The Heart PDF - Updated October 2023

The Heart Updated October, 2023 Dr. Mohamed Hindawy THE HEART General characters of the muscular tissue:  The muscle cell is elongated and so termed the Myofiber (muscle fiber), it contains Myofibrils that are bundles of Myofilaments (contractile threads). The cytoplasm is called Sarcoplasm, it...

The Heart Updated October, 2023 Dr. Mohamed Hindawy THE HEART General characters of the muscular tissue:  The muscle cell is elongated and so termed the Myofiber (muscle fiber), it contains Myofibrils that are bundles of Myofilaments (contractile threads). The cytoplasm is called Sarcoplasm, its sER is called Sarcoplasmic Reticulum, the plasma membrane is called the Sarcolemma and mitochondria are called sarcosomes. (NB. Sarco- = Flesh ‫(لحم‬  Muscle fibers may be: 1- Striated (show registering transverse dark & light bands) or non-striated. 2- Voluntary (under control of will) or involuntary. Types of muscle fibers: 1) Skeletal: (striated & voluntary except in Pharynx, upper esophagus & cremastric muscle → involuntary). 2) Cardiac: (striated & involuntary). 3) Smooth: (non-striated & involuntary). CARDIAC MUSCLE FIBERS (Cardiomyocytes)  FUNCTIONALLY: They have intermediate characters between skeletal muscle Fs. (contractions are Structure of a cardiac muscle Fiber:  Nuclei:  large, oval, central, mono- or binucleated.  The Sarcoplasm:  it is acidophilic and contains: 1. ORGANELLES: A) Myofibrils: oriented longitudinally, but branching and anastomosing → irregular cross striation. B) Mitochondria: 40% of cytoplasmic volume (compared to 2-10% in sk. Ms), they are located between myofibrils & at the perinuclear region. C) Sarcoplasmic reticulum (SR): sER, its terminal cisternae store Ca2+ for muscle contraction. 2. INCLUSIONS: A) Lipid droplets & Glycogen granules. B) Lipochrome (Lipofuscin) pigment granules: wear & tear pigment that increase with advance of age.  Transverse Tubules & Sarcoplasmic Reticulum:  1.The cardiac muscle fiber contains Diads (Dyads) # Location: at the Z-lines (compared to Triads at the A-I junctions in sk.ms). # Structure: each is formed of a thick TT (Transvers Tubule) derived from the sarcolemma & a thin TC (Terminal Cisterna) derived from the sarcoplasmic reticulum. # Function: conduct the depolarization wave from the sarcolemma into the TT → little cytosolic Ca2+ influx via TT→ triggers the pump of much Ca2+ from the TC into the cytosol → contraction. 1Page strong & utilize much energy) and smooth muscle Fs. (contractions are rythmic & involuntary).  HISTOLOGICALLY: Each fiber is formed of a chain of short cylindrical cells (in sk.ms. each fiber is formed of only one cell), the ends of the cardiomyocytes branch and interdigitate with similar ends of adjacent cells, giving a false impression of a syncytium ‫سيـنسيشيوم‬. However, the cardiomyocyte is not a syncytium as the individual muscle cells are joined together end to end by INTERCALATED DISCS. The Heart Updated October, 2023 2+ diad → Ca pump → contraction. 2. After recovery from the preceding contraction, a slow leak of Ca2+ from the sarcoplasmic reticulum into the cytosol → a succession of autonomous contractions. The rate of that inherited rhythm is modulated by external autonomic & hormonal stimuli. Dr. Mohamed Hindawy  Intercalated discs: are dark staining discs of sarcolemma at the sites of interdigitating cardiac muscle cells. Myofibrils * EM Structure: Each Myofibril consists of consecutive sarcomeres, separated by Z discs & containing myofilaments. (a) The myofibril shows alternating Light bands (Isotropic or I- band) & Dark bands (Anisotropic or Aband). The lateral registration of sarcomeres in adjacent myofibrils causes the entire muscle fiber to exhibit a characteristic cross striation pattern. (‫)أى أن فى اللييفات كلها تكون النطاقات الغامقة مواجهة لبعضها البعض فقط و كذلك النطاقات الباهتة تكون مواجهة لبعضها فقط‬ (b) Each dark band is bisected by a light disc called H-disc which is bisected by a dark line called M-line. Each light band is bisected by a dark disc called Z-disc (Z for Zigzag). (c) The Sarcomere: It is the functional contractile unit of a myofibril that extends between two successive Z-discs. It includes a whole dark band & ½ light band on either side. * Molecular structure: The myofilaments: are thread-like organelles which are longitudinally arranged inside the myofibrils. 