Sarah Margareta Ibrahim▯ Monday, January 28th 2013
Lecture 10 - Cardiovascular Physiology (Part 4 of 9)
(1) Electrical System of the Heart
Your heart is an electrical organ like your brain is an electrical organ. Main function of the
heart is not to generate an electrical signal - itʼs to pump (remember: pump, pipe ﬂuid).
The thing that sets the pump off and that coordinates the pump is an electrical system.
(2) The word “Electric”
The word electric comes from the greek word “electrum” (amber) or fossilized tree sap.
What happens if you take a piece of amber and rub it on a piece of cloth or fur?
Generates static electricity.
(3) Activation Sequence of the Heart
Every time your heart beats there are action potentials that sweep through the heart and
itʼs a fact that the cardiac cell has an action potential and this is the ﬁrst event that
eventually causes the contraction of the cell. No action potential= no contraction = no
heart beat = dead. So electrical system is very important. In our hearts thereʼs an area
high up in the right atrium called the sinoatrial node (SA node) or the sinus node. The
reason that the heart is beating right now is due to the presence of that small piece of
tissue. In humans itʼs about ½ cm x ½ cm x ½ cm. Thatʼs the pacemaker of the heart. In
that area of the heart, youʼll ﬁnd cells that are spontaneously active - they beat
spontaneously and spontaneously generate action potentials, these are pacemaker
cells. In contrast to the rest of the heart where the cells are not pacemaker cells. About
30 years ago we found out how to isolate single cells from heart and if you isolate a
single cell from the sinus node and you look at it under a microscope youʼll see itʼs
contracting all by itʼs self. If you take a regular old cell from the atrial muscle or
ventricular muscle what do you see? Nuttin. The cells in the SA node are where every
heartbeat starts. The action potentials then spread out from cell to cell to cell. The action
potential will ﬁnd itself in the muscle cells of the right atrium, it will then spread through
those cells and over to the muscle cells in the left atrium. Depolarization (activation)
wavefront that spreads through the entire heart. Remember the ﬁbrous ring? (That ring
that we cut open across to look at the valves). Remember that the ring is not made up of
muscle cells!! Therefore, they arenʼt electrically excitable and they donʼt have action
potentials. The action potentials have to get from atrium to the ventricles. Thereʼs
something called the specialized ventricular conducting system that does this for you.
Thereʼs a small bit of tissue called the atrioventricular node (AV node) made of cardiac
cells (so muscle cells) but itʼs specialized like the sinus node. The function of sinus node
and AV node is not to generate force. The atrial muscle and the ventricular muscle
generates force. Everything in yellow is part of the specialized system of the heart thatʼs
involved in the start (generation) and conduction of the action potential to the muscle.
The action potentials, as they travel through the right atrium, before they have a chance
to come over to the left atrium, they start to move through the AV node. They move down
the AV node to the Bundle of His (named after William His). Action potentials move down
the Bundle of His and then the Bundle of His breaks up (bifocates) into two branches -
the bundle branches (the left bundle branch and the right bundle branch). Action
potential will simultaneously travel down both bundle branches. Those branches break
up into a ﬁne network called Purkinje ﬁbers (those little things at the ends). Are these
Purkinje ﬁbers neurons? No! Theyʼre specialized cardiac ﬁbers. The purpose of this
system that branches is to bring the action potentials, more or less simultaneously to the
endocardial surface of the muscle (remember this is the muscle just below the
endocardium). The action potentials show up more or less at the same time along all of
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the endocardial surface and then they move through the thickness of the wall of the
ventricles endocardium to epicardium at the same time and thatʼs how the action
potential is propagated through the ventricular muscle. Sometime after action potential
has started you get a chain of events and you get a contraction of muscle. This is the
normal activation sequence of the heart: SA node -> Right atrium -> Left Atrium -> AV
node -> Bundle of His -> Bundle branches -> Purkinje ﬁbers -> ventricular muscle. Know
(4) Purkinje Fibre Network
Thereʼs a particular stains that you can use that preferentially goes to the Purkinje ﬁbers.
