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Structure of cardiac muscle excitation contraction coupling properties of cardiac muscle
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Structure of cardiac muscle excitation contraction coupling properties of cardiac muscle


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  • 1. RK Goit, Lecturer Department of Physiology Structure of Cardiac Muscle Excitation-Contraction Coupling Properties of Cardiac Muscle
  • 2. • heart is composed of 3 major types of cardiac muscle: atrial muscle, ventricular muscle, & specialized excitatory & conductive muscle fibers • atrial & ventricular types of muscle contract in much the same way as skeletal muscle, except that the duration of contraction is much longer • excitatory & conductive fibers contract only feebly • cardiac muscle fibers are made up of many individual cells connected in series & in parallel with one another
  • 3. • cardiac muscle fibers arranged in a latticework, with the fibers dividing, recombining, & then spreading again • cardiac muscle is striated in same manner as in skeletal muscle • cardiac muscle has myofibrils that contain actin & myosin filaments almost identical to those found in skeletal muscle – these filaments lie side by side & slide along one another during contraction in the same manner as occurs in skeletal muscle
  • 4. Cardiac Muscle as a Syncytium • cardiac cells are so interconnected that when one of these cells becomes excited, the action potential spreads to all of them, from cell to cell throughout the latticework interconnections • heart actually is composed of two syncytiums: the atrial syncytium, which constitutes the walls of the two atria, & the ventricular syncytium, which constitutes the walls of the two ventricles • division of the muscle of the heart into two functional syncytiums allows the atria to contract a short time ahead of ventricular contraction
  • 5. properties of heart can be divided into 2 groups: Beating heart Quiescent heart • Automaticity • Rhythmicity • Contractibility • Excitability • Conductivity • Distensibility • Functional syncitium • Long refractory period • Extrasystole & compensatory pause • All or none law • The staircase phenomenon • Length-tension relationship • Summation of subminimal stimuli
  • 6. Properties of cardiac muscle • Automaticity – capability of contract even in the absence of neural control • Rhythmicity – heart beats are extremely regular • Contractibility – cardiac muscle contracts in response to a stimulus • Excitability – ability of the cardiac muscle to respond to different stimuli
  • 7. • Conductivity – impulses produced in the SA node is conducted by the specialized conducting pathway • Distensibility – occurs due to compliance of the cardiac muscle • Functional syncytium – due to the presence of numerous gap junctions
  • 8. • Long refractory period
  • 9. • Extrasystole & compensatory pause – when the ventricle is stimulated in the relaxation period (relative refractory period), the cardiac muscle may contract – It may occurs because a papillary muscle may fire an impulse before normal impulse reaches the ventricles
  • 10. • All or none law – if the heart is stimulated with subthreshold stimuli no response is seen – a threshold stimulus is the weakest stimulus that evokes a response – amplitude of contraction in response to the suprathreshold stimuli remains the same as that with the threshold stimuli
  • 11. • Staircase phenomenon – if a quiescent ventricle is stimulated repeatedly such that the interval between consecutive stimuli is less than 10 s, the first 3-4 contractions are progressively more forceful
  • 12. • Length-tension relationship (Frank-Starling law) – the force of contraction of cardiac muscle is directly proportional to the initial length of the muscle fibers • Summation of subminimal stimuli – when subminimal stimuli are applied repeatedly, the stimuli summate & produce a response
  • 13. • Frequency force relationship – changes in cardiac rate & rhythm also affect myocardial contractility
  • 14. • Load velocity relationship
  • 15. References • Ganong Review of Medical Physiology, 23/E • Textbook of Medical Physiology, 12/E Guyton & Hall • Understanding Medical Physiology, 4/E Bijlani & Manjunatha
  • 16. Thank You