Lecture 2: The Action Potential

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Psy 111 Basic concepts in Biopsychology Lecture
3 The Action Potential
Website http//mentor.lscf.ucsb.edu/course/fall/p
syc111/
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Electrochemical Equilibrium Membrane
Permeability for an Ion.
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Resting Membrane Potential Summary
Vm (61.54) log PK K0out PNa Naout PCl
Cl-out PK K0in PNa Nain PCl
Cl-in
At rest, membrane has a large relative
permeability to K.
For our purposes there is a population of K
channels that are always open i.e. non-gated
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Membrane Potentials in Excitable Cells
Recordings from
Hepatocyte
Today we focus on what is happening when the
membrane is active!
Neuron
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Objectives

• Describe the electrical properties of excitable
  membranes.
• Describe the properties and stages of the action
  potential.
• Define generator potential and threshold and the
  relation between the two.
• What is the relation between magnitude of
  generator potential and magnitude of neuronal
  response?
• Discuss relation of changes in membrane potential
  and relative permeabilities during the stages of
  the action potential.
• Identify the voltage-gated channels involved in
  the action potential and the properties of these
  channels.
• Describe relation between vg-Na channels and
  voltage sensitivity.
• Describe the positive feedback in generation of
  action potentials.
• Discuss role of vg-Na channel inactivation in
  conduction of action potentials.
• Explain the factors determining action potential
  conduction speed.

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The Action Potential

• Properties and description
• Relative permeablities
• Ion channels gating
• Conduction

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Properties and description of the AP
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Properties and description of the AP
Generation of AP requires input
depolarization Generator Potential
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Properties and description of the AP

• Generator potential must be of threshold value
  to generate AP
• Larger generator potentials increase frequency
  (not size) of APs

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Properties and description of the AP
Increasing Generator potential increase AP
frequency -due to transient Relative
refractoriness a.ka. Relative refractory
period. Frequency can be increased only to
maximum 1000/Sec. -due to transient Absolute
refractoriness a.k.a. Absolute refractory
period
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Properties and description of the AP
Falling phase or repolarization corresponds to
absolute refractory period
Undershoot or hyperpolarization corresponds to
relative refractory period
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APs, Relative Permeabilites, Currents
Vm (61.54) log PK K0out PNa Naout
PK K0in PNa Nain
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• Rising Phase
• Inc. Na relative permeability
• Opening extra Na channels

Relative Permeability K Na Rest 40 1 Rise
40 400 Fall 100 1 Rest 40 1
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• Falling Phase
• Inc. rel perm of K
• Closing extra Na channels
• Opening more K channels

Relative Permeability K Na Rest 40 1 Rise
40 400 Fall 100 1 Rest 40 1
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Relative PermeabilityDynamics in an AP.
Relative Permeability K Na Rest 40 1 Rise
40 400 Fall 100 1 Rest 40 1
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Ion Channels mediating APs
Two types of voltage-gated ion channels mediate
changes in membrane permeability during the
action potential Inactivating Voltage-gated Na
Channel Delayed (Rectifying) Voltage-gated K
Channel
Gated channels are those that close or open
resulting in changes in relative permeablity of
the membrane to specific ions.
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Ion Channels

• Channels are a family of structurally and
  functionally similar proteins.
• Form a pore allowing ions to move across membrane
• Comprised of membrane-spanning domains (either
  one polypeptide or separate subunits).

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vg-Na Channels

• Inactivating
• Voltage-gated Na
• Channel
• single polypeptide
• four domains
• 6 transmembrane spans/domains
• voltage-sensitive
• selective pore
• inactivation gate

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vg-Na channels
Inactivating Voltage-gated Na Channel voltage-se
nsitive activation of selective pore is due to
modest depolarization
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Selective Pores
Only Na fits

• Na vs. K ions
• Equal charge
• Different hydrated sizes
• allows Na selective pore.

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vg-Na Channels
Inactivating Voltage-gated Na
Channel -voltage-sensitive activation
gate -inactivation gate
Removal of inactivation occurs only when membrane
repolarizes (ie at-65mV)
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vg-Na channels Blockade.

• Produces reversible lesion by preventing
  depolarization results in blockade of action
  potentials.
• e.g. tetrodotoxin (TTX) from pufferfish or fugu
  contains

Also site of action for local aneasthetics
lidocaine, cocaine (not for rewarding
properties)
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vg-K channels

• Delayed (Rectifying) Voltage-gated K Channel
• four polypeptides
• four subunits
• 6 transmembrane spans/domains
• voltage-sensitive
• slow activation
• selective pore

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vg-K channels

• Selective pore as for non-gated K channel
• Similar vg mechanism as vg-Na channels

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Ion Channels in APs
Rest ng-K channels open vg-Na channels
closed vg-K channels -closed
Rise ng-K channels open vg-Na channels
open vg-K channels -closed
Fall ng-K channels open vg-Na channels
inactivate vg-K channels open
Rest ng-K channels open vg-Na channels
close vg-K channels close
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Conduction of the Action Potential
-movement of AP from place to place along the
membrane.

• AP Generation in the axon hillock
• AP Conduction down the axon
• Conduction velocity and myelination

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Generation of AP in Axon Hillock
Axon Hillock (Initial Segment) -high in vg Na
Channels -high sensitivity (relative to dendrites
cell body) to generator potentials -amplifies
generator potential
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Generation of AP in Axon Hillock
Generator Potential reaches axon hillock
Act like a domino effect
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Generation and conduction of APs

• Density of voltage-gated Na channels determines
  sensitivity to depolarization
• Allows generation of AP (hillock)
• Conduction of AP (axon)

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Ions, Channels, Microgradients
e.g. Na channels

• Channel Closed
• (at rest)

• Channel Opens
• (ions rush in)

3. Channel Closes (ions diffuses)
Botton (Inside) View
Side View
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AP Generation and Conduction
Generator Potential
x
x
But why does it go in one direction?
X inactivation gate
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Conduction of AP down Axon
Again, goes in one direction due to inactivation
gate on proximal (toward soma) side of axon
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Conduction Speed
Speed is determined by conductance which is
proportional to volume, larger is faster and less
leakage (i.e. surface area versus volume).
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Conduction Speed
Myelin serves to insulate axon reduces
leakage-gtstronger repulsion down axon-gtfaster
conduction