Introduction to the Action Potential Flashcards
The
separation of charges makes each cell a
battery
the cells’ resting potential is a power source that generates
action potentials
are proteins in the cell membrane that confer permeability to ions
Ion channels
K+
channels are always open regardless of membrane potential. t/f
True
channels that are gated (opened and closed) by the membrane potential
voltage gated ion channels
Two types of voltage gated ion channels generate action potentials. Please enumerate
voltage gated
sodium channel and the other is a voltage gated potassium channel.
three main
features of the channel (voltage gated sodium channels)
- central pore that is selectively permeable to Na+ ions;
- activation gate
- inactivation gate
hat is closed at the resting potential
but is opened by depolarization (when the inside of the cell becomes less negative than rest);
voltage gated Na channel - activation gate
Na channel that is open at rest but is closed by
depolarization.
inactivation gate
inactivation gate is also referred to as a
“ball and chain”
has a structure such that a length of amino acids forms a chain-like structure that is linked to
another group of amino acids that forms a ball-like structure.
inactivation gate
when neurons are at
rest, at about -70 mV, what is the status of voltage gated Na channels
activation gate is closed and the inactivation is open.
The whole
idea of depolarization is to
(status of voltage gated Na channels )
open the activation gate and close the inactivation gate
However,
an important feature of the two gates is that when the axon is depolarized, the opening of the
activation gate is just slightly faster than the
closing of the inactivation gate
After the activation gate has been open for about ___, the slower inactivation gate
swings shut, thereby plugging the pore and preventing the further influx of Na+ ions into the cell.
1.0 ms
as a selflimiting
gate that allows the pore to be open for only about a 1.0 ms and no longer.
inactivation gate
membrane just under the open Na channel will approach this potential
sodium equilibrium potential, ENa,
sodium equilibrium potential, ENa, which is about
+55 mV (inside positive).
during depolarization, this brief period of less than 1.0 ms (1/1000 of a second); the
membrane potential spikes from a resting value of -70 mV to +55 mV, a net change of
125
mV
This spike in the membrane potential, from -70 to +55 mV, is what comprises the ___ of action potential
upstroke
is a stereotyped event
that is triggered by depolarization of the membrane
action potential, or spike
Typically the first depolarization that starts the entire cycle is due to the influx of positive
charges caused by the release of transmitter at the synapse. But once the spike or action
potential is first initiated at the axon hillock, it is reproduced over and over again at each
point along the axon, because of the presence of what structure
voltage gated sodium channels
The rapid re-polarization of the membrane
potential is accomplished by the
opening of voltage-gated potassium channels- efflux of K+ ions.
their
purpose is for the rapid efflux of positive charges from the inside of the cell to the outside.
voltage gated potassium channels
The reason that positive charges can be carried in by one ion (Na+)
and removed by another ion (K+) is that the opening (or closing) of channels depends ONLY
on the
charge inside the membrane
VOLTAGE GATED CHANNELS
OPERATE THOUGH
EQUAL OPPORTUNITY” CHARGES
they care only about charge and
are oblivious to whether the charge was carried by K+, Na +, or Ca++ ions
Na+ channels open
upstroke
downstroke
Na+ inactivation gates are closing
K channels are opening
Only K+ channels are open. membrane is at Ek
undershoot
both Na and K are closed
RM
begins less than 1.0 ms after the activation gates open.
Repolarization
because of the massive increase in potassium permeability, the
axonal membrane at that location approaches the potassium equilibrium potential, EK. This
phase of the action potential is called
the undershoot,
for that brief period the membrane potential is even
more negative than the resting potential.
the undershoot,
sets an upper limit on the rate at which action potentials can be produced.
refractory
period
Since the
refractory period is about
about 1.0 ms,
highest possible firing rate of any neuron is
1000
spikes/sec.
is an all or none event and has a threshold
action potential
When an action potential occurs, it always drives the membrane potential to
+55 mV.
However, depolarizations are graded
and can be small or large or have any value in between. T/F
True
Neurons, however, need a certain level of
depolarization to evoke an action potential, called the
“threshold level”.
Rather, the depolarization
needed to generate an action potential is produced by the
opening of many sodium channels at
the same time.
There is a range of small depolarizations
where the Na+ entry into the cell, while increased over resting, will still be less than the exit of
K+ ions from the cell. This range of depolarizations is referred to as being
“subthreshold”
no action potentials are evoked by these small depolarizations.
“subthreshold
The additional depolarization, caused by the slightly greater influx
of Na+ than efflux of K+, will then open a few more Na+ channels that will bring in more Na+
and initiate the
positive feedback
net result of positive feedback is
rapid, nearly simultaneous
opening of many sodium channels that drives the membrane potential to the sodium equilibrium
potential, ENa, which is an action potential.
if the cell reaches a depolarization where
the entry of Na+ is exactly equal to the exit of K+, and if, in the next instant one more K+ leaves
the cell than Na+ enters, the membrane potential will begin to return to the
resting value.
that value of membrane potential at which the
Na+ current entering the cell just exceeds the K+ current leaving the cell.
threshold
threshold
For most neurons this
is in the range
10 to 15 mV depolarized from rest
