Download LECTURE.6.ActPotential

LECTURE 6: ACTION POTENTIAL INITIATION AND PROPAGATION
REQUIRED READING: Kandel text, Chapter 9
Action potential (AP) is a brief spike of strong membrane depolarization
at a point along the axon caused by inward current flow
The AP is triggered by membrane depolarization that exceeds a certain threshold.
The depolarization trigger may result from:
1. Excitatory synaptic input in the dendrites or soma, leading to
AP initiation at the base of the axon
2. The action potential at an upstream region of axon, leading to
AP propagation along the axon
ACTION POTENTIAL INITIATION:
SODIUM CHANNEL DENSITY AT BASE OF AXON AND CHANNEL GATING KINETICS
CREATE A TRIGGER ZONE FOR LARGE INWARD CURRENT
KANDEL FIG 9-6
DEPOLARIZE
DEPOLARIZE
SODIUM
CURRENT
SODIUM
CURRENT
DEPOLARIZE
CHANNELS
OPEN
CHANNELS
OPEN
When excitatory synaptic currents depolarize
cell enough to activate small percentage of
sodium channels, high channel density
at initial segment gives sufficient
inward sodium current to further depolarize
region, thereby opening more channels
more rapidly ----> ----->
Trigger for explosive opening of all sodium
channels, large inward currents, and rapid
swing in Vm to positive value.
VIEWING ACTION POTENTIAL BY WHOLE-CELL PATCH CLAMP
SIZE OF EXCITATORY INPUT (SYNAPTIC) CURRENT DETERMINES SPEED OF INITIATION
50
100
150
GRANULE NEURON IN CEREBELLUM FIRES ACTION POTENTIALS SOONER
WITH GREATER INPUT DEPOLARIZING CURRENT
CURRENT CLAMP
Current clamp consists of a current generator which commands a specified
current that runs through the patch pipette back to bath ground,
i.e., across the cell membrane.
Instrument also records voltage from pipette to ground = Vmembrane
ICOMMAND
CURRENT
SOURCE
ICOMMAND
patch
pipet
ICOM
Imem
ICOMMAND
CELL
Imem
+
Icap
+ pA
ground
VOLTAGE
MONITOR
=
0 pA
bath
(grounded)
VMEM
Icap
Vrest
ACTION POTENTIAL DOWNSTROKE
SODIUM CHANNEL INACTIVATION AND POTASSIUM CHANNEL ACTIVATION
KANDEL FIGURE 9-3
SODIUM CHANNELS INACTIVATE
POTASSIUM OUTWARD CURRENT
Currents analyzed by V-clamp
NOTE HYPERPOLARIZATION
HYPERPOLARIZATION OF DOWNSTROKE REQUIRED FOR
RECOVERY OF SODIUM CHANNELS AND THEIR AVAILABILITY
FOR RE-FIRING
CONDUCTION ALONG UNMYELINATED AXON: REVISITED
A
ONE-WAY CONDUCTION OF
ACTION POTENTIAL DOWN AXON
SPEED OF CONDUCTION DETERMINED BY
RaxialCmembrane TIME CONSTANT OF AXON.
THIN AXONS CONDUCT AT ~ 1 mm/msec
B
2 4 6 8 1012
msec
C
WHY DOESN’T ACTION POTENTIAL AT
POINT “C” RETRIGGER A SECOND
ACTION POTENTIAL AT “A”,
WHERE CHANNELS HAVE RETURNED TO
RESTING STATE?
