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Cardiac muscle physiology
This document provides an overview of cardiac muscle structure and function. It defines key terms related to the properties of cardiac muscle such as rhythmicity, excitability, conductivity, and contractility. It describes the cardiac syncytium and normal conduction pathway in the heart. It explains excitation-contraction coupling in cardiac muscle and compares it to skeletal muscle. It also compares action potentials in the sinoatrial node and ventricular muscle. Finally, it discusses the significance of the plateau and refractory period in ventricular muscle action potentials.
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Cardiac muscle physiology
- 1. Dr. Kanimozhi Sadasivam,MD Associate Professor SRM Medical College & RC, Chennai Cardiac muscle
- 2. Learning objectives • Definethe terms; Rhythmicity, Excitability, Conductivity and Contractility. • Describe cardiac syncytium. • Outline the normal pathway of the cardiac impulse. • Describe the excitation-contraction coupling in cardiac muscles and compare it to excitation-contraction coupling in skeletal muscles. • Compare and contrast action potential in sino-atrial node and ventricular muscle. • Explain the significance of the plateau and refractory period in ventricular muscle action potential.
- 3. The Heart • Heartis a muscular organ that pumps blood throughout the circulatory system • It is situated in between two lungs in the mediastinum • It is made up of four chambers, two atria and two ventricles • The musculature of ventricles is thicker than that of atria. Force of contraction of heart depends upon the muscles
- 4. The Heart: Coverings Pericardium – a double serous membrane Visceral pericardium Next to heart Parietal pericardium Outside layer Serous fluid fills the space between the layers of pericardium
- 5. The Heart: HeartWall Three layers Epicardium Outside layer This layer is the parietal pericardium Connective tissue layer Myocardium Middle layer Mostly cardiac muscle Endocardium Inner layer Endothelium
- 6. The Heart: Chambers Slide11.6Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Right and left side act as separate pumps Four chambers Atria Receiving chambers Right atrium Left atrium Ventricles Discharging chambers Right ventricle Left ventricle
- 7. The Heart: Valves Slide11.8Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Allow blood to flow in only one direction Four valves Atrioventricular valves – between atria and ventricles Bicuspid valve (left) Tricuspid valve (right) Semilunar valves between ventricle and artery Pulmonary semilunar valve Aortic semilunar valve
- 8. THE CARDIAC MUSCLE •Myocardium has three types of muscle fibers: • i. Muscle fibers which form contractile unit of heart (99%) • ii. Muscle fibers which form pacemaker • iii. Muscle fibers which form conductive system 8
- 9. • Striated andresemble the skeletal muscle fibre • Cardiac muscle fibre is bound by sarcolemma. It has a centrally placed nucleus. Myofibrils are embedded in the sarcoplasm. • Sarcomere of the cardiac muscle has all the contractile proteins, namely actin, myosin, troponin and tropomyosin. • Sarcotubular system in cardiac muscle is slightly different to that of skeletal muscle. Muscle Fibres which Form the Contractile unit 9
- 10. Sarcotubular system incardiac & skeletal muscle Cardiac muscle Skeletal muscle Location of T tubules At Z line At A-I junction Diameter of T tubules More (5times) Less L tubules Narrow tubular cistern Large dilated cistern Association of T tubule ( Tubule & cistern) Diad (1 Tubule & 1cistern) Triad (1 Tubule & 2cistern) Sarcomeric organisation Less regular More regular
- 11. • Exhibit branching •Adjacent cardiac cells are joined end to end by specialized structures known as intercalated discs • Within intercalated discs there are two types of junctions – Desmosomes – Gap junctions that allow action potential to spread from one cell to adjacent cells • Heart function as syncytium when one cardiac cell undergoes an action potential, the electrical impulse spreads to all other cells that are joined by gap junctions so they become excited and contract as a single functional syncytium Atrial syncytium and ventricular syncytium
- 12.
- 13. 13 Structure of CardiacMuscle Cell
- 14. Orientation of cardiacmuscle fibres: • Unlike skeletal muscles, cardiac muscles have to contract in • more than one direction. • Cardiac muscle cells are • striated, meaning they will only contract along their long axis. • In order to get contraction in • two axis, the fires wrap • around.
- 15. Muscle Fibres whichForm the Pacemaker • Some of the muscle fibres of heart are modified into a specialized structure known as pacemaker. • These muscle fibres forming the pacemaker have less striation. • They are named pacemaker cells or P cells. • Sino-atrial (SA) node forms the pacemaker in human heart.
