BIOL 460 1st Edition Lecture 20 Outline of Last Lecture I Cardiac Cycle Continued II Valve sounds III Electrical Activity of the Heart IV Electrical Activity of cells of SA node a Pacemaker potential Outline of Current Lecture Current Lecture 1 Electrical activity of myocardial cells a Graph in notes ventricular b We have atrial myocardium and ventricular c Myocardial cells do have resting membrane potential of about 90 d SA node generates AP which are spread though heart eventually they encounter myocardial cells in ventricle i When they do the AP which is depolarization the AP depolarizes myocardial cells to threshold and you get sudden depolarization ii Caused by opening of Na voltage gated channels iii Opened in response to action potential generated by SA node that are spread out throughout heart iv Followed by a phase called plateau phase last 300ms v Plateau phase is where membrane potential becomes more negative and levels out at about 10 vi Caused by two types of channels 1 Open Ca voltage gated channels Ca into cells inside more positive 2 Open K voltage gated channels slow K out makes inside more negative 3 Cancel each other out so membrane pot doesn t change much These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute vii Followed by repolarization K voltage gated channels faster K out Ca close at end of plateau phase and more K open 2 Conducting tissues of the heart or Cardiac conduction system a The SA node pacemaker i Initiates action potential 1 Spread out over two atria at a speed of about 90 cm s a Through atria myocardia atria contract forcing blood into ventricles b Ventricular myocardia is electronically isolated by fibrous skeleton of heart i Need way to spread from atria to ventricular myocardia ii We use AV node slows the pace of action potentials to about 4 cm second iii This to allow the atria to pump blood into ventricles before ventricles contract iv This delay is about 12 2 seconds 2 AP go from Av node into the bundle of His AV bundle Bundle pierces fibrous skeleton of heart and goes into ventricles of heart Branches into right and left bundle branches These bundle branches branch into perkenji fibers Perkinji fibers terminate in papillary muscles 3 Once AP leaves AV node they get faster and faster Once in perkenji fibers they are going 500 cm s 3 Excitation contraction coupling a AP spread into myocardial cells from SA node b Spreads over sarcolemma and down to T tubules c This opens the usual Ca voltage gated channels d Ca voltage gated channels and a little bit puff of Ca goes into cells down electrochemical gradient e Most Ca comes from SR f Release channels in SR i Ca ligand gated release channels ii Called Ca induced Ca release mechanism g Steep concentration gradient allowing Ca to flow into sarcoplasm from SR h Ca binds to troponin C of troponin C tropomyosin complex i No Ca mechanically coupled release channels i This causes systole j To cause diastole i At end of systole AP stop and Ca voltage gated channels close ii Ca ATPase pumps primary active transport pump majority of Ca in sarcoplasm back to SR k On pm of cardiac cells are Ca Na exchangers i Couple downhill movement of Na into cell with uphill movement of Ca out of the cell ii This is secondary active transport Anti port iii Called cotransport proteins 4 Unlike skeletal muscle heart cannot sustain contraction a Skeletal muscles can by asynchronous activation of motor units i Also show summation b Cardiac muscle is organized into myocardium can t sustain contraction because of different motor units c Can t sustain or undergo summation because they have long refractory period i Refractory period is almost same length as systole 300ms ii Necessary to fill up with blood 5 Electrocardiogram a Visual image of electrical image of heart b machine called electrocardiograph c DOES NOT look like AP i Bc leads are on skin and heart s ap cause ion currents are picked up via electrocardiogram d Most important lead is between right arm and left leg across heart i Unipolar arm and ground ii Different leads to let you see different conditions e Electrocardiogram parts i P wave Caused by Atrial depolarization ii QRS complex caused by ventricular depolarization 1 When cells of ventricles 2 Occurs when half of ventricular myocardium is depolarized and half is not iii Between QRS and T wave is st segment corresponds to plateau phase of action potential which is duration of ventricular systole iv T wave ventricular repolarization max amplitude when half atria are depolarized and half are polarized v No atrial repolarization because mass of ventricles is enormous so QRS mask atrial repolarization f PR interval time between atrial systole and ventricular systole 1 2 s g Can detect Cardiac arrhythmias i Bradycardia less than 60bpm 1 Induced by SA node sinus bradycardia 2 Normal in athletes ii Tachycardia more than 100 bpm at resting 1 At rest is abnormal 2 Caused by drugs or abnormally fast ectopic pacemaker iii Flutter extremely rapid rates 200 300 bpm 1 Does pump blood but not much a because not much time to fill with blood b Can degenerate into fibrillation some groups of myocardial cells are generating action potentials and contracting while others aren t i heart doesn t pump blood just quivers c Can have atrial flutter usually degenerates into atrial fibrillation Can live for years like this because ventricles are already 80 full of blood without atria pumping d Ventricular flutter life threatening purpose of defibrillator Parts of myocardium contracting where as other parts aren t circus rhythm i Normally when ap is sent up heart it doesn t turn around and come back due to long refractory period but scar tissue non conductive slows progression of AP through myocardium once wave spreads across myocardium some have left so continuous circling quivers ii Also from enlarged heart due to large ventricles can cause it to take long time for AP to cross it and some cells may be non refractory causing flutter iv AV node block AV blocked 1 Av node has to pass the two atria into the ventricular myocardium If Av node is damaged it can t pass wave of AP from atrial myocardium to ventricular 2 There are three levels of AV node block a 1 degree if PR interval is greater than 12 2 seconds b 2 degree AV node is so damaged sometimes not passed from atria to ventricle P wave no QRS occasionally c 3 degree AV node is so