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Basic Electrophysiology
Valeria Fu
November 3, 2014
Neurons
• Cells in the nervous system
• Vary in structure and properties
• Their fundamental task is
signaling
• Neurons respond to stimuli,
conduct impulses and send
signals
• Neurons encode information by a
combination of electrical and
chemical signals.
Electrical Signaling in Neurons
Electrical Signaling in Neurons
Presynaptic neuron
Postsynaptic neuron
• Electrical signals in the presynaptic neuron cause the release of
neurotransmitter; the neurotransmitter binds to receptors of the
postsynaptic neuron and triggers electrical signal (synaptic potentials)
Plasma Membrane
• The entire neuron is enclosed by a plasma
membrane
• The plasma membrane:
• a double layer (bilayer) of phospholipid
molecules
• provides a resistance to the flow of ions
entering or leaving the cell (resistor)
• Allows to store charge inside the cell
(capacitor).
• Separating electrical charges between
extracellular and intracelluar spaces
Membrane Potential
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Uneven distribution of ions inside and outside of the membrane
Sodium (Na+) and Chloride (Cl-) are more concentrated outside the cell.
Potassium (K+) and organic anions (A-) are more concentrated inside the cell
Inside cell membrane, it is negatively charged.
The electrical potential difference across the membrane at any moment in time
is known as the Membrane Potential (Vm)
• Vm = Vin - Vout
Resting Membrane Potential
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When the cell/axon is not conducting impulses; it is said to be at rest
Resting potential ranges from about -60 mV to -70 mV
Membrane is selectively permeable to K+
Permeable to Na+ is low
• To maintain the steady resting membrane potential, the charge separation across
the membrane must be constant:
• Influx of positive charge ≈ efflux of positive charge
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Resting state of the cell is achieved by:
1. Gibbs-Donnan Equilibrium
2. Equilibrium Potential & Nernst Equation
3. ATP-dependent 3 Na+-K+ Pump
4. Ion Channels
1. Gibbs-Donnan Equilibrium
• Uneven distribution of charged ions on one side of a semipermeable membrane
• Diffusion occurs when ions move from areas of high concentration to low
concentration down a concentration gradient (Chemical driving force).
• Concentration of 2 ions of opposite sign on each side (extracellular and intracellular) of
the membrane are affected by the electrostatic repulsion (same sign repulses) and
attraction (opposite sign attracts).(Electrical driving force)
• At rest, K+ diffuses down to the concentration out of the cell
(Chemical driving force)leaving surplus of anion (A-) inside
the cell
• Excess cation outside the cell pushes K+ back to the cell
(Electrical driving force)
• Until K+ concentration inside and outside the cell are in
equilibrium (-75mV)
• Resting membrane potential (Vr ) settles at K+ equilibrium
potential (Ek)
• Vr = Ek
2. Equilibrium Potential & Nernst
Equation
• membrane potential where the net flow through any open channels is zero
• Depends on the concentration gradient of an ion
• Equilibrium potential of an ion can be calculated from the Nernst equation
derived by Walter Nernst (1888):
• Ex = RT In [X+]o
ZF [X+]i
• Ex = membrane potential at which ion X is in equilibrium
• R = gas constant
• T = temperature
• Z = charge on the ion
• F = Faraday constant
• [X+]o & [X+]I = concentration of ion inside and outside of the cell
• For monovalent ions at room temperature, the Nernst equation reduces to:
• Ex = 58 log10 [X+]o
[X+]i
Goldman-Hongkin Katz Equation
• Changing extracellular concentration of each ion has a strong effect on
resting membrane potential and could be calculated by:
• Permeability ability ratios for membrane at rest :
• PK:PNa:PCl = 1/0.04/0.45
3. ATP-dependent Na+-K+ pump
• 3 Na+ transport outward for every 2 K+ inward
• And contributes to the resting potential
• Consequences:
• 1. causes a net transfer of charge across the
membrane. Pump is electrogenic: cause to cell to
hyperpolarize
• 2. passive movements of Na+ and K+ are unequal
• 3. decreases the osmolarity of the intracellular fluid
and balances the effect of impermeant anion in the
cytosol
4. Ion Channels
• Neuronal signaling is based on the movements of ions across cell membrane.