- 16. Muscle Fibres whichForm Conductive System • Conductive system of the heart is formed by modified cardiac muscle fibres • Impulses from SA node are transmitted to the atria directly. However, the impulses are transmitted to ventricles through various components of conducting system
- 17. Conducting system ofheart
- 18. • Electrical – Excitability(Bathmotropic action) – Auto rhythmicity – Conductivity (Dromotropic action) • Mechanical – Contractility (Inotropic action) – Refractory period – Staircase / treppe effect
- 20. Action potential- thechange in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell.
- 21. 1. Autorhythmicity myogenic(independent of nerve supply) due to the specialized excitatory & conductive system of the heart intrinsic ability of self-excitation (waves of depolarization) cardiac impulses Definition: the ability of the heart to initiate its beat continuously and regularly without external stimulation 21
- 22. Have two importantfunctions 1. Act as a pacemaker (set the rhythm of electrical excitation) 2. Form the conductive system (network of specialized cardiac muscle fibers that provide a path for each cycle of cardiac excitation to progress through the heart) Autorythmic fibers Forms 1% of the cardiac muscle fibers 22
- 23. Sinoatrial node(SA node) Specialized region in right atrial wall near opening of superior vena cava. Atrioventricular node (AV node) Small bundle of specialized cardiac cells located at base of right atrium near septum Bundle of His (atrioventricular bundle) Cells originate at AV node and enters interventricular septum Divides to form right and left bundle branches which travel down septum, curve around tip of ventricular chambers, travel back toward atria along outer walls Purkinje fibers Small, terminal fibers that extend from bundle of His and spread throughout ventricular myocardium Locations of autorhythmic cells
- 24. Mechanism of Autorhythmicity Autorhythmic cells do not have stable resting membrane potential (RMP) Natural leakiness to Na & Ca spontaneous and gradual depolarization Unstable resting membrane potential (= pacemaker potential) Gradual depolarization reaches threshold (-40 mv) spontaneous AP generation 24
- 25. Prepotential / pacemakerpotential/ Diastolic potential
- 26. Rate of generationof AP at different sites of the heart RATE (Times/min) SITE 70 - 80SA node 40 - 60AV node 20 - 35AV bundle, bundle branches,& Purkinje fibres SA node acts as heart pacemaker because it has the fastest rate of generating action potential Nerve impulses from autonomic nervous system and hormones modify the timing and strength of each heart beat but do not establish the fundamental rhythm. 26
- 28. • Non-SA nodaltissues are latent pacemakers that can take over (at a slower rate), should the normal pacemaker (SA node ) fail 29
- 29. Demonstration of Propertiesof Cardiac Muscle • The properties of cardiac muscle are demonstrated using a quiescent heart. • A quiescent heart is a heart which has stopped beating but is still alive. • Such a preparation can be obtained by tying a Stannius Ligature in the frog’s heart.
- 30. Autorhymicity- effect ofStannius Ligature in the frog’s heart
- 31. 2. Excitability Definition: Theability of cardiac muscle to respond to a stimulus of adequate strength & duration by generating an AP • AP initiated by SA node travels along conductive pathway excites atrial &ventricular muscle fibres 32
- 32. Action potential incontractile fibers
- 33. AP-contraction relationship: • APin skeletal muscle is very short-lived-AP is basically over before an increase in muscle tension can be measured • AP in cardiac muscle is very long-lived – AP has an extra component ,which extends the duration . – The contraction is almost over before the action potential has finished.
- 36. Refractory Period • Itis that period during which a second stimulus fails to evoke a response. • Absolute Refractory Period : It is that period during which a second stimulus however high it is fails to evoke a response. • Relative Refractory Period : It is that period during which a second stimulus evokes a response if it is sufficiently high.
- 37. Refractory period • Longrefractory period (250 msec) compared to skeletal muscle (3msec) • During this period membrane is refractory to f urther stimulation until contraction is over. • It lasts longer than muscle contraction, prevents tetanus • Gives time to heart to relax after each contract ion, prevent fatigue • It allows time for the heart chambers to fill during diastole before next contraction
- 38. Normal Cardiogram • Itis a recording of the mechanical activity of the heart • Systole- Contraction Diastole- Relaxation
- 39. Extra systole • Itis an extra contraction seen when the second stimulus falls during the relative refractory period. • Systole-Down stroke • Diastole-Up stroke
- 40. Compensatory Pause • Whenan external stimulus is applied during the later 2/3 of diastole an extra contraction is observed. • This is followed by a compensatory pause. • Extra systole + Compensatory pause = 2 cardiac cycles
- 41.
