action potentials
are a sequence of rapidly occurring
events
that decrease and eventually reverse the
membrane potential
this is referred to as depolarization
they then eventually restore it back to
its resting
state which is referred to as
repolarization
action potentials are what result in
nerve impulses action potentials can
only occur if the membrane potential
reaches
threshold threshold is usually
approximately
negative 55 millivolts
if a stimulus is at sub
threshold strength meaning there's not a
very strong stimulus
it can cause a local graded potential
but if it does not go up to threshold
it will not lead to an action potential
if the stimulus reaches threshold
strength
it will cause an action potential and a
nerve impulse will be able to be
propagated
down the axon
if i use a super strong stimulus
or a supra threshold strength
this does not actually change the size
or amplitude of the action potential
like what we saw in graded potential
instead it affects the frequency
of the action potential please
note that action potentials are all or
none
and one size fits all
the voltage across the membrane of an
action potential
can be correlated with the ion channels
found in the membrane
in the resting state the membrane
potential is established by leakage
channels
and the sodium potassium pump also in
the membrane we have voltage-gated
channels specifically
sodium and potassium voltage-gated
channels
but during rest these are closed
also notice that there are two gates on
the sodium channel
there's an activation gate and an
inactivation gate
this actually is going to be very
important for establishing an
action potential recall that at resting
membrane potential
the voltage of the membrane is negative
70 millivolts
when we depolarize to threshold
this happens when the activation gate
of the voltage-gated sodium channel
opens
allowing sodium to come into the cell
the voltage-gated potassium gate however
remains
closed remember the
membrane potential at threshold needs to
get to at least
negative 55 millivolts
eventually the membrane potential will
go up to
positive 30 millivolts that is the peak
of an action potential after the peak of
the action potential
repolarization begins we begin our
descent
back toward the resting potential
this can happen when the inactivation
gate of the sodium channel closes
thus stopping sodium from coming in as
well
the voltage-gated channel of potassium
is going to allow potassium to leave the
cell
thus positive charges are leaving and
we're now starting to return
back to the negative state after we
reestablish
our resting membrane potential the
activation gate of the sodium channel is
going to close
the inactivation gate is going to open
and as well the potassium channels are
also going to close
we are now back to our original resting
membrane potential
of negative 70 millivolts
in order for communication to occur from
one body part to another
action potentials must travel all the
way
from the trigger zone of the axon to the
terminal of the axon or the axon
terminals
action potentials do not die out and are
able to keep their strength over the
entire length
of the neuron there are two ways
in which the action potential is
propagated depending on the presence
or absence of a myelin sheath
we have continuous conduction which
happens
when there is no myelin sheath and then
we have
saltatory conduction which occurs in the
presence of a myelin sheath
or in the white matter
in continuous conduction the action
potential is going to proceed
down the membrane toward the axon
terminals
however in saltatory conduction the
action potential
is able to jump from node to node
down the myelinated tissue these
nodes are referred to as nodes of
ranvier
they are spaces in between the
oligodendrocytes or the schwann cells
and in these spaces that is where the
sodium channels are going to open up
this allows for the nerve impulse to
kind of hop
down the neuron as opposed to continuing
along the neuron smoothly if you compare
the distance of an action potential at
the same time point
so in this case 10 milliseconds after
the beginning of an action potential
you'll find that in saltatory conduction
the action potential gets further toward
the axon terminal
than it does in continuous conduction
there are several factors that are going
to affect the propagation of speed
you have axon diameter where the larger
the diameter of the axon
the faster the impulse will go
the amount of myelination where the more
myelination you have
the faster the impulse will go and
finally
temperature where the higher the
temperature the faster the impulse
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