Unformatted text preview: CARDIAC MUSCLE
- Control of Contraction - Specialized Conduction System of the Heart - Autonomic Influence - Electrocardiogram Cardiac Muscle Cell Anatomy Striated like skeletal, but smaller cells and have a single nucleus Branched Cell junction = Intercalated disk Desmosomes Gap junctions T-tubules (larger than skeletal) Sarcoplasmic reticulum (not as extensive as skeletal) Mitochondria! Figure 14-7h Figure 14-10b Table 12-3 Cardiac Cell Types Contractile cells Autorhythmic cells (pacemakers) smaller with few contractile fibers Action potential is initiated in pacemaker cells and spreads via gap junctions from one contractile cell to the next Cardiac Excitation-Contraction Coupling Action potential propagates along sarcolemma Opens voltage-gated calcium ion channels Calcium influx RyR (ryanodine receptor) channels on SR membrane open due to binding of calcium Calcium released from SR Calcium binds to troponin, etc. (90% of intracellular calcium is from SR, 10% is extracellular) Relaxation Ca2+-ATPase pumps calcium back into SR Cation exchange at sarcolemma Intracellular calcium is exchanged with extracellular sodium Na+- Ca2+ Antiport (1 Ca2+ out for every 3Na+ in) 3Na+ removed via Na+/K+-ATPase Figure 14-11 - Overview Can increase the force of contraction by increasing the amount of calcium influx from the extracellular fluid The more calcium influx that occurs, the more calcium that is released from the SR, the more calcium binding to troponin, the more crossbridges that can be formed between actin + myosin, the more tension developed. Action Potential in Pacemaker Cells (Slow AP) Spontaneously depolarize
-60 mV (pacemaker potential) to threshold (-40 mV) Na+ influx exceeds K+ efflux (If channels) As depolarization continues, If channels close and some Ca2+ channels open At threshold:
More Ca2+ channels open rapid depolarization Repolarization
Ca2+ channels close Slow K+ channels open - K+ efflux Figure 14-15 - Overview Frequency of spontaneous depolarization of these cells establishes heart rate the frequency of contraction of the heart Autonomic Influence on Heart Rate Sympathetic Increases HR Cathecholamines: NE (neurons) + EPI (adrenal medulla) Bind to 1-adrenergic receptors on pacemaker cells cAMP 2nd messenger system changes ion channels Increased membrane permeability to both Na+ and Ca2+ More rapid depolarization to threshold to increase rate of action potentials from pacemaker cells Figure 14-16a Parasympathetic ACh binds to muscarinic receptors cAMP 2nd messenger system changes ion channels Hyperpolarization takes longer to reach threshold Depolarization to threshold takes longer Increased K+ permeability Decreased Ca2+ permeability Figure 14-16b Action Potential in Contractile Cells (Fast AP) RMP at -90 mV Rapid depolarization Fast voltage-gated Na+ channels open Na+ influx Close at +20 mV Na+ channels close Some K+ efflux Slight repolarization Plateau Voltage-gated Ca2+ channels open (Ca2+ influx) Decrease in K+ permeability (less efflux) Ca2+ channels close Slow K+ channels open (efflux) Rapid repolarization Figure 14-13 Action potentials in cardiac muscle cells (200 msec) is much longer than skeletal muscle cells (5 msec) Long absolute refractory period in cardiac muscle cells means that summation of contractions is not possible No Tetanus! Important for pumping activity of heart Figure 14-14 - Overview Table 14-3 Electrical events precede mechanical events
Depolarization Contraction Repolarization Relaxation Electricity spreads from one cardiac muscle to the next via gap junctions Defined pathway for flow of electricity results in all atrial cardiac muscle cells contracting simultaneously, then in all ventricular cardiac muscle cells contracting simultaneously. Figure 14-7f Figure 14-7g Conduction Pathway Sinoatrial (SA) Node Right atrium where superior vena cava enters Main pacemaker (fastest rate of spontaneous depolarization) Fast conduction Slower conduction Can not spread from atrial muscle to ventricular muscle because of CT barrier; only way to spread electricity to ventricles is through AV Node Conduction slows here to allow atria to completely contract before ventricles contract SA Node to AV (atrioventricular) Node Depolarization across atrial muscle AV Node Bundle of His (Atrioventricular or AV Bundle) Electricity flows down in between R + L ventricles Purkinje fibers (fast conduction!) R + L Bundle Branches Purkinje fibers spread out into contractile cells in walls of ventricles Figure 14-17 Figure 14-18 Figure 18.14a AV Node also has pacemaker activity Slower rate then SA Node SA Node 70 per minute AV Node 50 per minute Fastest pacemaker sets the pace ECG (Electrocardiogram) Measurement of the electrical activity of the heart Surface electrodes used (ok because our extracellular fluid is a good conductor of electricity) Summation of all action potentials occurring at a given time Indicates contraction and relaxation of the heart (mechanical events) Figure 14-22 - Overview P wave Atrial depolarization Ventricular depolarization QRS complex (Atrial repolarization occurs at the same time) T wave Ventricular repolarization Positive or negative deflections represent the location of the electrodes measuring the electricity Variety of leads that can be used (I, II, III, 6 V leads, others) Figure 14-19 Figure 14-20 Figure 14-21 - Overview (1 of 9) ...
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This note was uploaded on 04/29/2009 for the course BIOL 425 taught by Professor Tondi during the Fall '08 term at George Mason.
- Fall '08