L9 - CARDIAC MUSCLE 2.pdf

ITM 101 | HISTOLOGY LESSON # 7 – CARDIAC MUSCLE 1st YEAR | A.Y. 2023-2024 | SEPTEMBER 21, 2023 DR. JOANNA MELISSA MARQUEZ - The sarcoplasm is more abundant and a longitudinal...

ITM 101 | HISTOLOGY LESSON # 7 – CARDIAC MUSCLE 1st YEAR | A.Y. 2023-2024 | SEPTEMBER 21, 2023 DR. JOANNA MELISSA MARQUEZ - The sarcoplasm is more abundant and a longitudinal CARDIAC MUSCLE striation is more apparent owing to the separation of - The human heart beats at a rate of 60 to 100 times a minute bundles of myofibrils by rows of mitochondria. throughout life. Its contraction is myogenic. It is - The myofibrils diverge around a centrally placed nucleus, independent of nervous stimulation. outlining a fusiform axial region of sarcoplasm rich in - All cardiac myocytes are capable of spontaneous rhythmic organelles and inclusions. depolarization and repolarization of their membrane. - A small Golgi complex is located near one pole of each elongate nucleus. - Lipid droplets are common in this region and, in older animals, granular deposits of lipochrome pigment may be abundant. - A unique feature of cardiac muscle is the occurrence of transverse intercalated discs Transverse Intercalated Discs - Found at regular intervals along the length of the fibers. They are relatively inconspicuous in routine preparations but are heavily stained by iron haematoxylin, appearance will become more visible. - a step-like configuration in section. - In the pattern of cross-striations, the intercalated discs are invariable and occurs at the I-bands Picture A shows the cardiac muscle cells containing myofibrils composed of myofilaments arranged in sarcomeres, T tubules to transmit the impulse from the sarcolemma to the interior of the cell, numerous mitochondria for energy, and intercalated discs that are found at the junction of different cardiac muscle cells. B. A photomicrograph of cardiac muscle cells shows the nuclei and intercalated discs. C. An intercalated disc connects cardiac muscle cells and consists of desmosomes and gap junctions. Desmosome and gap junctions are important to facilitate synchronous repolarization and depolarization, to form one contraction. - Its step-like appearance is nothing like that in an actual tissue sample. CARDIAC MYOCYTE - Consists of separate cellular units about 80 um in length and 15 um in diameter - The strands are predominantly parallel. - The cardiac myocytes are joined end-to-end at junctional specializations called intercalated discs. - The individual myocytes branch and form oblique interconnections with neighboring strands. - This is what makes cardiac muscle unique, it is branched. Allowing them to contract uniformly. “Cardiac myocyte has a pattern of cross-striations similar to that of skeletal muscle but branching and interconnection of neighboring fibers is evident” - There is a variation in terms of the size of the myocytes. 101 INTRODUCTION TO MEDICINE | HISTOLOGY AYURA 2027 1 ITM 101 | HISTOLOGY LESSON # 7 – CARDIAC MUSCLE 1st YEAR | A.Y. 2023-2024 | SEPTEMBER 21, 2023 DR. JOANNA MELISSA MARQUEZ ULTRASTRUCTURE OF CARDIAC MUSCLE The mitochondria of cardiac muscle have numerous cristae that often show a periodic angulation that gives them a zig-zag form. As a rule, the mitochondria are about the length of a sarcomere (2.5 um), but they may be as long as 7-8 um. Under the electron microscope a distinctive feature of cardiac muscle in cross section is the absence of separate myofibrils. “In skeletal muscles there are bundles of myofibrils while in here there are only 1” Longitudinal section of a small area of cardiac muscle Instead, the cross section of a myocyte is occupied by a showing a loose network of tubular elements of the continuum of myofilaments, interrupted here and there by sarcoplasmic reticulum. mitochondria and profiles of sarcoplasmic reticulum that penetrate into the cylindrical mass of myofilaments from its periphery. Oval and irregular shaped figure is the mitochondria with visible cristae. Tiny dots surrounding mitochondria is the myofilament Glycogen tends to be more abundant in cardiac than in skeletal muscle. It occurs in the form of 30-40-nm dense particles located in the areas of intermyofilament sarcoplasm that contain mitochondria, but particles may also be found aligned in rows between myofilaments (Fig. 10-44). These are more numerous in the I-bands than in the A-bands. Glycogen and lipid are both important energy sources for the contractile activity of the myocardium. ULTRASTRUCTURE OF CARDIAC MUSCLE T-tubules of cardiac muscle In longitudinal sections, these incursions appear as slender fusiform areas of sarcoplasm containing mitochondria and circular profiles of sarcotubules. Locally, these may appear to define the lateral limits of myofibrils of varying width, but this is misleading because they are of limited longitudinal extent and, at their ends, lateral continuity of the mass of myofilaments is again evident. 101 INTRODUCTION TO MEDICINE | HISTOLOGY AYURA 2027 2 ITM 101 | HISTOLOGY LESSON # 7 – CARDIAC MUSCLE 1st YEAR | A.Y. 2023-2024 | SEPTEMBER 21, 2023 DR. JOANNA MELISSA MARQUEZ differ significantly from those of skeletal muscle. TRANSDUCTION OF EXCITATION located at the level of the Z-discs instead of at the A-I In both, transduction of excitation from the sarcolemma to junctions the reticulum takes place at rows of intramembrane there is only one per sarcomere. particles called feet or spanning proteins bridging the gap These are of greater diameter between the apposed membranes penetrate deep into the cell where they communicate with In addition, there are small saccules or cisternae of the occasional tubules of slightly smaller diameter that course superficial reticulum that are connected directly to the parallel with the long axis of the cell. sarcolemma by junctional feet. These are sometimes referred to as the corbular sarcoplasmic reticulum. The transverse tubules are lined by a layer continuous with the external lamina of the sarcolemma This system of branching tubules is called the transverse-axial-tubular