2Page *Definition: cylindrical contractile elements that are longitudinally oriented in the sarcoplasm of the cardiac muscle fiber. The Heart Types of myofilaments: Extension Ends Proteins Updated October, 2023 Thin Filaments From Z-line to the border of H-disc One end is free & the other is attached to Z-disc. Actin, tropomyosin & troponin Dr. Mohamed Hindawy Thick (Myosin) Filaments Span the A-band only, interdigitating with the free ends of the thin filaments Both ends are free Myosin only A-Band NB: * Overlap area: is dark as it contains two types of filaments (thin + thick). * H- disc: is light as it contains only one type of filaments (thick). * M-line: is dark as it contains two types of filaments (Thick + myomesin). I- Band: is light as it contains only one type of filaments (thin). Z-disc: is dark as it contains many types of filaments: ❶ Interdigitating thin filaments. ❷ α-Actinin (which attach thin filaments to the Z-discs). The Intercalated Discs *Definition: the site of interdigitating cell membranes of two adjacent cardiac muscle cells within the muscle fiber. Structure [Types of junctions in] of the intercalated disc: A: Fascia adherens: (In transverse part): (Z-disc ‫)تقوم مقام ال‬:  Anchoring sites for myofibrils (represent half a Z-line). Thin filaments at the ends of terminal sarcomeres are inserted into these junctions → transmit contractile forces from cell to cell (i.e. provide MECHANICAL COUPLING of cardiac muscle fibers). B: Gap (nexus) junctions: (In both longitudinal & transverse parts):  They provide narrow (2 nm) gaps traversed by intercommunicating channels between the cells → allow rapid transmission of action potentials among adjacent cells → the muscle fibers contract simultaneously as one unit (i.e. provide ELECTRICAL COUPLING of cardiac muscle fibers). C: Macula adherens = Desmosomes: (In adjoining sites between fascia adherens & gap junctions):  They provide anchorage for intermediate filaments of the cytoskeleton that prevents separation of cells during contraction (i.e. provide WELDING SPOTS among cardiac muscle fibers). Repair of the Cardiomyocytes (Cardiac Muscle cells) Satellite (stem) cells are absent. So, when cardiac muscle fibers are injured, they heal by fibrosis. Structure of the Wall of Heart ** ANATOMICALLY: it is formed of 2 thin-walled atria & 2 thick-walled ventricles. 3Page *LM: they appear as a straight or step-like dark lines across the myocardial fibers. The Heart Updated October, 2023 Dr. Mohamed Hindawy ** HISTOLOGICALLY: the heart wall is formed of three layers: A. THE EPICARDIUM (outer layer):  Visceral layer of the pericardium which invests the heart.  It is formed of: 1. Simple squamous epithelium (mesothelium): An outer layer of flat cells that secrete a little amount of serous fluid which lubricates the movement of the epicardium on the parietal layer of pericardium → ↓ friction. 2. Subserous layer of fibro-elastic CT: that blends with the endomysium of myocardium. It contains coronary blood vessels (vasa vasorum of the heart), autonomic nerves & fat cells. B. THE MYOCARDIUM (middle layer): It is formed of: 1. Cardiac muscle fibers (Main bulk): extensively branching & anastomosing fibers that are woven around the cardiac chambers. 2. Endomysium: areolar CT separating the cardiac myocytes. It is richin capillaries and lymphatics. C. THE ENDOCARDIUM (inner layer):  It lines the chambers & covers the cardiac valves.  It is formed of four layers (from inside to outward): 1. Endothelium: Simple squamous endothelial layer (most inner). 2. Sub-Endothelial Layer: a delicate CT layer (with fine collageneous fibers). 3. fibro-elastic layer: a robust layer of dense CT (with thick collagenous & elastic fibers). 4. A layer of areolar CT: which contains blood vessels, nerves & Purkinje fibers (branches of conducting system of the heart). Little amount of adipose tissue may be seen. NB: layers number 2& 3 accommodate movement of myocardium without damaging the endothelium. Valves of the Heart ** Anatomical Types: 1. Tricuspid valve (formed of 3 cusps): present between the right atrium & ventricle. 2. Mitral valve (formed of 2 cusps): present between the left atrium & ventricle. 3. Aortic & pulmonary valves. ** Histological Structure: ‫هام‬  It is a leaflet of endocardium being formed of: 2- Sub-Endothelial Layer 3. Fibro-elastic layer. Dense CT layer (with much collagenous & elastic fibers). Cardiac valve layers 1. Endothelium: a continuation of the endocardial simple squamous endothelium. It covers both surfaces of the valve. 2. Sub-Endothelial Layer: a continuation of the subendothelial layer of endocardium. Formed of delicate CT layer rich in elastic fibers. 3. Lamina Fibrosa: a tough central collagenous fibrous sheet that forms the main bulk. 4. The Valve Annulus: a fibrous ring formed by condensation of the lamina fibrosa at the orifice of valve. It is rich in capillaries & macrophages. Annulae of the four valves together form a central fibrous cardiac skeleton which is continuous with the collageneous tissue of the myo-, endo- & epi-cardia. 5. Chordae Tendinae: collagenous strands which merge with the fibrous laminae of the valve leaflets & connect the valves to the papillary muscles (extensions of ventricular myocardium) which, via the chordae tendinae, stabilize the cusps of the mitral & tricuspid valves. 