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Cardiac Muscle: Structure and Properties

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CARDIOVASCULAR SYSTEM: CARDIAC MUSCLE: STRUCTURE AND PROPERTIES For: Semester II CC2TH/ GEN 2TH Prepared and Compiled By: OLIVIA CHOWDHURY DEPARTMENT OF PHYSIOLOGY SURENDRANATH COLLEGE April 29, 2020 OLIVIA CHOWDHURY •Anatomy of The Heart April 29, 2020 OLIVIA CHOWDHURY •The Layers Of The Heart Three layers: • Epicardium . Pericardium – a double serous membrane . Visceral pericardium (Next to heart) . Parietal pericardium (Outside layer) . Serous fluid fills the space between the layers of pericardium . Connective tissue layer • Myocardium . Middle layer . Mostly cardiac muscle • Endocardium . Inner layer . Endothelium April 29, 2020 OLIVIA CHOWDHURY • The Heart Valves Allows blood to flow in only one direction Four valves: Atrioventricular valves– between atria and ventricles Bicuspid/ Mitral valve between LA and LV Tricuspid valve between RA and RV Semilunar valves between ventricles and arteries Pulmonary semilunar valve Aortic semilunar valve April 29, 2020 OLIVIA CHOWDHURY •Direction Of Blood Flow In The Heart April 29, 2020 OLIVIA CHOWDHURY Right side of the heart: • receives venous blood from systemic circulation via superior and inferior vena cava into right atrium • pumps blood to pulmonary circulation from right ventricle Left side of the Heart: • receives oxygenated blood from pulmonary veins • pumps blood into systemic circulation April 29, 2020 OLIVIA CHOWDHURY •The Cardiac Muscle Myocardium has three types of muscle fibers: Muscle fibers which form contractile unit of heart Muscle fibers which form the pacemaker Muscle fibers which form conductive system April 29, 2020 OLIVIA CHOWDHURY •The Cardiac Muscle Striated and resemble the skeletal muscle fibre Sarcomere is the functional unit Sarcomere of the cardiac muscle has all the contractile proteins, namely actin, myosin, troponin tropomyosin. Cardiac muscle fibre is bound by sarcolemma. It has a centrally placed nucleus. Myofibrils are embedded in the sarcoplasm. Sarcoplasmic reticulum is less abundant than in skeletal muscle. Sarcolemma of cardiac muscle has specialized ion channels that skeletal muscle does not have: voltage-gated Ca2+ channels. April 29, 2020 OLIVIA CHOWDHURY • The Cardiac Muscle: Presence Of Intercalated Discs Fibers are branched Adjacent cardiac cells are joined end to end by specialized structures known as intercalated discs Heart functions as a syncytium (Atrial syncytium and ventricular syncytium) The discs contain several gap junctions providing cytoplasmic continuity. Rapid transmission of impulse. when one cardiac cell undergoes an action potential, the electrical impulse spreads to all other cells that are joined by gap junctions April 29, 2020 OLIVIA CHOWDHURY •Muscle Fibres which Form 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. April 29, 2020 OLIVIA CHOWDHURY •Muscle Fibres Which Form 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 April 29, 2020 OLIVIA CHOWDHURY •Conducting System Of The Heart April 29, 2020 OLIVIA CHOWDHURY • Properties of Cardiac Muscle Autorhythmicity Excitability (Bathmotropic action) Conductivity (Dromotropic action) Contractility (Inotropic action) April 29, 2020 OLIVIA CHOWDHURY • Autorhythmicity Definition: the ability of the heart to initiate its beat continuously and regularly without external stimulation 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 April 29, 2020 OLIVIA CHOWDHURY •Locations Of Autorhythmic Cells 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 April 29, 2020 OLIVIA CHOWDHURY spread throughout ventricular myocardium 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/ prepotential) Gradual depolarization reaches threshold (-40 mv) spontaneous AP generation April 29, 2020 OLIVIA CHOWDHURY •Rate Of Generation Of AP At Different Sites Of TheHeart SA node acts as the pacemaker because it has the fastest rate ofgenerating action potential April 29, 2020 OLIVIA CHOWDHURY April 29, 2020 OLIVIA CHOWDHURY •Autorhymicity-Effect Of Stannius Ligature In Frog’s Heart • 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. April 29, 2020 OLIVIA CHOWDHURY • Excitability Definition: The ability 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 April 29, 2020 OLIVIA CHOWDHURY •Action Potential In Contractile Fibers April 29, 2020 OLIVIA CHOWDHURY •Refractory Period It is 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 strong , 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. April 29, 2020 OLIVIA CHOWDHURY • Long refractory period (250msec) compared to skeletal muscle (3msec) • During this period membrane is refractory to further stimulation until contraction is over. • It lasts longer than muscle contraction, prevents tetanus • Gives time to heart to relax after each contraction, prevents fatigue • It allows time for the heart chambers to fill during diastole before next contraction April 29, 2020 OLIVIA CHOWDHURY •Contractility Definition: Ability of cardiac muscle to contract in response to stimulation. All Or None Law • The response to a threshold stimulus is maximal. If the stimulus is below threshold there is no response • 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. April 29, 2020 OLIVIA CHOWDHURY Treppe or Staircase Phenomenon • When stimuli of same strength are applied at short intervals, a gradual increase in the height of contraction is observed for the first few contractions, after which there is no further increase. This gradual increase in the force of contraction is called staircase phenomenon. • This is due to the BENEFICIAL EFFECT- decrease in viscosity, mild increase in temperature and increase in the level of calcium ions. April 29, 2020 OLIVIA CHOWDHURY Summation of Subliminal Stimuli When a stimulus of subliminal strength is applied the cardiac muscle does not show any response. When a series of subliminal stimuli are applied in succession, the muscle responds with a contraction. It is due to the summation of all the subliminal stimuli that produce a threshold stimulus. April 29, 2020 OLIVIA CHOWDHURY •Conductivity Definition: property by which excitation is conducted through the cardiac tissue April 29, 2020 OLIVIA CHOWDHURY 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 Thus, the velocity of impulses is maximum in Purkinje fibers and minimum in the AV node April 29, 2020 OLIVIA CHOWDHURY •Effects of Autonomic Nerve Activity on the Heart April 29, 2020 OLIVIA CHOWDHURY THANK YOU! April 29, 2020 OLIVIA CHOWDHURY.
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