system (TATS). The calcium-binding protein calsequestrin is localized in the junctional saccules and in the corbular reticulum. ➔ The importance of the corbular sarcoplasmic reticulum is that aside from the calcium coming from outside, although limited, it has its own stock of calcium. The longitudinal sarcoplasmic reticulum is less elaborate As in skeletal muscle, contraction is dependent on free than that of skeletal muscle. It consists of a calcium ions in the sarcoplasm. But cardiac muscle, with subsarcolemmal network of tubules 20-35 nm in diameter relatively small saccules in place of terminal cisternae, has that extends into deep clefts within the column of more limited intracellular reserves of calcium. myofilaments. During depolarization of the sarcolemma and its Its pattern varies at different levels in the sarcomere, being invaginations, there is an influx of extracellular calcium. close-meshed adjacent to A-bands and more loosely This is followed and supplemented by release of organized at I-bands. intracellular calcium stored in the reticulum. Calcium from these two sources activates sliding of the filaments and consequent contraction. SMALL FLATTENED SACCULES The structure of cardiac muscle in the atria and ventricles Terminal cisternae and triads are not found in cardiac of the heart is similar, but the atrial myocytes have a muscle. Their functional counterparts are relatively small smaller average diameter and the transverse-axial tubule flattened saccules that establish junctional contacts with system is poorly developed. Such tubules are seen only in the transverse-axial system of tubules at the level of the the largest of the atrial myocytes. It is possible that in the Z-discs. more slender myocytes there is less need for transverse The total area of junctional contact of these saccules is tubules for inward conduction of excitation. considerably less than that of the terminal cisternae with ➔ Transverse-axial tubule system (TATs) less the T-tubules of skeletal muscle. developed in the atrial compared to the ventricular. ➔ Ventricles have a more taxing or tedious job which is while atrial myocytes are less in terms of diameter and less development of TATs. The spread of action potential is reported to be more rapid in atrial myocytes than in those of the ventricles. The contractile elements are identical in their ultrastructure, but minor differences have been discovered at the molecular level. The heavy chains of myosin molecules occur in two isoforms, a-HMC and B-HMC. In the atrium, a-HMC is more abundant, whereas in the ventricle, B-HMC is the predominant form. 101 INTRODUCTION TO MEDICINE | HISTOLOGY AYURA 2027 3 ITM 101 | HISTOLOGY LESSON # 7 – CARDIAC MUSCLE 1st YEAR | A.Y. 2023-2024 | SEPTEMBER 21, 2023 DR. JOANNA MELISSA MARQUEZ THE INTERCALATED DISK MYOCARDIAL ENDOCRINE CELLS At the intercalated discs, the conjoined myocytes Myocytes in the atrium synthesize and secrete peptide have a highly irregular surface with multiple ridges hormones involved in the regulation of blood volume and and papillary projections on the end of one cell the electrolyte composition of the extracellular fluid. fitting into complementary grooves and pits on the The myoendocrine cells are specialized myocytes localized mainly in the right and left atrial appendages, but they are other also found scattered within other areas of the atria and In this mosaic of junctional specializations, only the along the conductive system in the ventricular septum desmosomes are typical in respect to their shape. Contain membrane-bounded dense secretory granules 0.3-0.4 pm in diameter. The granules contain the precursor of a family of biologically active polypeptides collectively called cardiodilatins (CDD) or atrial natriuretic polypeptides (ANP). Zonulae adherens are not circumferential. The term fascia adherens is a more appropriate. Fascia adherens is more flat and narrow The myofilaments of the conjoined cells terminate in a very dense layer of sarcoplasm. Actin-binding proteins a-actinin and vinculin can be found in this PURKINJE MYOCYTES layer. They serve to bind the ends of the myofilaments to the sarcolemma. are relatively short (~50um) compared to ordinary The plakoglobin, and adhesive glycoprotein (A-CAM) myocytes (~80um) but are nearly twice their are localized in the narrow cleft between the diameter (30um) (Fig 10=51) membranes. Intercalated discs are not found but there are large The fascia adherens, comprising the greater part of gap junctions both at the ends and sides of the cells. the disc, is interrupted in certain areas by typical tHeire ultra-structure and membrane properties desmosomes. Intermediate filaments of the favor rapid impulses conduction cytoskeleton may attach there. The atrioventricular bundle contains an admixture of cellular elements. transitional myocytes extend from the node into its initial portion, but more distally, Purkinje myocytes predominate. More extensive junctions of this kind are found on the longitudinal segments of the step-like intercalated discs. The diffusion of ions through the junctions permits coordination of the activities of the myo-cytes enabling cardiac muscle to behave physiologically as though it were a syncytium. The very firm attachment of myocytes at the intercalated discs ensures transmission of the traction generated by the individual cells throughout the myocardium. 101 INTRODUCTION TO MEDICINE | HISTOLOGY AYURA 2027 4 ITM 101 | HISTOLOGY LESSON # 7 – CARDIAC MUSCLE 1st YEAR | A.Y. 2023-2024 | SEPTEMBER 21, 2023 DR. JOANNA MELISSA MARQUEZ INNERVATION OF THE MYOCARDIUM Although the initiation of each heartbeat is myogenic, the heart is innervated and its rate is modulated by the autonomic nervous system. Parasympathetic nerve fibers from the vagus and fibers from the sympathetic trunk form extensive plexuses at the base of the heart. Ganglion cells and numerous nerve axons are found in the wall of the right atrium, especially in the region of the sinoatrial and atrioventricular nodes. 101 INTRODUCTION TO MEDICINE | HISTOLOGY AYURA 2027 5