The Neuronal Membrane at Rest
Transcript of The Neuronal Membrane at Rest
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M A R Y E T B O Y L E , P H . D .D E P A R T M E N T O F C O G N I T I V E S C I E N C E
U C S D
The Neuronal Membrane at Rest
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Understanding the ionic basis for the resting potential
Describe the concept of electrochemical equilibrium and relate this concept to the resting membrane potential of
neurons.
Explain why the permeability of the neuronal plasma membrane at rest to K+ and the concentrationgradient of this ion across the neuronal plasma membrane account
for the resting membrane potential of neurons.
Use the NERNST EQUATION to predict the resting membrane potential of neurons given knowledge of the
concentration gradients of permeant ions..Learning
objectives
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Overview - Neurons generate electrical signals
Problem: neurons are poor at conducting electricity.
Solution: evolved mechanism to overcome limitations.
How: electrical signals are based on the flow of ions across the plasma membrane.
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Action potential in the nervous system
Collect distribute integrate
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Consider the following experimental setup:
record
stimulate
Microelectrode to measure membrane potential
Microelectrode to inject current
neuron
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The experiment:
record
stimulate
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Comparison of electrical problems…
Copper wire
Electrical charge is carried by free electrons
Well insulated‐plastic coating; air
Great conductor
Cytosol in Axon
Electrical charge is carried by ions.
Not perfectly insulated; salty extracellular fluid will
conduct electricity
Less than perfect conductive medium!
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Two basic concepts critical to understanding neuronal signaling…
Charge
Ions are needed to carry the electrical charge
Concentration gradient is a means to store energy
FlowPermeability mechanism to redistribute ions across the
membrane
Ion flow regulation and management
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Essential Resting Membrane Potential Properties:
Salty fluids on either side of the membrane
The membrane
Proteins that span the membrane
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Distinguishing between the role of channels and pumpsin establishing the concentration gradient.
Two permeability mechanisms:
PUMPS are used to create ion gradients – actively moves ions
against gradient
CHANNELS allow ions to diffuse down concentration gradient
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Water Key ingredient in intracellular and extracellular fluid Key feature – water is a polar solvent – which means thatother polar molecules tend to dissolve in water.
Cytosolic and Extracellular Fluid
Net negative charge
Net positive charge
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Why does NaCl crystal dissolve in water?
NaCl dissolves in water because the polar water molecules have a stronger attraction for the electrically charged sodium and chloride ions than the ions do for one another.
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Cations: ions with a net positive chargeAnions: ions with a net negative charge
Ions: Atoms or molecules with a net electrical charge
cations • Na+, K+, Ca++
anions • Cl-
Major charge carriers in the neuron:
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Hydrophilic v. Hydrophobic
‐philic
Dissolve in water
Uneven electrical charge
NaCl
‐phobic
Do NOT dissolve in water
Shared electrons are distributed evenly
lipids
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Phospholipid bilayer: the cell membrane
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• Neuronal membrane is a sheet of phospholipids two molecules thick.
• Hydrophilic heads face the outside and inner watery environments
• Hydrophobic tails face each other.
Polar “head” containing phosphate
Non-polar ‘tail” containing hydrocarbon
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H3N+
H
COO-
R
C
Central alpha carbonAmino group
Hydrogen atom
Carboxyl group
Variable R group
Recall, the basic Structure of an amino acid
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Other amino acids
Strongly hydrophilicR groups
Strongly hydrophobic
R groups
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Peptide bonds and polypeptides
Four levels of protein structure
primary structure
secondary structure
tertiary structure
quarternary structure
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Thinking about channels
Nonpolar R groupshydrophobic
Polar R groups(Hydrophilic)
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Diameter of pore
R-Groups lining
the pore
Gating Ion pumps
Ion specificity is determined by:
Ion channels can be opened and
closed by changes in the
local micro-environment of the membrane.
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The Movement of Ions- Factor #1: Diffusion
Dissolved ions distribute evenly Ions flow down concentration gradient when:
Channels permeable to specific ionsConcentration gradient across the membrane
Impermeable membrane Na+ and Cl- channels Equal distribution of ions
A channel across a membrane is like a bridge across a river.
A gated channel is a like a
drawbridge.
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The Movements of Ions- Factor #2: Electricity
Cl-Na+
Movement of ions are influenced by electrical fields.
Recall: opposite charges attract and like charges repel.
Place wires from two terminals of a battery in a solution containing
dissolved NACl.
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The movement of electrical charge is called electrical current (I) and measured in units called
amperes (amps).
By convention, current flows from + to –
Two important factors determine how much current
will flow:
Electrical potential&
Electrical conductance
Current flow
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Electrical potential - voltage
Electrical potential is the force exerted on a charged particle
It reflects the difference in electric potential per unit charge between the
anode and the cathode
More current will flow as this difference is increased.
Voltage is represented by the symbol V and is
measured in volts.
The difference in electrical potential
between the terminals of a car battery is 12V.
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Electrical conductance (g)
Is the relative ability of an electrical charge to migrate from one point to another.
It is represented by the symbol g and measured in units called Siemens (S).
