How Passive & Active Properties of
Neurons work together
• Ligand-gated channels & Graded Potentials
• Threshold
• Voltage Gated Channels
• Action Potentials
Passive Spread of
Potentials
&Neuronal
Conduction
Speeds
All Cells have a “Resting
Potential”
Resting potential
depends in part on
the Na/K
ATPase
All Cells have a “Resting
Potential”
Resting potential
depends on the Na/K
ATPase and LEAK
cation channels.
“leak”
Squid Giant axon is 1 mm in diameter with
conduction velocities 10x its smaller axons.
Nernst Equation
∆G = RT ln(Cin Cout ) + ZℑVm
Vm = RT ln (Cout / Cin)
ZF
Nernst Equation
Vm = (RT/zF) ln([Na+]out/[Na+]in)
= (.025 V) ln (150/15)
= .057 V or 57 mV
= ENa+ at 18oC
Vm = EK+ = (RT/zF) ln([K+]out/[K+]in)
= (.025 V) ln (5/150)
= -85 mV
1
Neural Communication
Depends on Membranes
Gated ION
Ligand Gated
Channels
Produce Graded
Potentials
• Amplitude is Proportional to amount of
NT released
•Signal Decrements with distance
•Inhibitory Potentials (IPSPs)
•Excitatory Potentials (EPSPs)
“leak”
The Nernst Equation is a Model for Vm
Vm = E for all permeable ions
For > 1 ion use a modification
of Nernst called the Goldman
Equation
Depolarization
Eions = (RT/F) ln pK+[K+]out + pNa+[Na+]out
pK+[K+]in + pNa+[Na+]in
Opening Channels affects Vm
If a channel is equally
permeable to Na+ and
K+ then…..
How could an
IPSP occur?
EK,Na = (.025) ln
1[5]out + 1[150]out
1[150]in+ 1[15]in
= (0.25) ln [155/165]
= - 1.56 mV
2
Opening Channels affects Vm
If the net effect of channels opening is to
bring Vm above Threshold
an AP is produced.
EK,Na = (.025) ln
1[5]out + 1[150]out
1[150]in+ 1[15]in
= (0.25) ln [155/165]
= - 1.56 mV
Action Potentials
Stimulus
Voltage-Gated Na+ Channels
Decision to produce a signal
made here
•This is the
Trigger Zone
What is the maximum ∆Vm ?
If
[Na+]out = 150 mM
[Na+]in = 15 mM
Assume T of 18oC
•The signal is an
Action Potential
Voltage-Gated Na+ Channels
Voltage-Gated Na+ Channels
PNa+ >>>>> P K+
Change in Vm Î ENa+
Initiates a “Domino Effect”
- Positive Feedback
- Regenerative Property of AP
3
Voltage-Gated K+ Channels
Repolarization
Membrane Permeability during an
Action Potential
Depolarization
K+ K+
Depolarization
K+ K+
Repolarization
- Negative Feedback
- Self Limiting Property
- “All or None” Property
Fig. 7.13
Voltage-Dependent
Na+ Channel
Voltage-Dependent
Na+ Channel
Fig. 7.11
Voltage-Dependent
Na+ Channel
Voltage-Dependent
Na+ Channel
4
Voltage-Dependent
Na+ Channel
Refractory Period
Voltage-Dependent
Na+ Channel
Membrane Permeability explains
Refractory Period
Inactivation
X
Repolarization
Channel
Inactivation
also explains
Unidirectional
Propagation
5