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بسم الله الرحمن الرحيم

بسم الله الرحمن الرحيم. ﴿و ما أوتيتم من العلم إلا قليلا﴾. صدق الله العظيم الاسراء اية 58. Properties of Cardiac Muscle. By Dr. Abdel Aziz M. Hussein Lecturer of Medical Physiology Member of American Society of Physiology. Cardiac Properties. Cardiac Properties.

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بسم الله الرحمن الرحيم

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  1. بسم الله الرحمن الرحيم ﴿و ما أوتيتم من العلم إلا قليلا﴾ صدق الله العظيم الاسراء اية 58 Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  2. Properties of Cardiac Muscle By Dr. Abdel Aziz M. Hussein Lecturer of Medical Physiology Member of American Society of Physiology Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  3. Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  4. Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  5. Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  6. Cardiac Properties Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  7. Cardiac Properties Impulses originate regularly at a frequency of 60-100 beat/ min SAN AVN

  8. Cardiac Rhythmicity Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  9. Rhythmicity • Def. • Rhythmicity is the ability of heart to beat regularly • Automaticity is the ability of heart to generate impulses without external stimuli • Autorhythmicity is the ability of cardiac ms to generate impulses without external stimuli at regular rate Draabdelaziz Hussein, Mansoura Faculty of Medicine

  10. Rhythmicity • Origin • Myogenic not neurogenic. • The nerves control the rate but do not initiate the beat cocaine • Evidences: • a) Local anesthetics e.g. cocaine block the nerve but not stop the heart Draabdelaziz Hussein, Mansoura Faculty of Medicine

  11. Rhythmicity • Evidences: b)Transplanted heart (no nerve supply) continues to beat. c) Heart of human fetus start to beat before development of nerves Draabdelaziz Hussein, Mansoura Faculty of Medicine

  12. Rhythmicity • Nature: • Most cardiac fibers have the ability of self excitation especially nodal and conducting fibers 110 b/min 90 b/min • SAN has the greatest rhythm, so it is called "pace maker of the heart 45 b/min 35 b/min 25 b/min Draabdelaziz Hussein, Mansoura Faculty of Medicine

  13. Rhythmicity Mechanism of Autorhythmicity of SAN: • RMP of SAN is low about -55mv to -60 mv. • This is due to natural leakiness to Na ions Action Potential (AP): Prepotential or pacemaker potential (phase 4) 2. Upstroke (phase 0) 3. Repolarization (phase 3) Draabdelaziz Hussein, Mansoura Faculty of Medicine

  14. Pacemaker Potential It is a gradual rise of membrane potential from the resting level of -55 mv to the firing level or the threshold voltage of -40 mv It occurs during diastole, so it is also called the diastolic depolarization (DD) Mechanism: Na influx through funny (slow) Na channels 2. Ca influx through T (=transient) -type Ca channel 3. Decreased K+ efflux Draabdelaziz Hussein, Mansoura Faculty of Medicine

  15. Pacemaker Potential Importance of Pacemaker potential: 1. It is the cause of Rhythmicity. 2. The rate of slope of the prepotential determines the heart rate. The more rapid is the slope, the more will be the heart rate. Draabdelaziz Hussein, Mansoura Faculty of Medicine

  16. Action Potential 2. Upstroke (phase 0): • From membrane potential (– 40 m.v) to (+ 10 m.v) Mechanism: • Inward Ca++ current through L (long lasting ) Ca++ channels Draabdelaziz Hussein, Mansoura Faculty of Medicine

  17. Action Potential 3. Repolarization (phase 3): • From membrane potential (+10 m.v) to (-60 m.v) Mechanism: • increase K+ efflux Draabdelaziz Hussein, Mansoura Faculty of Medicine

