Class Notes (810,488)
Canada (494,139)
Physiology (885)
PHGY 210 (301)
Lecture 10

Lecture 10.pdf

5 Pages
Unlock Document

McGill University
PHGY 210
Michael Guevara

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 ▯ 1 Sarah Margareta Ibrahim▯ Monday, January 28th 2013 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 this!! (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 v. Closely). *Three spaces in body: Intracellular (in cell), interstitial space (between cells) and the intravascular space. (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
More Less

Related notes for PHGY 210

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.