4Page Endocardium layers 1- Endothelium The Heart Updated October, 2023 Dr. Mohamed Hindawy The Impulse Generating and Conducting System of the Heart It is a vascular neuro-muscular structure among the contractile cardiac muscle fibers. Components: 1. Sinoatrial (S-A) node the pacemaker of the heart (in the right atrium). Three internodal tracts propagate cardiac impulse to right atrium. Bachmann's bundle ‫ حزمة باكمان‬is a branch of the anterior internodal tract that supplies the wall of the left atrium. 2. Atrioventricular (A-V) node. (in the interatial septum) The excitatory cells of both S-A & A-V nodes are small specialized myocardial fibers with much gap junctions → fast transmission of the depolarization waves. These cells contain little contractile filaments and are embedded in dense vascular collageneous tissue containing numerous autonomic nerve fibers. 3. A-V bundle of His, its right and left branches and their ramification (Purkinje fibers), the latter pass in the sub-endocardial supporting CT before penetrating the ventricular myocardium. Moderator band is a branch of the right bundle branch into the ventricular cavity that supplies the wall of the right ventricle. 4. The depolarization wave propagates among adjacent cardiomyocytes via gap junctions of the intercalated discs. The Moderator Band **Structure: 1. Purkinje fibers: much & grouped. 2. Connective tissue: i) Endomysium: areolar CT surround the individual Purkinje fibers. ii) CT sheath: surround the band. iii) It contains fat cells & blood capillaries. 3. Endocardium: covers the moderator band. 5Page **Definition: A bundle of cardiac muscle fibers into the ventricular cavity that encloses a branch from the right bundle branch to supply impulses to the wall of the right ventricle. The Heart Updated October, 2023 **Differences between the cardiac muscle and Purkinje fibers: Dr. Mohamed Hindawy ‫هام جدا‬ As compared to ordinary contractile cardiac muscle fibers, Purkinje fibers are characterized by:  Intercalated discs are ABSENT, but has numerous gap junctions (for rapid conduction) & desmosomes (for welding).  T. tubules are ABSENT.  Faster in Conduction due to: 1. Much Gap junctions. 2. Larger diameter.  Paler in color due to: 1. Large amount of glycogen which dissolve during preparation → vacuolation. 2. Few number of myofibrils which are more peripheral in position.   mitochondria ENDOCRINE CARDIOMYOCYTES ** Definition: atrial cardiomyocytes that partly act as endocrine cells. ** Site: among right atrial myocytes. However, all right atrial myocytes are potentially hormone-secreting. ** Types of polypeptide hormones produced: 1. Cardiac natriuretic peptides (cNPs): that include atrial natriuretic factor (ANF) and brain (ventricular) natriuretic peptide (BNP) → maintenance of extracellular fluid volume & blood pressure. 2. Other polypeptide hormones → act upon the myocardium in a paracrine or autocrine fashion. 6Page ** EM: These cells have both: 1. Structural features of contraction (myofibrils, dyads, mitochondria …. etc) 2. Structural features of polypeptide hormone-producing cells; ↑rER, well developed Golgi complex & atrial-specific granules (storage granules) at the perinuclear region near the Golgi complex. The Heart Updated October, 2023 Dr. Mohamed Hindawy Mark (T) for True and (F) for false for each of the following and correct the false statements 7Page 1. Injury of the myocardium is followed by fibroblast activation and fibrosis. ( ) 2. Mitochondria of the cardiac muscle cell aggregate mainly under the sarcolemma. ( ) 3. Lipochrome pigment is a common finding in myocardium of the elderly. ( ) 4. Actin filaments are inserted into desmosomes of the intercalated discs. ( ) 5. I-band is light as it contains one type of filaments. ( ) 6. Epicardium is covered with endothelium. ( ) 7. The three layers of the wall of the heart are continuous with the three layers which form the walls of blood vessels entering and leaving the heart. ( ) 8. Valves of the heart are covered with mesothelium. ( ) 9. A- band is dark as it contains myosin (thick) filaments. ( ) 10. Thin filaments are composed of one type of protein; while thick filaments include three types. ( ) 11. There are triads for initiation of cardiac muscle contraction. ( ) 12. Gap junctions between the cardiac muscle cells facilitate cardiac impulse conduction. ( ) 13. Cross striations are attributed to longitudinal orientation of the myofibrils inside the cardiac muscle fibers. ( ) 14. Mitochondria account for 40% of the cytoplasmic volume in the cardiac muscle cell. ( ) 15. Purkinje fibers are non-contractile as they contain few myofibrils. ( ) 16. Fascia adherents are located in the longitudinal segments of the intercalated discs. ( ) 17. Transverse tubules are much wider than terminal cisterns in cardiac muscle fibers. ( ) 18. Diads are located at the A-I junctions of the sarcomeres in the cardiac muscle fibers. ( ) 19. The cardiac muscle fiber is composed of a branching chain of short cylindrical cells. ( ) 20. Myofibrils in Purkinje fibers are located under the sarcolemma. ( ) 21. Diads are seen in both ordinary cardiac muscle fibers and Purkinje fibers. ( ) 22. Some specialized cardiac muscle fibers have endocrine function. ( )