
• Nernst = -94 mv
potential
-K+
Pump

K+
leak channel
Leaks K+
even in a resting cell
May also leak Na+
slightly
100x more permeable to K+
than Na+

Membrane Potential
Diffusion Potential
potential
• Na ratio 0.1
• Nernst = +61 mv
• Goldman = -86 mv
-K+
Pump

Membrane Potential
Diffusion Potential
potential
-K+
Pump
• Provides additional contribution to resting potential
• Causes continual loss of + charges from inside of the cell
• Additional -4 mv

The Na+
-K+
Pump
Present in all cell membranes
Continually transports Na+
outside
and K+
inside
Electrogenic pump – more positive
charges are pumped to the outside
than to the inside = net positive
inside

Resting Stage
Resting membrane potential before
the action potential begins
Membrane is “polarized” during this
stage

Excitation
Process of eliciting the action
potential
Any factor that causes Na+
to begin to
diffuse inward through the
membrane in sufficient numbers can
set off automatic regenerative
opening of the Na channels
May be mechanical, chemical, or
electrical
Mechanical – touch receptors,
mechanoreceptors
Chemical – neurotransmitters from
one neuron to the next in the brain
Electrical – electrical current to
transmit signals between successive
muscle cells in the heart and
intestine

Threshold for Excitation
The level required to elicit an action
potential
Subthreshold potentials do not elicit
a full action potential
Weak stimulus may not be enough
the excite a fiber

Depolarization Stage
Membrane suddenly becomes
permeable to Na+
Na+
diffuse to the interior of the axon
Potential is rising rapidly in the
positive direction
May cause overshoot in some (large)
nerve fibers

Refractory Period
• While the membrane is still depolarized, a new action potential cannot occur
• Na and Ca channels are inactivated and no amount of excitatory signal
applied will open inactivation gates
• Absolute refractory period
• Period during which a second action potential cannot be elicited, even with
a strong stimulus

Repolarization Stage
Na+
channels begin to close and K+
channels open
Rapid diffusion of K+
to the exterior
re-establishes the normal negative
resting membrane potential

Voltage-Gated Sodium Channel
Activation
• Membrane potential becomes less
negative than resting state
• Activation gate opens between -70
mv and -50 mv
• Na+
can pour inward increasing
Na+
permeability to as much as
500-5000 fold

Voltage-Gated Sodium Channel
Inactivation
• Inactivation gate closes a few
10,000ths of a second after
activation gate opens
• Na+
can no longer pour to the
inside of the membrane
• Membrane potential begins to
return toward the resting
membrane state (repolarization)

Voltage-Gated Potassium Channel
Resting state
• K+
channel is closed
• Depolarization causes conformational
change opening gate allowing K+
to
diffuse towards the outside of the cell
• Opens around the same time as Na+
inactivation gate closes speeding
repolarization process
• Full recovery of resting membrane
potential within another few
10,000ths of a second

Voltage-gated Calcium Channel
• Serves along with (or instead of) Na+
in some cells to
cause most of the action potential
• Internal < external
• Depolarization causes influx of calcium into the cell
• Known as “slow channels”
• Slower than Na+
channels
• Provides a more sustained depolarization

Voltage-gated Calcium Channel
• Deficit of calcium ions lead to hyperexcitability of Na+
channels
• Ca2+
binds to the exterior surfaces of the Na+
channel
protein molecule altering the electrical state of the
sodium channel, thereby lowering the voltage level
required to open the sodium gate
• Fall of 50% below normal may start causing
spontaneous discharges leading to muscle “tetany”

Chvostek’s sign and Trousseau’s
sign

Initiation of the Action Potential
1. Positive-Feedback Cycle opens the Na+
channels
2. Threshold for initiation of the Action Potential
• Usually around -65 mv
“All or Nothing Principle”
- Once the threshold is reached, Action potential is
propagated regardless of the strength of the stimulus

Direction of Propagation
• No single direction of propagation
• Action potential travels in all
directions away from the stimulus
• Propagation happens until the
entire membrane is depolarized
• Transmission of the depolarization
along a nerve or muscle fiber is
called a nerve or muscle impulse

Plateau in some Action Potentials
• Plateau greatly prolongs the
period of depolarization
• Usually occurs in heart muscle
fibers (0.2-0.3 s)
• Fast channels (Na+
) cause spike
• Slow channels (Ca2+
) cause
plateau

Repetitive Discharge
Occurs normally in the heart, most
smooth muscle, and in many
neurons
1) Rhythmical beat of the heart
2) Rhythmical peristalsis of the
intestines
3) Rhythmical control of breathing

• “Resting” membrane potential is
only -60 to -70 mv – sodium and
calcium channels are not totally
closed
1. Some Na+
& Ca2+
flow inward
2. Increased membrane voltage,
increasing membrane
permeability
3. More ions flow inward
4. Permeability increases more until
an action potential is generated

• Increased permeability to K+
during repolarization causes
Hyperpolarization
• During hyperpolarization, self-re-
excitation will not occur

Special
Characteristics
Nerve trunks

Myelination
• Larger fibers have a covering
called Myelin sheath
• Act as an insulating layer around
the nerve fibers, allowing for rapid
and efficient transmission of
electrical impulses along the axon
• Nodes of Ranvier
• found about every 1-3 mm
• Small uninsulated area

Saltatory Conduction
• Action potentials occur only at the
nodes
• Nerve impulse jumps along the
fiber
• Increases velocity of nerve
transmission by as much as 5-50x
• Conserves energy for the axon
because only the nodes
depolarize, little energy
expenditure during re-
establishing resting membrane
potential
• Velocity:
• Myelinated: 100m/sec

Thank you!
Karlomagno C.
Alejandrino MD-MBA,
FPNA
kalejandrino@ateneo.edu

Comprehensive Overview of Membrane and Action Potentials in Neurons