Download Electrophysiology - University of Nevada, Las Vegas

Mammalian Physiology
Resting Membrane Potentials
Action Potentials
UNLV
UNIVERSITY OF NEVADA LAS VEGAS
PHYSIOLOGY, Chapter 2 & 3
Berne, Levy, Koeppen, Stanton
Objectives
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Describe the basics of electrophysiology
Describe the ionic basis of the resting membrane potential
Describe the ion movements in an action potential
Describe factors determining conduction velocity
Describe the function of the myelin sheath
Basic Concepts
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Ohm’s Law
I=
E
R
– I = current – movement of electrical charge
– E = voltage – electrical potential (difference in charge between 2 points)
– R = resistance – hindrance to charge movement
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High electrical resistance – insulators
Low electrical resistance – conductors
Basic Concepts
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Resting membrane potential
– Difference in ion concentration
• Current carried by Na+ and K+
– Difference in membrane permeability
• Selectively permeable membrane responsible for potential difference
• RMP represents a charge or voltage difference across the membrane
• Permeability changes with electrical activity
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Excitable cells – nerve cells, muscle cells
Basic Concepts
Separation of charge is defined as potential energy (the potential to do work),
thus this separation of charge is termed “membrane potential” or Voltage and in
cells is measured in millivolts (mV).
If the charge is allowed to move across the membrane then the potential energy
is turned into work, thus when ions or electricity flows you have “current” flow.
If something slows or hinders the flow of
ions or electricity this is termed “resistance”
Typical Ion Distribution
Concentration of Ions (mmols/liter)
Ion
Na+
ClK+
Extracellular
150
110
5
Intracellular
15
10
150
Equilibrium Potential
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Consider forces acting on ions – electrical charges & concentration
gradients
Membrane is selectively permeable
– K+ is freely permeable
– Na+ is relatively impermeable
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K+ - driving force down concentration gradient out of cell – seeks
chemical equilibrium
Impermeable negatively charged anions exert electrical force on K+
tending to keep K+ inside the cell
Resting membrane potential is established when inward electrical
gradient for K+ is balanced by outward chemical gradient for K+
When these forces balance – no net K+ movement
Equilibrium constants for ions can be calculated by Nernst
equation - Keq
Cell Membrane
Outside
Inside
Na+
Na+
[150]
[15]
K+
K+
[5]
[150]
Cl-
Cl-
[110]
[10]
Cell Membrane
Anions -
Na/K ATPase pump
Nernst Equation
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Nernst equation can be used to predict direction of ion flow
– If measured potential = calculated potential, ion is in equilibrium
and no net ion flow occurs
– If measured potential is same sign as calculated potential, but
larger, electrical force is greater than chemical force and ions will
flow in direction of electrical force
– If measured potential is same sign as calculated potential, but less,
chemical force is greater than electrical force and ions will follow
chemical force
– If measured potential is opposite sign than calculated potential,
electrical and chemical forces are in the same direction – ion
cannot be in equilibrium
Nernst Equation
Chemical forces acting on an ion = RT ln [A]/[B]
Electrical force acting on an ion = zF(EA– EB)
At equilibrium, RT ln[A]/[B] + zF(EA– EB) = 0
or
(EA – EB) = RT ln[B] converting to log10 and standardizing for RT
(2.303 RT/F = 60 mv)
zF [A]
EA – EB = -60 mv log [A]I
z
[B]o
(equation 2-4)
Nernst Example
Keq = -60 log [0.1]
1
[0.01]
=
-60 log 10 = -60 mv
General rule:
An electrical potential difference of 60 mv is needed to balance a 10 fold
concentration difference
Nernst Example
K+o = 5 meq
K+I = 150 meq
Na+o = 150 meq
Na+I = 15 meq
Keq = ?
