LECTURE 6: ACTION POTENTIAL INITIATION AND PROPAGATION
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Transcript of LECTURE 6: ACTION POTENTIAL INITIATION AND PROPAGATION
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
When excitatory synaptic currents depolarizecell enough to activate small percentage of
sodium channels, high channel density at initial segment gives sufficient
inward sodium current to further depolarizeregion, thereby opening more channels
more rapidly ----> ----->Trigger for explosive opening of all sodiumchannels, large inward currents, and rapid
swing in Vm to positive value.
DEPOLARIZE
DEPOLARIZE
DEPOLARIZE
CHANNELSOPEN
CHANNELSOPEN
SODIUMCURRENT
SODIUMCURRENT
VIEWING ACTION POTENTIAL BY WHOLE-CELL PATCH CLAMPSIZE OF EXCITATORY INPUT (SYNAPTIC) CURRENT DETERMINES SPEED OF INITIATION
GRANULE NEURON IN CEREBELLUM FIRES ACTION POTENTIALS SOONERWITH GREATER INPUT DEPOLARIZING CURRENT
15050 100
CURRENT CLAMP
Current clamp consists of a current generator which commands a specifiedcurrent 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
+-patchpipet
CELL
bath(grounded)
CURRENTCURRENTSOURCESOURCE
ICOMMAND
VOLTAGEVOLTAGEMONITORMONITOR
Icap
Imem
ground
ICOMMAND
ICOMMAND Imem Icap= +
ICOM 0 pA
+ pA
VMEM Vrest
ACTION POTENTIAL DOWNSTROKESODIUM CHANNEL INACTIVATION AND POTASSIUM CHANNEL ACTIVATION
SODIUM CHANNELS INACTIVATEPOTASSIUM OUTWARD CURRENT
NOTE HYPERPOLARIZATIONCurrents analyzed by V-clamp
KANDEL FIGURE 9-3
HYPERPOLARIZATION OF DOWNSTROKE REQUIRED FOR RECOVERY OF SODIUM CHANNELS AND THEIR AVAILABILITY
FOR RE-FIRING
CONDUCTION ALONG UNMYELINATED AXON: REVISITED
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
WHY DOESN’T ACTION POTENTIAL ATPOINT “C” RETRIGGER A SECOND
ACTION POTENTIAL AT “A”,WHERE CHANNELS HAVE RETURNED TO
RESTING STATE?
BECAUSE POTASSIUM CHANNELS STILL OPEN AT POINT “B” PROVIDE A
SHORT CIRCUIT AGAINST BACK PROPAGATION
A
B
C
D
msec2 4 6 8 1012
CONDUCTION ALONG UNMYELINATED AXON: POTASSIUM CHANNEL SHUNT PREVENTS BACK PROPAGATION
+ + - -- -
- -- -+ ++ ++ +
Point AREST
Point DREST
Point CSODIUM
CURRENT
Point BPOTASSIUM
CURRENT
leak
leak
leak
leak
Na Na Na Na KKKK
RaxialRaxial Raxial
Vm0
-70
A B D EC
Since gaxial > gleak ,
point D undergoes significant passive depolarization
leading to AP
Since gK (at B) >> gaxial ,
point B (and point A) do notundergo much
passive depolarization Back inhibition Forward propagation
BLOCKING POTASSIUM CHANNELS CAUSES BACKFIRING/REFIRING OF ACTION POTENTIALS
OTHER CHANNELS AND CURRENTS MODIFYTHE INTRINSIC FIRING PROPERTIES OF NEURONS
KANDEL FIGURE 9-11
Activation And Inactivation Voltage DependenceActivation And Inactivation Voltage DependenceAnd Kinetics Determine Time Window And Kinetics Determine Time Window
For Channel ConductanceFor Channel Conductance
RATERATE
MEMBRANE VOLTAGE (mV)MEMBRANE VOLTAGE (mV)- 70- 70 - 35- 35 00
ACTIVATION
INACTIVATIONINACTIVATION
FGF-HOMOLOGOUS FACTORS (FHFs):A FAMILY OF NEURONAL PROTEINS THAT BIND SODIUM CHANNELS
Induce long-term,use-dependent
channel inactivation
Raise voltage at whichintrinsic fast inactivation
of channels occursControl
neuronalexcitability
FHFsFHFs Cytoplasmic Subunits Modulating Sodium Channel InactivationCytoplasmic Subunits Modulating Sodium Channel Inactivation
FHF Genes, Isoforms and ExpressionFHF Genes, Isoforms and Expression
-trefoil core ~150 aa-trefoil core ~150 aa 25-30 aa25-30 aaFHF1AFHF1B
FHF2AFHF2B
FHF4AFHF4B
66 aa66 aa
62 aa62 aa
64 aa64 aa
4 aa4 aa
9 aa9 aa
69 aa69 aa
FHFs are broadly expressed in neurons of CNS and PNS.FHFs are broadly expressed in neurons of CNS and PNS.