• Hydrophilic pores through which ions flow from extracellular space to intracellular
space or vice versa down their concentration gradients.
• There are gated and non-gated ion channels in membrane.
• Voltage-dependent gated ion channels:
• 1. Probability of opening of channels is strongly influenced by voltage only
• 2. Opening and closing of channels are influenced by changes in membrane
potential close to normal resting potential.
Ohm’s Law
• Current flow (I) between extraceullar space and intracellular space depends on the
voltage difference (V) and Resistance to current flow (R)
• I= V
R
• Voltage difference (Vm – Ex) is called Driving Force
• Conductance (G) is the inverse of resistance:
• G = 1/R
• Ix = (Vm – Ex) Gx
• X = ion
Equivalent Electrical Circuit
• Membrane is a Capacitor and a Resistor connected
in parallel.
• Neuronal signaling is based on the movements of
ions across cell membrane.
• We can calculate the number of ions must move
through membrane in order to give rise to the
membrane potential:
• V = Q/C
• Q = charge
• C = capacitance
• V = electrical potential
Neuronal Signal Transmission
• Neurons use a single type of signal to transmit information over a long distance.
Action Potentials
• When the dendrite/axon/nerve is conducting an impulse.
• Large, brief, invariant signals propagate along the axons
without decrement.
• “all-or-none”
• Opening and closing of a sodium channel and a potassium
channel in a precise timing give a transient changes in
membrane potential which allows electrical signals travels
along axons at speed up to 120 meter per second.
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Four Stages:
1. Resting Potential
2. Depolarization (Rising phase)
3. Repolarization (Falling phase)
4. Undershoot (After-hyperpolarization)
1. Resting Potential
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When the axon is at rest (-70mV)
Neither Na+ or K+ ion channel is open
Steady Na+ influx is balanced by steady efflux of K+
ATP-dependent Na+-K+ pump is at work
2.Depolarization (Rising phase)
• When the nerve fiber is stimulated, synaptic
inputs (Post-synaptic neuron ) to a neuron
cause the membrane to depolarize
(membrane potentials are less negative)
• A transient depolarizing potential (i.e.
excitatory synaptic potential) causes
opening of some voltage-gated Na+
channels.
• Increase membrane Na+ permeability and
allows influx of Na+ to further depolarize
the membrane
• Increase in depolarization allows influx of
more positive charge flow inside the cell.
• When depolarization of membrane exceeds
threshold, Action Potentials result.
Action Potentials
• To transfer information from one part of
the neuron to another
• ‘All-or None’ law:
• The strength of neuronal response is
independent of stimulus strength; Once
the stimulus is above threshold, it
produces full size action potentials to
minimize the possibility that the
information is lost along the way down
the axon.
Action Potentials
• Latency, the time delay from the onset of
stimulus to the peak of action potentials, is the
function of stimulus strength (strength-latency
relationship)
• The larger the depolarizing stimulus, the
greater the frequency of action potential firing
Frequency coding in axons
2. Depolarization (Rising Phase)
• Sodium conductance exceeds Potassium
conductance
• Net inward current drives the membrane
potentials toward Na+ equilibrium (+55mV)
• Sodium permeability decreases when the
action potential approaches the peak which
results from inactivation of sodium channel ->
no longer respond to depolarization
• Potassium conductance responds more slowly
and starts to increase when the action
potential is near to its peak.
3. Repolarization (Falling phase)
• Delay opening of Potassium channel
• Inactivation of Sodium channel
• Efflux of K+ increases carrying their
positive charge with them
• Thus lead to hyperpolarization
(membrane potential is more negative)
• Bring the membrane potential back to
resting state (-60 mV to -70 mV)
4. After-hyperpolarization
• Potassium conductance is higher than
normal
• Sodium conductance is lower than
normal
• Membrane potentials is driven closer to
the equilibrium of K+ (-90mV) than it is at
rest (60 – 70mV)
Refractory Period
• Immediately after an action potential
• Refractory period: threshold is higher than
normal to initiate another action
potential.
