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Chapter II Book Notes: Electrical Signaling of Nerve
Cells
9/29/16 3:11 PM
Electrical Potentials across Nerve Cell Membranes
When a neuron is at rest, there is a constant voltage across its membrane
which is called the resting membrane potential (-40 to -90mV)
• Electrical signals produced by neurons are responses to stimuli that
change resting membrane potential
Receptor potentials are due to the activation of sensory neurons by
external stimuli such as light, sound, or heat
• Receptor potentials can change resting membrane potential
because of sensory neurons
• Amplitudes are graded in proportion to the magnitude of the
sensory stimulus or synaptic potentials
Synaptic potentials allow transmission of information from one neuron to
another
• Brief change is resting membrane potential
• Serve as the means of exchanging information in complex neural
circuits
Action potentials conduct electrical signals over great distances despite
their own intrinsically poor electrical characteristics
• You can elicit an AP is to pass electrical current across the
membrane of the neuron
• Normal situations generate AP from synaptic or receptor potentials
• Hyperpolarization à makes membrane potential more negative

•

o Membrane potential would change in proportion to current
delivered
o These hyperpolarizing responses do not require specialties of
neurons so they’re called passive responses
Depolarizationà membrane potential of the nerve cell becomes

•

more positive
o If this depolarization hits the threshold potential, an AP is
fired
This is an active response that lasts about 1ms

•

Amplitude of the AP is independent of the magnitude of the current
to evoke it à all or none response

•

If the amplitude of stimulus is increased it means that multiple AP’s
will fire à intensity proportional to frequency

How Ionic Movements Produce Electrical Signals
Electrical potentials are generated across the membranes of neurons
because (1) there are differences in concentrations of specific ions across
cell membranes and (2) the membranes are selectively permeable to
some ions
Active transporters are proteins that establish the ion concentration
gradient because they move ions in or out of cells against their
concentration gradient
Ion channels are the cause of the membrane being selectively permeable
because they only allow certain ions to cross the membrane in the direction
of their gradient
• Transporters and channels work opposite of each other and thus
generate electrical signals
Example of tank with membrane in the middle that is only permeable to K+
ions
• K+ concentration is equal on each sideà no electrical potential
•

•

generated, no net flux of K+
K+ concentration increased by ten on left side à electrical potential
on left goes negative because ions flow to the right side down their
concentration gradient
Electrochemical equilibrium means there is an exact balance
between two forces (1) concentration gradient that causes the K+
movement from left to right (2) the opposing electrical gradient
that stops K+ from moving across the membrane

Forces that Create Membrane Potentials
The electrical potential generated across the membrane at electrochemical
equilibrium is called the equilibrium potential
• Conventionally defined as the potential difference between the
compartments

This can be predicted by the Nernst equation
o Ex = (RT/zF) ln ( [X]2 / [X]1)
o Ex is the equilibrium potential for any ion, X
o R = gas constant
o T = absolute temperature
o Z = valence of the permeant ion
o F = faraday constant (amount of charge in one mol)
o Simplfied = Ex = 58/z log ( [X]2 / [X]1)
o slope will be 58/z
If the right side was replaced with 10mM of Na+ and the left with 1mM of
Na+ ( ions were now sodium) the equilibrium potential would be +58mV
because the valence is +1
• Ca2+ ions it would be +29mV
• Cl – ion it would be +58mV again
•

Balance of chemical and electrical forces at equilibrium means that the
electrical potential can determine ion fluxes across the membrane while ionic
gradient can determine the membrane potential (voltage of the left side –
voltage of the right side)
• -58mV is the voltage needed to counter the difference in K+
concentrations on two sides of the membrane
• If the left side is initially made more negative that -58k that K will
flow right to left
• Battery off = net flux of K+ from left to right
• Battery on @ -58mV = No net flux of K+
• Battery on @ -116mV = net flux of K+ from right to left
Electrochemical Equilibrium in an Environment with more than one
Permeant Ion
Scenario:
10mM of K+ and 1mM of Na+ on left
10mM of Na+ and 1mM of K+ on right
• If the membrane was only permeable to K+, membrane potential =
-58mV

•
•
•

If the membrane was only permeable to Na+, membrane potential
= +58mV
If permeable to BOTH, it depends on how permeable the membrane
is to each ion ( it will either approach +58 or -58mV)
Use GOLDMAN Equation
o V = 58 log (Pk [K]2 + PNa [Na]2 + PCl [Cl]1/  Pk [K]1 + PNa [Na]1
+ PCl [Cl]2)
o V = voltage across membrane (left relative to right)
o P = permeability
o Negative ions are inverted

The Ionic Basis of the Resting Membrane Potential
Ion transporters creates substantial Transmembrane gradients for most ions
There is much more K+ on the inside
There is much more Na+ on the outside
Transporter dependent concentration gradients are the source of RMP and
AP
RMP of squid axon is ~ - 65mV, K+ is the ion that is closest to being in
electrochemical equilibrium when the cell is at rest
• This implies that the RM is most permeable to K+
• H&K found out that when extracellular K+ is raised to equal the
concentration of K+ inside the axon, the K+ equilibrium potential
goes to 0mV à so the RMP varied as predicted with log of [K+]
Value of RMPwas not exactly 58 because other ions have small
influence
H & K showed that the inside negative resting potential arises because
• (1) the membrane of the resting neuron is more permeable to K+
than anything else
• (2) more K+ inside than outside à K+ channels are open in resting
neurons à large K+ concentration is produced by membrane
•

transporters that selectively accumulate in K+ within neurons
The Ionic Basis of Action Potentials

What causes the membrane potential of a neuron to depolarize during an
action potential?
• Increased permeability to Na+ à
Lowering extracellular Na+ to lower the equilibrium potential of Na+ reduces
the rate of rise of the AP and its peak amplitude
• Direct relationship between the log of external Na+ and peak
amplitude à slope ~ 58mV
•

Lowering Na+ concentration has little effect on RMP because the
channels are not permeable yet

Membrane becomes very Na+ permeable during the rising phase or
overshoot phase
• Na+ selective channels that are essentially closed during resting
state begin to open
• Membrane pumps maintain a large electrochemical gradient for
Na+
• Membrane channels open à Na+ rushes inà depolarization to
•
•

•

approach ENa
Membrane permeability to Na+ is short so the membrane potential
rapidly repolarizes to resting levels = repolarization
Undershoot or hyperpolarization follows this because Na+
channels are inactivated and K+ channels are slower to close
because they are even more permeable than they are at rest
End of AP when K+ channels go back to normal

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9/29/16 3:11 PM



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