  18. Action Potential Draabdelaziz Hussein, Mansoura Faculty of Medicine

  19. mv +10 0 Repolarization Depolarization -10 Phase 4 (only in pacemaker cells Phase 3 -20 Phase 0 -40 R.M.P -60 Na+ Na+ Na+ ca++ ca++ ca++ ca++ Na+ Na+ ca++ ca++ Na+ ca++ Na+ K+ m ca++ ca++ ca++ ca++ h K+ K+ K+ K+ K+ K+

  20. Factors affecting Rhythmicity Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  21. Factors Affecting Rhythmicity • Chronotropism means an influence on the heart rate • +ve chronotropic → ↑ heart rate • -ve chronotropic → ↓ heart rate Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  22. Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  23. Autonomic N. System Sympathetic stimulation: • has a +ve chronotropic effect • Release noradrenalin → increases fiber membrane permeability to Na+ & Ca++→ rapid slope of Prepotential → threshold is reached rapidly → increase Rhythmicity Figure 14-17: Modulation of heart rate by the nervous system

  24. Autonomic N. System Parasympathetic stimulation: • has a -ve chronotropic effect • Acetylcholine released at the vagal endings →increase the permeability of fiber membrane to K+→ rapid K+ efflux → hyperpolarization→ decrease Rhythmicity Figure 14-17: Modulation of heart rate by the nervous system

  25. Physical Factors Warming: • Moderate warming : increases Rhythmicity due to; a)↓permeability of the membrane to K+ ions during the pacemaker potential, i.e. rapid slope of prepotential b)↑speed of ionic fluxes across the membrane during the action potential • Excessive warming (45 ) denatures the intracellular proteins and produces cardiac damage • Moderate coolingdecreases Rhythmicity • Excessive coolingstop Rhythmicity Figure 14-17: Modulation of heart rate by the nervous system

  26. Chemical Factors 1. Drugs and hormones: • Catecholamines have +ve chronotropic effect. • Cholinergic drugs (as methacholine) have –ve chronotropic effect • Thyroxine has a +ve chronotropic effect as it stimulates the metabolism of the SAN. • Digitalis depresses nodal tissue. It increases K+ efflux → hyperpolarization Figure 14-17: Modulation of heart rate by the nervous system

  27. Chemical Factors 2. Blood Gases: • O2 lack (hypoxia): • Mild hypoxia increases rhythmicity • Severe hypoxia decreases and stop rhythmicity • Hypercapnia: has a -ve chronotropic effect through acidemia • H ion concentration : • Acidosis decreases rhythmicity • Alkalosis increases rhythmicity • Severe acidosis and alkalosis decreases rhythmicity . Figure 14-17: Modulation of heart rate by the nervous system

  28. Chemical Factors 3. Inorganic ions : • K ions : • mild hyperkalemia have -ve chronotropic effect. (because excess K in ECF decreases K efflux during repolarization) • Mild hypokalemia has opposite effect • Ca ions: • Mild increase of Ca++ (hypercalcemia) decreases rhythmicity by activating K+ channels → hyperpolarization → so longer time is needed to reach threshold potential. • Mild hypocalemia has the opposite Figure 14-17: Modulation of heart rate by the nervous system

  29. Chemical Factors 3. Inorganic ions : • K ions : • mild hyperkalemia have -ve chronotropic effect. (because excess K in ECF decreases K efflux during repolarization) • Mild hypokalemia has opposite effect • Ca ions: • Mild increase of Ca++ (hypercalcemia) decreases rhythmicity by activating K+ channels → hyperpolarization → so longer time is needed to reach threshold potential. • Mild hypocalemia has the opposite Figure 14-17: Modulation of heart rate by the nervous system

  30. Toxins • Typhoid or diphtheria toxins have a –ve chronotropic effect, due to direct inhibitory action on nodal tissues Figure 14-17: Modulation of heart rate by the nervous system

  31. Dra abdelaziz Hussein, Mansoura Faculty of Medicine

  32. THANKS Dra abdelaziz Hussein, Mansoura Faculty of Medicine

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