Conductance depends on the number of
particles available to carry electrical charge‐
and the ease with which these particles can travel through
space
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Electrical resistance (R) relative inability of the charge to migrate.
Resistance is the inverse of
conductance.R =
R = Resistance is
represented by the symbol R and is measured in units called ohms (Ω)
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Ohm’s Law: V=IR
RIV
How would you express Ohm’s law using conductance (g) instead of resistance?
_____________
Express Ohm’s law as: I = _______
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How much current (I) will flow if:How much current (I) will flow if:
Recall: Ohm’s lawRecall: Ohm’s law1. g = 0 and V=100?
2. V = 0 and g=30?
Current (I) =conductance (g) * potential difference (V)
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Example 1:
What would happen if NaCl were dissolved
in equal concentrations on
either side of a phospholipid bilayer?
Na+Cl- Na+Cl-
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Example 1:
What would happen if NaCl were dissolved
in equal concentrations on
either side of a phospholipid bilayer?
Na+Cl-
Na+Cl-Na+Cl-
Na+Cl-
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Example 1:
What would happen if NaCl were dissolved
in equal concentrations on
either side of a phospholipid bilayer?
Na+
Cl-
Na+
Cl-
There will be a large potential difference across
the membrane.
No current will flow –because there are no channels to allow the migration of Na+ and Cl‐across the membrane.
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Example 2:
Now, what would happen ??
Na+
Cl-
Na+
Cl-Current
flow
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To drive ions across the membrane electrically:
(1) the membrane possesses channels permeable to that ion and
(2) there is an electrical potential difference across the membrane.
Current flow
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The Ionic Basis of The Resting Membrane Potential
A voltmeter measures the difference in electrical potential between the tip of a microelectrode inside the cell and a wire placed in the extracellular fluid.
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glass microelectrode
intracellular
ground
amplifierwire
Extracellular bath
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A- Any negative charged ion.
[20 inside] : [1 outside]Large concentration gradient exists between the inside of the cell and the outside.
No channel proteins.
Vm = 0 mVVm = ??? mV
No net movement of ions when separated by a phospholipid
membrane – and the K+:A- ratio remained the same.
[K+:A-=1] : [K+:A-=1 ]
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Model System:
inside outside
voltmeter
1mM KCl 1mM KCl
• Chamber with two compartments• Add channels
• Only permeable to K+• Each chamber is filled with 1mM KCl• K is free to move from one chamber to
the other• No next flux of K• Voltage recorded by voltmeter is: 0mV
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What would happen if :• Added 10mM KCl inside vs. 1mM KCl outside• Channels are only permeable to K+• K + is free to move from one chamber to the other
inside outside
10mM KCl 1mM KCl
inside outside
10mM KCl 1mM KCl
?mVInitial
conditions:
Vin‐out =
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Electrochemical Equilibrium:
• The flux of K+ from the inside is EXACTLYbalanced by an opposing membrane potential
• For example, as K+ diffuses down its concentration gradient an electrical force builds up that is exactly equal and opposite to the strength of the chemical gradient.
inside outside
10mM KCl 1mM KCl
?mVVin‐out =
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A- Any negative charged ion.
Selective permeability K+ can move between the inside and outside but A- cannot
ADD K+ channels
Initially, K+ ions would get out of the cell along the steep concentration gradient.
Vm = ??? mV
Eventually diffusion and electrostatic pressures are perfectly balanced and ….
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A- Any negative charged ion.
Selective permeability K+ can move between the inside and outside but A- cannot
Diffusional and electrical forces are equal and opposite.
Net movement across the membrane ceases.
Vm = ??? mV
EQUILIBRIUM POTENTIALEion represents the ionic
equilibrium potential.
Eion = Vm mV
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Equilibrium Potential(Eion)
No net movement of ions when separated by a phospholipid membrane
Equilibrium reached when K+channels inserted into the phospholipid bilayer
Electrical potential difference that exactly balances ionic concentration gradient
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4 points of Equilibrium Potentials
Large changes in VmMinuscule changes in ionic concentrations
Net difference in electrical chargeInside and outside membrane surface
Rate of movement of ions across membrane –ionic driving forceProportional Vm – Eion
Concentration difference known for an ion:Equilibrium potential can be calculated
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Distribution of Electrical Charge
The uneven charges inside and outside the neuron line up along the membrane because of electrostatic attraction across the very thin barrier.
Notice that the bulk of the cytosol and extracellular fluid is electrically neutral.
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Steps to establishing Eion
Steps to establishing the equilibrium potential of Na+
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The Nernst Equation
Calculates the exact value of the equilibrium potential for any permeant ion (mV) Takes into consideration:
R Gas constant z Valence of the permeant ion (electrical charge) T Temperature (absolute – degrees Kelvin) F Faraday constant (amount of electrical charge in a mole of a univalent ion [ion]o concentration of ion outside of the cell [ion]I concentration of ion inside of the cell
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Eion = Ionic equilibrium potential
R = Gas constant
T = Absolute temperature
z = Charge of the ion
F = Faraday’s constant
log = Base 10 logarithm
[ion]o = Ionic concentration outside the cell
[ion]i = Ionic concentration inside the cell
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EK = 61.54 mV log
ENa = 61.54 mV log
ECl = ‐61.54 mV log
ECa = 30.77 mV log
At body temperature (37°C), the Nernst
equation for the important ions, K+, Na+, Cl‐ and Ca++simplify to:
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=
= ‐1.3
EK = 61.54 mV ‐1.3
= 80 mV
If
and
then
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=
=
ENa = 61.54 mV
= mV
If extracellular sodium is 10x greater than the intracellular sodium:
andthe neuronal membrane is selectively permeable onlyto sodium ‐
What will the resting membrane potential be?
