What is an Action Potential?
Break Down:
Resting Membrane
Potential
AP Generation
Signal Propagation
Neurotransmitter Release
Signal Received (EPSP/IPSP)
Repeat
The arrival of an
action potential at
the terminal will
cause calcium, Ca+,
to enter the cell and
release vesicles
filled with
neurotransmitter.
vesicle
neurotransmitters
exocytosis
Synapse
Receptor
Release of
neurotransmitter
will produce a
change along the
post-synaptic
membrane of the
next neuron in the
chain
…and so on…
Neurotransmitter
Receptor
(ion channel)
• Neurotransmitter engages a receptor (lock-and-key)
• Receptor may be an ion channel
• If it opens an ion channel…
EPSP
Sodium ion flow inward is responsible for the
generation of an EPSP.
IPSP
Chloride ion flow inward is usually responsible for the
generation of an IPSP
Synaptic Release (Movie)
Multiple Inputs
Post-synaptic membrane may
receive an excitatory postsynaptic potential (EPSP) and
become depolarized
or
Post-synaptic membrane may
receive an inhibatory postsynaptic potential (IPSP) and
become hyperpolarized
or BOTH
Multiple excitatory and inhibitory inputs onto
dendrites and the soma summate.
Spatial Summation
EPSP summation is
decrimental- it is
proportional to input
Temporal Summation
Temporal and Spatial Summation (Movie)
Summary
• Neurotransmitter released causes
change in ion permeability on postsynaptic membrane.
• Depending on ion, causes an EPSP or
IPSP
• EPSPs and IPSPs are summated
spatially and temporally
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 voltageactivated 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
Review
Good animated review at
http://outreach.mcb.harvard.edu/animations/actionpotential.swf
What is a membrane potential?
-difference in voltage across the membrane
What is the value of the RMP of a neuron?
-it’s -70mV
What inside the cell is large, immobile, and
negative?
-proteins, AWhat does selective permeability mean?
is different from positive ion
ion .
is negative.
and negative
Which way is concentration force of
Which way is electrical force of
?
?
Define the Chemical and Electrical forces on these
ions in a neuron at RMP (-70inside)
More
Concentration Electrical
concentrated gradient pushes gradient
on the…
it…
pushes it…
Cl-
Outside
IN
OUT
K+
Inside
OUT
IN
Na+
Outside
IN
IN
Determine Chemical and Electrical forces on K+
at different membrane potentials (*remember it’s
still highly concentrated on the inside)
Chemical
Electrical
force pushes force pushes
it…
it…
At RMP (-70 mV)
OUT
IN
At peak (+35 mV)
OUT
OUT
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
What Phase?
EPSPs
Return
to
Rapid
RMP
Repolarization
(generator
Threshold
Hyperpolarization
RMP
Depolarization
potential)
What channels are open?
Which ions are moving?
Which direction?
Na+/K+
non-v-gated
pump
working
and
v-gated
(Na+
K+
K+
K+
Na+
leak
OUT
OUT
IN
↨
out and
Na+K+
IN in)
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
• 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 3:2 by the pump
restores the correct balance.
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
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
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 wouldn’t be any hyperpolarization from
K+ efflux following it until after the AP is started by
the electrode.
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.
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
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 postsynaptic membrane, ie EPSP or IPSP (Ex. Ach →
NA+ influx and GABA →Cl- influx)
699 Research
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Psych >Undergrad >Research >link to research opportunities
or ask a Prof
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before grad school or
employment.
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• Honors thesis