What is an Action Potential?

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What is an Action Potential?
Break Down Resting Membrane Potential AP
Generation Signal Propagation Neurotransmitter
Release Signal Received (EPSP/IPSP) Repeat
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The arrival of an action potential at the
terminal will cause calcium, Ca, to enter the
cell and release vesicles filled with
neurotransmitter.
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vesicle
neurotransmitters
exocytosis
Synapse
Receptor
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Release of neurotransmitter will produce a change
along the post-synaptic membrane of the next
neuron in the chain and so on
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• Neurotransmitter engages a receptor
  (lock-and-key)
• Receptor may be an ion channel
• If it opens an ion channel

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EPSP
Sodium ion flow inward is responsible for the
generation of an EPSP.
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IPSP
Chloride ion flow inward is usually responsible
for the generation of an IPSP
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Synaptic Release (Movie)
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Multiple Inputs
Post-synaptic membrane may receive an excitatory
post-synaptic potential (EPSP) and become
depolarized
or Post-synaptic
membrane may receive an inhibatory post-synaptic
potential (IPSP) and become hyperpolarized
or BOTHMultiple excitatory and inhibitory inputs
onto dendrites and the soma summate.
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Spatial Summation
EPSP summation is decrimental- it is proportional
to input
Temporal Summation
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Temporal and Spatial Summation (Movie)
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Summary

• Neurotransmitter released causes change in ion
  permeability on post-synaptic membrane.
• Depending on ion, causes an EPSP or IPSP
• EPSPs and IPSPs are summated spatially and
  temporally

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Summary - Action Potential

• Initially the cell is resting at around -70 mV.
• Cell receives EPSPs and IPSPs from other neurons.
• The cell becomes excited (depolarized) enough
• Threshold (-55 mV) is reached, voltage-gated Na
  channels open and action potential is sent down
  axon from the axon hillock.
• Inward rush of Na depolarizes adjacent area of
  axon and preps new AP site.
• The AP peaks (35 mV), Na channels close and
  voltage-activated K channels open.
• K efflux follows Na influx down the axon and
  causes hyperpolarization that prevents the AP
  from traveling backwards.
• AP reaches axon terminal. Ca enters and
  releases vesicles filled with neurotransmitter.
• NT crosses the synapse and binds receptor on
  post-synaptic cell

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Review
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• Good animated review at http//outreach.mcb.harvar
  d.edu/animations/actionpotential.swf

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• What is a membrane potential?
• -difference in voltage across the membrane
• What is the value of the RMP of a neuron?
• -its -70mV
• What inside the cell is large, immobile, and
  negative?
• -proteins, A-
• What does selective permeability mean?

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• is different from positive ion and
  negative ion . is negative.

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Define the Chemical and Electrical forces on
these ions in a neuron at RMP (-70inside)
More concentrated on the Concentration gradient pushes it Electrical gradient pushes it
Cl- Outside IN OUT
K Inside OUT IN
Na Outside IN IN
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Determine Chemical and Electrical forces on K at
different membrane potentials (remember its
still highly concentrated on the inside)
Chemical force pushes it Electrical force pushes it
At RMP (-70 mV) OUT IN
At peak (35 mV) OUT OUT
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Depolarize or Hyperpolarize?

• Membrane potential travels from -70 ? -55
• Depolarize
• Membrane potential travels from -70 ? 10
• Depolarize
• Membrane potential changes from -70 ? -80
• Hyperpolarize
• Na enters cell Depolarize
• Negative Cl- enters resting cell (-70)
  Hyperpolarize

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What Phase?
EPSPs (generator potential)
Return to RMP
Rapid Depolarization
RMP
Repolarization
Threshold
Hyperpolarization
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What channels are open? Which ions are moving?
Which direction?
non-v-gated and v-gated Na IN
Na/K pump working (Na out and K in)
K leak ?
Na IN
K OUT
K OUT
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The permeability (leaking) of which ion is proportional to the RMP K
Which ion enters the cell, depolarizes the membrane and starts the AP Na
Which ion repolarizes the membrane by leaving the cell K
Hyperpolarization is the function of which ion channels remaining open K
What channel/pump maintains and reestablishes the RMP? Na/K pump
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• K leaks out across the membrane more easily than
  Na leaks in
• The Na/K pump trades 3 Na for 2 K, does this
  really restore balance?
• Yes, because the neuron does not stay at RMP for
  long, it will fire APs, and eventhough there is
  an efflux of K from the neuron, there is so much
  Na influx that a 32 by the pump restores the
  correct balance.

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• True or False

Ion distribution at resting in the axon is like
that of the dendrites and soma.
TRUE Electrical and chemical gradients act
differently on ions in the axon than in the soma.
FALSE K efflux follows Na influx as it
proceeds down the axon. TRUE Na starts
entering the next site while K is
hyperpolarizing the old site. TRUE
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• Myelination pushes the field of depolarization
  from Na entry further than it would go in an
  unmyelinated axon. TRUE
• In saltatory conduction, the depolarization jumps
  over the outside of the myelin. FALSE
• Na is already depolarizing the next node while
  K is beginning to efflux. TRUE
• An AP starts out at full force and gets weaker as
  it travels down the axon. FALSE

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• Depolarization from Na is not localized, but
  also effects nearby membrane. What does it do
  there? What is the importance?
• Depolarizes neighboring membrane to threshold so
  that it opens v-gated Na channels and fires AP
  next.
• If you put an electrode into the middle of an
  axon and stimulate (depolarized it), the AP would
  go in what direction? Why?Both. There
  wouldnt be any hyperpolarization from K efflux
  following it until after the AP is started by the
  electrode.

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• Entry of which ion besides Na is important for
  release of vesicles containing neurotransmitter?
  
• Calcium, Ca
• Name two ways in which EPSPs and IPSPs summate
• Temporal and spatial
• If there are enough EPSPs, what happens?
• An AP is sent from the axon hillock.

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EPSP or IPSP?

• GABA receptor lets Cl- into cell IPSP
• Ach receptor lets Na into the cell EPSP
• Ach receptor lets Ca into the cell EPSP
• Glycine receptor lets Cl- into the cell IPSP
• A different GABA receptor causes K to leave the
  cell IPSP

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• Be able to
• Determine which direction K, Na, and Cl- would
  travel at various membrane potentials (ex. -70,
  -55, 0, 30)
• Describe which ions are moving and why at each
  stage of the AP
• Predict change in AP profile under influence of
  neurotoxins (Ex. TTX that blocks v-gated Na, a
  toxin that blocks K efflux)
• Predict the effect of a neurotransmitter on the
  post-synaptic membrane, ie EPSP or IPSP (Ex. Ach
  ? NA influx and GABA ?Cl- influx)

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