If you take a ventricle, stain it and open it up ﬂat, youʼll see something like the ﬁgure. All
along the endocardial surface you see these Purkinje ﬁbers coming out of the bundle
branches at the top. The Purkinje ﬁbers then go into the endocardial muscle. These are
bringing the action potentials to the endocardial muscle and then the action potentials
will travel through the endocardial muscle, through the mid-myocardial muscle and
eventually end up in the epicardial muscle. Then everything is over and you wait for a
fraction of a second until the whole cycle starts over again with the next spontaneous
beat starting in the SA node.
(5) Intercalated Disk
How do the action potentials travel from cell to cell? Remember that cardiac muscle cell
is 100 microns long and 20 microns in diameter. Itʼs a cylinder. Where the two ends of
the cell comes together you see a line in the picture. These arenʼt just lines, itʼs a
specialized junction and there are various things in that junction so that when they are
activated and contract, the cell will contract along and so they are sort of pulling against
each other fast, one to the next to that they donʼt come apart. If you go down to a higher
magniﬁcation even though it looks like a straight line in this picture itʼs not...
(6) Nexus or Gap Junction
Itʼs a very meandering structure. What you see in this image is a very small part of an
intercalated disk running from bottom to top. At the top you have one cell and at the
bottom you have another cell. Space in between cell membrane is the interstitial space.
Whatʼs happened to the interstitial space at the black line? Itʼs gone! Itʼll go away and itʼll
come back. This black line is a specialized space and itʼs called the nexus or gap
junction. This is where the membranes of the two cells almost fuse (they come together
*Three spaces in body: Intracellular (in cell), interstitial space (between cells) and the
(7) Connexons or Hemi-Junctions or Hemi-Channels
The white objects in this image are hemi-junctions (half-junctions or half-channels or
connexons). Thatʼs the hexagonal array of particles. Itʼs a protein molecule. These kind
of ﬂoat around in the membrane of the two cells and when they come together they dock
together (each of the half channels) to make a complete gap junction ﬂow channel.
In the cross-section, you see one half channel and the other half channel and theyʼve
docked but thereʼs a hole in the middle - thereʼs a pore there! This pore is quite big. Itʼs
ﬁlled with the ﬂuid with the cytoplasm of the cell. About 10,000 daltons can pass though
those channels. But weʼre talking about action potentialʼs propagation so what do we
need to move from cell to cell? Charged objects - ions! So things like sodium, potassium,
calcium etc. Have no problem passing through these channels. They arenʼt like the
sodium or potassium channels in the membrane of the cell which will only pass sodium
or potassium - these are non-selective, theyʼre just huge holes in the membrane.
Because itʼs electrical, ions can ﬂow from interior of one cell to interior of other cells
▯ 2 Sarah Margareta Ibrahim▯ Monday, January 28th 2013
through this channel, thatʼs the method of propagation through which the action potential
occurs. If you break those channels apart propagation is going to cease in the heart.
(8) Local Circuit Currents
How does this work? This involves the concept of local circuit currents - there are
currents that are generated locally that ﬂow in a circuit. Action potential is creeping
through these ﬁve cells in the ventricle. The cells on the right are resting from the
previous heartbeat and they havenʼt been activated yet - theyʼve repolarized after
previous heartbeat and are back to their resting potential. But depolarization has
reached cell A. Cell A has had the action potential, been activated/depolarized. What will
the voltage be across the membrane of the cell if itʼs in the resting state? -70. The one
thatʼs been activated is sitting at +20 or +30. If you are a potassium ion and youʼre sitting
on the left side of the gap junction ion between cell A and cell B what do you do? Youʼre
carrying a positive charge so youʼll be attracted by those bad boys with the negative
charge across the gap. So youʼll move because youʼre being repelled by the positive
charges in your environment. Same thing holds true for sodium and calcium. Thatʼs
inside the cell though. What about in the interstitial space? The sodium sees the line of
negative charges on the opposite cell so it will move in the opposite direction (but
remember this is outside the cell). So there will be a ﬂow of charge and this is called a
current and itʼs local and itʼs happening right at this junction between a cell that has been
activated and a cell that has not been activated. Itʼs in a circuit (moves in a circle).
If you were a chloride ion sitting on cell A (outside) you would move to the outside of cell
B. Every charged particle is going to move whether itʼs