but because potassium channels become
more active so potassium losses become
very very heavy there is a heavy
potassium efflux and as more and more
potassium is going out it means more and
more positive charges are going out it
means cell will again start developing
electronegativity because cell is losing
the positive potassium so when
potassium keep on moving out what really
happens that cells due to loss of
positively charged potassium become
progressively electro negative and
potassium is progressively being lost
potassium is being lost until membrane
potential again becomes at which value?
resting membrane potential value that is
minus 90 milli volts this is that right
so what happened during this time what
happens really that electrically
membrane has come back to its negative
polarization if membrane has come back
to its negative polarization it means
membrane is repolarized due to potassium
efflux membrane has gone to the
phenomenon of repolarization let us
revise it again what really happens this
was a resting myocardial cell it was
having resting membrane potential of
minus 90 milli volt is that right when
it was appropriately stimulated some
cations trickled in entered in these
cations took the resting membrane
potential at threshold potential at
threshold potential voltage-gated sodium
channels open lot of sodium comes in and
rest and then potential goes towards the
it loses all the electronegativity and goes
towards the positive side this
phenomenon is called membrane is
undergoing which phenomenal
depolarization membrane is depolarized
after depolarization in ventricular and
atrial cells repolarization process
start and repolarization process
initially
potassium goes out but very soon calcium
start coming in for a brief time
potassium is going out but at the same
time calcium is coming in so cations are
lost as well as cations are gained so
there is no voltage difference we say
that electrical potential is showing
plateau phase but eventually calcium
channels shut down and potassium
channels become progressively more open
so lot of potassium leaks out and due to
heavy losses of positive ions right
membrane become progressively electronegative
inside and potassium keep on
going out until the membrane voltage
goes back to its resting negatively
polarized state so we say that there is
membrane has gone under the phenomenon
of repolarization so these two processes
together they are called depolarization
and repolarization is that right so by
stimulation we say this cell number one
has undergone process of depolarization
and after the depolarization it has
undergone repolarization is that right
once this cell has cell went under
depolarization once this cell it was
undergoing depolarization due to sodium
and flats right thus sodium which was
coming to the sell some of this sodium
through gap junctions these are very
special connections between the adjacent
myocardial cells adjacent myocardial
cells neighboring myocardial cells have
electrical windows so what happens when
one cell undergo depolarization the
cations which have come into the cell
some of these cations trickle to the
next cell
here they go to the next cell when's
cations from the first stimulated cell
prickle into next cell what really
happens then resting membrane potential
of the next cell will reach up to yes
please reach up to threshold and then
what will happen voltage-gated sodium
channels will open into this then same
thing will happen for short time
voltage-gated calcium and potassium
channels will be open simultaneously
eventually calcium channel stop but
potassium channel keep on working and
losses of potassium eventually take the
membrane potential back to its resting
situation so it means depolarization of
one cell led to the depolarization of
second cell and when second cell is
depolarized when second cell gate gains
lot of cations you trickles the cations
into third cell and then third cell
undergoes the process of depolarization
and repolarization it means that this
fluctuating potential of the membrane
right the membrane fluctuate from
resting and potential to depolarization
and then it fluctuates back to
repolarization this depolarization and
repolarization they are moving in the
cell membranes because depolarization
and reposition the moving so this
fluctuation total electrical fluctuation
of depolarization and repolarization
because it is mobile it is called action
potential as the right let me write it
more clearly that there is yes resting
membrane potential and with proper
stimulation there is stimulation resting
minus potential will lead to the
membrane to yes please depolarization
then depolarization will stimulate
initially calcium and potassium channels
but eventually potassium channels become
very active and a lot of potassium goes
out and that leads to repolarization
and as depolarization and
repolarization the moving on the
membrane so both of them together they
are called action yes please action
potential is that right? so in next
discussion when I say their cells are
undergoing depolarization it means cell
is gaining cations usually sodium in
case of atrial and ventricular cells the
undergo during depolarization the gain
sodium cations right and if there is
special point if there is depolarization
of SA node and AV node remember
depolarization of SA node and AV node is
not sodium-dependent that is
calcium-dependent so it means when I say
that atrial tissue or ventricular
tissues are drawing depolarization it
means those tissues are gaining sodium
and if I say SA node and AV node are
depolarizing it means they are gaining
calcium but anyway depolarization is
whenever cell is appropriately
stimulated a lot of sodium or calcium
come in and depolarizes the cell but
whenever cell is depolarized it is
followed by which activity
repolarization and this depolarization
in one cell stimulates the next cell
through the gap junctions then next cell
become depolarized and second cell will
depolarize the third cell and third cell
will depolarize the next cell so in this
way in the heart because every cell has
electrical windows or gap junctions with
the next cell so electrical stimulation
of one cell can lead to the electrical
stimulation of the next cell am i clear
if this basic concept is clear now we'll
talk about that little bit more in
detail right
eventually what we have to understand
today we have to understand that how
electrical activity of the heart can
eventually be presented as ECG pattern
is that right now we develop some more
basic concepts let's suppose here is
your heart these are your atrial
chambers and here we have your yes please
ventricular chambers right of course
here should be SA node sinoatrial node
and in between the atrial and ventricle
this is fibrous what is this fibrous
anulus right and only electrical
connection between the atrial and ventricle
is what is this? AV node the only
electrical window between the atrial
myocardium and ventricular myocardium is
AV node is it right? so this is SA node
here it is AV node this is atrial
myocardium and here it is ventricular
myocardium right now you see