Keq = -60 log [150]
[5]
= -60 log 30 = -60 x 1.5 = -90 mv K+
Keq = -60 log [15]
[150]
= -60 log 0.1 = -60 x -1 = +60 mv Na+
-90 mv is resting membrane potential of excitable cells
So, resting membrane potential is a result of K+ permeability
Resting Membrane Potential
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K+ permeability allows K+ to follow concentration gradient
There is some Na+ leakage into the cell – enough to raise Keq to
-70 mv
This would be expected to cause K+ to leave the cell (RMP<Keq
for K+)
If not checked, K+ and Na+ would both reach chemical and
electrical equilibrium
Na/K ATPase pump maintains concentration gradient
– Electrogenic pump (3 Na out/2 K in)
An Action Potential
Action Potential is a brief reversal of membrane potential polarity due to
changes in ion permeability when cell is depolarized to threshold
Action Potentials
Action potentials from different cell types have different durations
Characteristics of Action Potential
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Threshold is 10-15 mV depolarized (ie. –55 to –60 mV)
All or none response
– Once membrane reaches threshold, amplitude is independent of
strength of initiating stimulus
– Action potential cannot be summed
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Nerve has a refractory period – period when a subsequent
stimulus will not generate another action potential
Action potential is conducted without decrement (amplitude is
constant)
Duration of action potential is constant
Signaling is accomplished by changing frequency of action
potentials
Action Potential
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When membrane potential is
raised to threshold, voltage-gated
Na+ channels open and Na+ enters
cells following electrical and
chemical gradients
As inside of cell becomes positive,
K+ efflux increases and Na+ entry
slows (K+ voltage-gated channels
open, increasing K+ efflux)
As positive charges leave cell,
potential returns toward 0 and
voltage-gated Na+ channels close
while K+ channels remain open;
Na+ entry ceases while K+ efflux
continues
Slow closure of K+ voltage-gated
channels leads to overshoot
Resting membrane potential is
restored by Na/K ATPase pump
Ion Conductance in an Action Potential
Action potential is sum of Na+ and K+ conductances – note the
differential time course of ion movement
Action Potential Summary
Refractory Period
Action potential moves from the initial area of depolarization outward, not back
toward the center due to the inactive state of the voltage-gated Na+ channels
Electrotonic Conduction
Action is propagated by cycles of depolarization
– depolarization of membrane generates an
action potential, producing local currents which
bring adjacent regions to threshold, generating
action potentials which produce local currents
which bring adjacent regions to threshold….etc.
Conduction Velocity
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Conduction velocity is determined by membrane capacitance
(Cm) and electrical resistance to current flow
Membrane potential is the charge stored by the membrane
capacitor
Membrane capacitance is the amount of charge that must flow
to depolarize the membrane
The larger the capacitance, the greater the amount of charge
that must flow and the slower the conduction velocity
Capacitance is a function of the membrane surface area that
must be depolarized
Conduction Velocity
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Electrical resistance determines how rapidly charge can flow
Resistance to current flow is a function of resistance to current
flow across the membrane (Rm) and resistance to current flow
along the cytoplasm of the nerve (Rin)
– Currents that flow across the membrane are lost from the cable
– Currents that flow through the longitudinal resistance carry the
signal along the cable
The effective resistance is
proportional to the geometric
mean of Rm and Rin
Rm·Rin
Fiber Size and Conduction Velocity
Time constant for conduction is Cm x
Conduction velocity =
Cm x
Rm · Rin
1
Rm · Rin
Nerve or muscle cell can be viewed as a cylinder
Surface area = 2π·r·l
Cross-sectional area = π r2
Capacitance (Cm) is proportional to surface area
Membrane resistance (Rm) is inversely proportional to surface area
Internal resistance (Rin) is inversely proportional to cross-sectional area
Doubling of nerve radius will increase Cm by 2, decrease Rm by 2 and
decrease Rin by 4, so product of Cm x Rm · Rin = 2 x 1
= 1
2x4
2
This means conduction velocity increases by a factor of 2 when radius doubles
Conclusion: larger fibers have faster conduction velocities
Myelin Sheath
Schwann Cells:
-in the PNS Schwann
cells myelinate axons
which greatly
increases conduction
velocity
-Schwann cell plasma
membranes are lipid
dense and wrap
around the axon to
form the myelin
sheath
-Schwann cells wrap
themselves around
the axon at regular
intervals leaving gaps
or bare spots on the
axon called “Nodes of
Ranvier”
Myelin Sheath and Conduction Velocity
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Myelin sheath increases conduction velocity without increasing
nerve size
Increases length constant of nerve
– Increases Rm by blocking current flow across membrane
– Ratio of Rm/Rin is greater
– Less dilution of ions across large membrane area
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Decreases capacitance of membrane
– Myelin sheath decreases surface area that must be depolarized
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Restricts generation of action potentials to nodes of Ranvier
– Na+ and K+ are concentrated at isolated sites
Myelin Sheath and Conduction Velocity
Myelinated axons have greater
conduction velocities than
unmyelinated nerves with100
times diameter
10 µm myelinated fiber = 50 m/sec
500 µm unmyelinated fiber = 25 m/sec
Saltatory Conduction
The action potential appears to jump from node of Ranvier to node of Ranvier.
Only the membrane at the node of Ranvier depolarizes, not the membrane
under the myelin sheath. There are no ion channels under the myelin sheath.
The jumping or saltatory conduction is much faster than depolarizing the entire
membrane.
Action potential doesn’t really jump – rather ions accumulate at nodes of
Ranvier – increased conductance – build-up of ions means faster movement of
ions across membrane = faster signal conduction