Recording of Membrane Potential
Oscilloscope display
Stimulating
electrode
Recording
oscilloscope
mV
+ 60 -
+ 30 -
Insert
electrode
0- 30 -
Electrotonic potential
- 60 - 90 -
Nerve cell
Resting
potential
Local Electrical Circuit
Stimulation
+
+
+ + + ++ + + + + + + + + + + + + + + + - - - - - - - + + + + + + + + + + + + + + + + + + + +
Membrane capacitance
---------------------------++++++------------------------The resting membrane potential
+ +
Intracellular axial resistance
Membrane Potential in
Response to Current Injection
mV
Outward
-45 -50 -55 -
Membrane
-60 potential
Current 0
-65 -70 -
0.5 nA
Inward
Time
-75 -
Time
Current-Voltage (I-V) Relationship
I (nA)
DV =
+1
Outward
+2
Slope dV/dI = Rin
- 80
- 70
- 50
-1
Inward
Hyperpolarization
-2
Depolarization
- 40
I x Rin
Rin (input resistance)
can be defined by slope
of the I-V curve. The I-V
curve shown here is
linear; Vm changes by
10 mV for every 1 nA
change in current,
yielding a resistance of
10 mV/1nA, or 10 x
106W.
Capacitive property of
neural membrane
Membrane potential
Applied current
Time
Time
Current flow across the neural membrane
ionic and capacitive current
Capacitive
current
Ionic
current
K
Na
K
Na
K
- --
+++
Na
K
Electrical equivalent circuit for examining
the effects of membrane capacitance
Extracellular side
Capacitive
current
Ionic
current
Current
generator
_ _
Rin
Ii
Cm
++
Ic
Im
Cytoplasmic side
Electrical equivalent circuit for examining
the effects of membrane capacitance
Extracellular side
Im
Ic
Current
generator
Rin
Cin
Cytoplasmic side
Ii
RESTING STATE: No current flow through capacitor or resistor.
Electrical equivalent circuit for examining
the effects of membrane capacitance
Extracellular side
Im
Ic
Current
generator
Rin
- + + Cin
Cytoplasmic side
Ii
INITIAL STEP: V = 0 and no current flow through the resistor. Im = Ic
Electrical equivalent circuit for examining
the effects of membrane capacitance
Extracellular side
Im
Ic
Current
generator
Rin
- + + Cin
Cytoplasmic side
Ii
Vm increase and drive the current to flow through the resistor. Im = Ii + Ic
Electrical equivalent circuit for examining
the effects of membrane capacitance
Extracellular side
Im
Ic
Current
generator
Rin
- + + Cin
Cytoplasmic side
Ii
Vm increase and drive the current to flow through the resistor. Im = Ii + Ic
Electrical equivalent circuit for examining
the effects of membrane capacitance
Extracellular side
Im
Ic
Current
generator
Rin
- + + Cin
Cytoplasmic side
Ii
Capacitor is fully charged and no more current flow through capacitor. The
system approach steady state and all current flow through the resistor. Im = Ii
Electrical equivalent circuit for examining
the effects of membrane capacitance
Extracellular side
Im
Ic
Current
generator
Rin
- + + Cin
Cytoplasmic side
Ii
The process is reversed after no current is applied.
Membrane capacitance and time
course of potential change
b
a
Membrane
potential (D Vm)
63%
Time constant (t)
Out
Im
Ii
Membrane
current (Im)
In
0
Ionic current (Ii)
Ic
Capacitive
current (Ic)
Neuronal process as a co-axial fiber
Current Generator
RECF
Rm
Ra
Neuronal process as a co-axial fiber
Inner conductor
(cytoplasm)
Outer conductor
(ECF)
Inner layer insulation
(membrane)
Cytoplasm
Membrane
Outer layer insulation
(ECF)
Neuronal process as a co-axial fiber
Outer resistance (raECF)
Membrane
Tranmembrane
resistance (rm)
Axial resistance (ra)
Extracellular fluid (outer conductor)
rm
cm
Membrane
ra
Cytoplasm (inner conductor)
Neuronal process as a co-axial fiber
Current Generator
Rm
Ra
The Length Constant
100%
100
37%
10
l
0
distance
1
stimulation
0
1
2
3
4
Propagation of action potential
The continuous conduction
+50
Direction of propagation
+50
0
0
- 60
- 60
+++++++++++
___________
1
_____
+++++
2
++++
____
3
+++
___
1
_____
+++++
2
++++++++++++++
______________
3
Effect of myelination
• Increase membrane resistance
• Decrease membrane capacitance
Less charge loss in charging
capacitor and leakage across
membrane, therefore increase the
length constant.
Propagation of action potential
The saltatory conduction
+++
___
____
+++
++++
____