D
BECAUSE POTASSIUM CHANNELS STILL
OPEN AT POINT “B” PROVIDE A
SHORT CIRCUIT
AGAINST BACK PROPAGATION
CONDUCTION ALONG UNMYELINATED AXON:
POTASSIUM CHANNEL SHUNT PREVENTS BACK PROPAGATION
Point B
POTASSIUM
CURRENT
Point A
REST
Point C
SODIUM
CURRENT
Point D
REST
++
++
--
++
--
--
++
--
Raxial
Raxial
Raxial
A
Since gaxial > gleak ,
point D undergoes significant
passive depolarization
leading to AP
Since gK (at B) >> gaxial ,
point B (and point A) do not
undergo much
passive depolarization
Vm
B
C
D
E
0
-70
Back inhibition
Forward propagation
BLOCKING POTASSIUM CHANNELS CAUSES BACKFIRING/REFIRING OF ACTION POTENTIALS
OTHER CHANNELS AND CURRENTS MODIFY
THE INTRINSIC FIRING PROPERTIES OF NEURONS
KANDEL FIGURE 9-11
Activation And Inactivation Voltage Dependence
And Kinetics Determine Time Window
For Channel Conductance
ACTIVATION
RATE
INACTIVATION
- 70
- 35
0
MEMBRANE VOLTAGE (mV)
FGF-HOMOLOGOUS FACTORS (FHFs):
A FAMILY OF NEURONAL PROTEINS THAT BIND SODIUM CHANNELS
FHFs
Cytoplasmic Subunits Modulating Sodium Channel Inactivation
Raise voltage at which
intrinsic fast inactivation
of channels occurs
Control
neuronal
excitability
Induce long-term,
use-dependent
channel inactivation
FHF Genes, Isoforms and Expression
66 aa
FHF1A
FHF1B
FHF2A
FHF2B
b-trefoil core ~150 aa 25-30 aa
4 aa
62 aa
9 aa
64 aa
FHF4A
FHF4B
69 aa
FHFs are broadly expressed in neurons of CNS and PNS.
Generally, different classes of neurons express different profile of FHFs
FHF expression commences during neuronal maturation and is
stably maintained
Sodium Channels in Fhf1-/-Fhf4-/- Granule Cells
Inactivate at More Negative Voltage and
Inactivate Faster At Specific Voltages
Voltage Dependence
Time Constants at Specific Voltages
KO
WT
WT
KO
WT
FHF1+4 KO
V1/2 = -59.1 +/- 4.8 mV V1/2 = -72.8 +/- 4.3 mV
n = 8 cells
n = 9 cells
P < 10-4
(from Goldfarb et atl, Neuron, 2007)
Fhf1-/-Fhf4-/- Granule Cells In Cerebellar Slices Cannot
Fire Repetitively In Response To Sustained Current Injection
(from Goldfarb et atl, Neuron, 2007)
WHOLE CELL PATCH-CLAMPED GRANULE NEURONS
IN ADULT MOUSE CEREBELLUM SLICES
SODIUM CHANNELS INACTIVATE AT MORE NEGATIVE POTENTIAL
IN FHF MUTANT NEURON
Wild Type
Fhf1-/-Fhf4-/-
WHOLE CELL PATCH-CLAMPED GRANULE NEURONS
IN ADULT MOUSE CEREBELLUM SLICES
IMPAIRED SODIUM CHANNEL RECOVERY IN FHF MUTANT NEURON
Wild Type
Fhf1-/-Fhf4-/-
ALTERED SODIUM CHANNEL RESPONSES IN Fhf1-/-Fhf4-/GRANULE CELLS CAUSES IMPAIRED EXCITABILITY
Normal sodium channel density and activation in mutant cells
Current-induced depolarization gives rapid 1st action potential
In mutant cells, downstroke of action potential does not lower voltage
far enough for many sodium channels to recover from inactivation,
and the rate of channel recovery is impaired
Subsequent action potentials blocked; no repetitive firing
“A-type” FHFs Induce Long-Term Inactivation of Sodium Channels
( from Dover et al, J. Physiology, 2010)
FHF
Isoform
Upshift in V1/2
Steady State
Inactivation
Induction of
Long-Term
Inactivation
1A
13 mV
++
2A
13 mV
+++
4A
16 mV
+++
1B
1 mV
-
2B
7 mV
-
4B
17 mV
-
Does Channel Fast Inactivation Limit Long-term Inactivation?
Mutant Channel Deficient
For Fast Inactivation
FHF2A Restores Inactivation
And Augments Long-Term Inactivation
( from Dover et al, J. Physiology, 2010)
Long-Term Inactivation Requires FHF2A Channel-Binding
and N-Terminal Effector Domains
( from Dover et al, J. Physiology, 2010)
Long-Term Inactivation Gating Particle Model
Antibody Inhibition of Channel Long-Term Inactivation
( from Dover et al, J. Physiology, 2010)
FHF N-Terminal Peptide Injection Recapitulates Long-Term Inactivation
( from Dover et al, J. Physiology, 2010)