Generally, different classes of neurons express different profile of FHFsGenerally, different classes of neurons express different profile of FHFs
FHF expression commences during neuronal maturation and isFHF expression commences during neuronal maturation and is stably maintainedstably maintained
Sodium Channels in Sodium Channels in Fhf1Fhf1-/--/-Fhf4Fhf4-/--/- Granule Cells Granule CellsInactivate at More Negative Voltage and Inactivate at More Negative Voltage and
Inactivate Faster At Specific VoltagesInactivate Faster At Specific Voltages
V1/2 = -59.1 +/- 4.8 mV
n = 8 cells V1/2 = -72.8 +/- 4.3 mV
n = 9 cells P < 10P < 10-4-4
(from Goldfarb et atl, Neuron, 2007)
WT FHF1+4 KO
KO
KO
WT
WT
Voltage Dependence Time Constants at Specific Voltages
Fhf1Fhf1-/--/-Fhf4Fhf4-/--/- Granule Cells In Cerebellar Slices CannotGranule Cells In Cerebellar Slices CannotFire Repetitively In Response To Sustained Current InjectionFire Repetitively In Response To Sustained Current Injection
(from Goldfarb et atl, Neuron, 2007)
WHOLE CELL PATCH-CLAMPED GRANULE NEURONSIN ADULT MOUSE CEREBELLUM SLICES
SODIUM CHANNELS INACTIVATE AT MORE NEGATIVE POTENTIALSODIUM CHANNELS INACTIVATE AT MORE NEGATIVE POTENTIAL IN FHF MUTANT NEURONIN FHF MUTANT NEURON
Fhf1Fhf1-/--/-Fhf4Fhf4-/--/-Wild TypeWild Type
WHOLE CELL PATCH-CLAMPED GRANULE NEURONSIN ADULT MOUSE CEREBELLUM SLICES
IMPAIRED SODIUM CHANNEL RECOVERY IN FHF MUTANT NEURONIMPAIRED SODIUM CHANNEL RECOVERY IN FHF MUTANT NEURONWild TypeWild Type Fhf1Fhf1-/--/-Fhf4Fhf4-/--/-
Normal sodium channel density and activation in mutant cells
Current-induced depolarization gives rapid 1st action potentialCurrent-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 firingSubsequent action potentials blocked; no repetitive firing
ALTERED SODIUM CHANNEL RESPONSES IN Fhf1ALTERED SODIUM CHANNEL RESPONSES IN Fhf1-/--/-Fhf4Fhf4-/--/- GRANULE CELLS CAUSES IMPAIRED EXCITABILITYGRANULE CELLS CAUSES IMPAIRED EXCITABILITY
““A-type” FHFs Induce Long-Term Inactivation of Sodium ChannelsA-type” FHFs Induce Long-Term Inactivation of Sodium Channels
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 -( from Dover et al, J. Physiology, 2010)
Does Channel Fast Inactivation Limit Long-term Inactivation?Does Channel Fast Inactivation Limit Long-term Inactivation?
Mutant Channel DeficientMutant Channel DeficientFor Fast InactivationFor Fast Inactivation
FHF2A Restores InactivationFHF2A Restores InactivationAnd Augments Long-Term InactivationAnd Augments Long-Term Inactivation
( from Dover et al, J. Physiology, 2010)
Long-Term Inactivation Requires FHF2A Channel-BindingLong-Term Inactivation Requires FHF2A Channel-Bindingand N-Terminal Effector Domainsand N-Terminal Effector Domains
( from Dover et al, J. Physiology, 2010)
Long-Term Inactivation Gating Particle ModelLong-Term Inactivation Gating Particle Model
Antibody Inhibition of Channel Long-Term InactivationAntibody Inhibition of Channel Long-Term Inactivation
( from Dover et al, J. Physiology, 2010)
FHF N-Terminal Peptide Injection Recapitulates Long-Term InactivationFHF N-Terminal Peptide Injection Recapitulates Long-Term Inactivation
( from Dover et al, J. Physiology, 2010)