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The Distribution of Ions Across The Membrane
K+ more concentrated on inside, Na+ and Ca2+ more concentrated outside
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What is the Equilibrium Potential for an Ion?
An equilibrium potential for an
ion is -
the membrane
potential that results if a
membrane is selectively
permeable to that ion alone
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The sodium-potassium pump
Enzyme ‐ breaks down ATP in the presence of internal Na Pump exchanges internal Na for external K
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Calcium Pump
An enzyme that actively transports Ca++out of the cytosol.
Additional mechanisms decrease intracellular Ca++ to a
very low level (0.0002mM)
Intracellular calcium‐binding proteins and
organelles –mitochondria and ER that sequester Ca++
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What would happen to Vm if the membrane were EQUALLY permeable to K+ and Na+?
Vm would equal some average of EK and ENa
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What would happen to Vm if the membrane were 40x more permeable to K+ than to Na+?
Vm would be between EK and ENa but much closer to EK than Ena this approximates what happens in real neurons.
The resting membrane potential of -65mV approaches, but does not achieve, the potassium equilibrium potential of -80mV.
This difference arises because, although the membrane at rest is highly permeable to K, there is also a steady leak of Na into the cell.
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Relative Ion Permeabilities of the Membrane at Rest
Recall, the Nernst equation is about Vm if a membrane is selectively permeable to that ion alone.
However, neurons are permeable to more than one type of ionMembrane permeability determines membrane potential
Goldman equationTakes into account permeability of membrane to different ions
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Calculating the Vm using the Goldman Equation
K+ • [K]o=5mM• [K]i=100mM
Na+ • [Na]o=150mM• [Na]i=15mM
Permeability @ rest
•K+ is 40 x > Na+
Assumptions:
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The Goldman Equation
is the membrane potential.and are the relative permeabilities to K and Na ions
Constants are the same as in the Nernst equation
If the resting membrane ion permeability is to K+
is 40 times greater than it is to Na+, then solving:
Vm = 61.54mV log
= 61.54mV log
= ‐65mV
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NATURE REVIEWS | NEUROSCIENCE VOLUME 3 | FEBRUARY 2002
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Choe, S (2002)
The four main classes of potassium channels. a | 2TM/P channels (which consist of two transmembrane (TM) helices with a P loop between them), exemplified by inwardly rectifying K+ channels and by bacterial K+ channels such as KcsA. b | 6TM/P channels, which are the predominant class among ligand‐gated and voltage‐gated K+ channels. c | 8TM/2P channels, which are hybrids of 6TM/P and 2TM/P, and were first found in yeast. d | 4TM/2P channels, which consist of two repeats of 2TM/P channels. 8TM/2P and 4TM/2P probably assemble as dimers to form a channel. 4TM/2P channels are far more common than was originally thought. These so called ‘leakage’ channels are targets of numerous anesthetics. S4 is marked with plus signs to indicate its role in voltage sensing in the voltage‐gated K+ channels.
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Crystallographic structure of the bacterial KcsApotassium channel (PDB 1K4C).In this figure, only two of the four subunits of the tetramer are displayed for the sake of clarity. The protein is displayed as a green cartoon diagram. In addition backbone carbonyl groups and threonine side chain protein atoms (oxygen = red, carbon = green) are displayed. Finally potassium ions (occupying the S2 and S4 sites) and the oxygen atoms of water molecules (S1 and S3) are depicted as purple and red spheres respectively.
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Choe, S (2002)
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Resting Membrane Potential is Close to Ek
Resting membrane potential is close to EK because it is mostly permeable to K+
Membrane potential sensitive to extracellular K+
Increased extracellular K+ depolarizes membrane potential
A tenfold change in [K+]o from 5 to 50mM, causes a 48mV depolarization of the membrane.
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Increasing extracellular K+ from 5mM to 50mM causes a 48mV depolarization
10x
∆ 48mV
Increasing extracellular K depolarizes the
neuron!
Increasing extracellular K depolarizes the
neuron!
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Regulating the External K Concentration
Blood‐Brain barrier Limits the movement of potassium into the extracellular fluid.
Potassium spatial buffering Astrocytes have membrane potassium pumps and channels that concentrate K+ in their cytosol.
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http://www.nytimes.com/2011/06/04/us/04kevorkian.html?pagewanted=all
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Concepts to Remember
Activity of the sodium‐potassium pump Large K+ concentration gradient Electrical potential difference across the membrane Similar to a battery
Potassium channels Contribute to resting potential
Roles of ion pumps