• Absolute refractory period: the threshold
is infinite, impossible to evoke another
action potential
• Relative refractory period: requires a
larger than normal stimulus to evoke an
action potential.
Review: Action Potential
Action Potential Propagation
• Passive spread (local potential): voltage
change spreads from one point to
another, but with attenuation with
distance.
• Vd = Voe -d/
• Even voltage changes at distance, local
depolarization is large enough to
spread to the adjacent region of the
axon to generate a full size action
potential
Action Potential Propagation
• Part of the inward Na+ current flows down to
interior of axon to produce local potential in
advance of an action potential
• Local potential depolarizes the membrane
• Activated voltage-gated Na+ channels
• When reach threshold, inward current
further depolarizes the membrane and acts
as a source for local potential change.
• The inward current flows downstream and
moves the action potential along the axon.
• Due to refractory period, inward current will
not initial another action potential in towards
the cell body.
• Therefore, action potentials propagate in
ONE direction.
Saltatory Conduction
• An axon is myelinated.
• In the myelinated area, there is no inward current of Na+ when the Na+
channels open because there is no extracellular Na+
• The only place that the myelinated axon comes to contact with extracellular
fluid is at the Node of Ranvier where the axon is unmyelinated.
• Action potential jumps from node to node to propagate down the axon.
This is called saltatory conduction.
Synaptic Potentials
• Action potential travels along the axon down to
the presynaptic terminal
• The depolarization of the presynaptic neuron
triggers the release of neurotransmitter in the
cleft.
• When the neurotransmitter binds to the
receptor of the post-synaptic neuron, it gives
rise to the synaptic potentials.
• In the central nervous system (CNS), glutamate
is the major excitatory neurotransmitter which
generate excitatory postsynaptic potentials
(epsp).
• While GABA or glycine is commonly the
inhibitory neurotransmitter which open the
channels of K+ and Cl-. In turn, it hyperpolarizes
the cell and makes depolarization to threshold
more difficult. This synaptic potential is called
inhibitory postsynaptic potentials (ipsp).
Passive Membrane Properties
• They are constant during neuronal signaling
• They affect the electrical signaling process:
• 1. Time course of electrical signals
• 2. Efficiency of signal conduction
Time Course of Electrical Signals
• Membrane acts as an electrical capacitor(Cm) and resistor (Rm)
• The shape of change of a potential (voltage) is determined by the fact that
membrane capacitance and resistance are in parallel.
• Charged (discharged) of membrane capacitance does not occur instantaneously.
• Time constant  .
•  = Rm Cm
• Voltage changes exponentially with time (t)
• Vt = Vo e –t/ 
• Voltage falls to 1/ e of its initial value Vo in a time equal to one time constant ().
• The longer the time constant, the longer duration of synaptic potential -> more
chance for temporal summation (small potentials adding together) -> higher
chance to drive membrane potential for an action potential
Efficiency of Signal Conduction
• 1. Axoplasmic resistance:
• the greater length of the axoplasmic core, the greater the resistance (ion collisions
along the dendrite) , the smaller the current.
• Vm = Imrm
• 2. Insulation of the membrane:
• the better the insulation, the further the current spread along the dendrite/axon,
the faster the velocity of action potential
• Length constant () = rm/ra
• Where rm = membrane resistance; ra = axial resistance
• Myelination of axon affects velocity of action potential
• 3. Axon Diameter:
• The larger the axon diameter, the lower resistance of axoplasm to flow of current;
more effective depolarization of membrane, the faster the velocity of action
potential.
Review
Review
Review
Review
Review
Presynaptic neuron
Postsynaptic neuron
References
• Principles of Neural Science (Kandel, Schwartz & Jessell)
• Molecular Neurobiology (Zach Hall)
• The Neuron (Levitan & Kaczmarek)
• Ionic Channels of Excitable Membranes (Bertil Hille)