- 42. 3. Contractility Definition: abilityof cardiac muscle to contract in response to stimulation 43 All Or None Law • The response to a threshold stimulus is maximal. If the stimulus is below threshold there is no response provided the physiological conditions remain constant • The cardiac muscle follows the all or none law as a whole. • In the case of skeletal muscle, all-or-none law is applicable only to a single muscle fiber.
- 43. Treppe or Stair-casePhenomenon • When stimuli of same strength are applied at short intervals, an increase in the height of contraction is observed. • This is due to the BENEFICIAL EFFECT - decrease in viscosity, mild increase in temperature and increase in the level of calcium ions.
- 44. Summation of Sub-minimalStimuli When a series of sub-minimal stimuli are applied to the cardiac muscle, it responds with a contraction once all the sub –minimal add up to produce a threshold stimulus.
- 45. Excitation-Contraction Coupling in CardiacContractile Cells Similar to that in skeletal muscles 46
- 46. The cardiac musclestores much more calcium in its tubular system than skeletal muscle and is much more dependent on extracellular calcium than the skeletal muscle. An abundance of calcium is bound by the mucopolysaccha -rides inside the T-tubule. This calcium is necessary for contraction of cardiac muscle, and its strength of contraction depends on the calcium concentration surrounding the cardiac myocytes. At the initiation of the action potential, the fast sodium channels open first, followed later by the opening of the slow calcium channels.
- 47. 4. Conductivity Definition: propertyby which excitation is conducted through the cardiac tissue 48
- 48. Tissue Conduction rate(m/s) Atrial muscle 0.3 Atrial pathways 1 AV node 0.05 Bundle of His 1 Purkinje system 4 Ventricular muscle 0.3-0.5 49 Thus, the velocity of impulses is maximum in Purkinje fibers and minimum at AV node
- 49. The atrial andventricular muscles have a relatively rapid rate of conduction of the cardiac action potential, and the anterior internodal pathway also has fairly rapid conduction of the impulse. However, the A-V bundle myofibrils have a slow rate of conduction because their sizes are considerably smaller than the sizes of the normal atrial and ventricular muscle. Also, their slow conduction is partly caused by diminished numbers of gap junctions between successive muscle cells in the conducting pathway, causing a great resistance to conduction of the excitatory ions from one cell to the next.
- 50. Criteria for spreadof excitation & efficient cardiac function 1. Atrial excitation and contraction should be complete before onset of ventricular contraction- ensures complete filling of the ventricles during diastole 2. Excitation of cardiac muscle fibres should be coordinated ensure each heart chamber contracts as a unit accomplish efficient pumping-smooth uniform contraction essential to squeeze out blood 3. Pair of atria & pair of ventricles should be functionally co- ordinated both members contract simultaneously - permits synchronized pumping of blood into pulmonary & systemic circulation 51
- 51. Summary • In whichphase of the ventricular muscle action potential is the potassium permeability the highest? • A) 0 • B) 1 • C) 2 • D) 3 • E) 4
- 52. • Which ofthe following statements about cardiac muscle is most accurate? • A) The T-tubules of cardiac muscle can store much less calcium than T-tubules in skeletal muscle • B) The strength and contraction of cardiac muscle depends on the amount of calcium surrounding cardiac myocytes • C) In cardiac muscle the initiation of the action potential causes an immediate opening of slow calcium channels • D) Cardiac muscle repolarization is caused by opening of sodium channels • E) Mucopolysaccharides inside the T-tubules bind chloride ions
- 53. • Which ofthe following structures will have the slowest rate of conduction of the cardiac AP? • A) Atrial muscle • B) Anterior internodal pathway • C) A-V bundle fibers • D) Purkinje fibers • E) Ventricular muscle
- 54. • What isthe membrane potential (threshold level ) at which the S-A node discharges? • A) −40 mV • B) −55 mV • C) −65 mV • D) −85 mV • E) −105 mV
- 55. • If theventricular Purkinje fibers become the pacemaker of the heart, what is the expected HR? • A) 30/min • B) 50/min • C) 65/min • D) 75/min • E) 85/min
- 56. • What isthe resting membrane potential of the sinus nodal fibers? • A) −100 mV • B) −90 mV • C) −80 mV • D) −55 mV • E) −20 mV
- 57.