Cardiac and Smooth Muscle PDF

_____________ LESSON 11 _____________ CARDIAC AND SMOOTH MUSCLE I. CARDIAC MUSCLE Heart muscle tissue is cardiac muscle or myocardium and is composed of muscle fibers of cylindrical morphology, about 15 µm in diameter and 85-100 µm in length. They are often bifurcated at their ends and are joined...

_____________ LESSON 11 _____________ CARDIAC AND SMOOTH MUSCLE I. CARDIAC MUSCLE Heart muscle tissue is cardiac muscle or myocardium and is composed of muscle fibers of cylindrical morphology, about 15 µm in diameter and 85-100 µm in length. They are often bifurcated at their ends and are joined to each other by intercalated discs, visible with the light microscope, especially by iron hematoxylin stains. Intercalated discs have transverse portions with numerous interdigitate papillary projections with adjacent fibers forming a complex union with desmosomes and fasciae adherens (due to the shape of a belt) and lateral portions rich in gap junctions. Thin myofilaments are attached to them to transmit the movement of contraction from one cell to the adjacent one. Between the adherens junctions of the intercalated discs there are numerous communicating junctions, which favour the passage of ions from one cell to the next, thus allowing the contraction to take place in a progressive and coordinated manner. The cardiac muscle fibers are arranged parallel to each other forming fascicles or parallel sheets to the surface of the endocardium and epicardium and surrounded by loose connective tissue rich in blood vessels. The muscle fibers are surrounded by the outer lamina and by a loose reticular connective tissue with a highly developed capillary network, but there are no satellite cells, so the cardiac muscle fibers cannot regenerate, but when they are destroyed, they are replaced by connective tissue. Like skeletal muscle, cardiac muscle fibers have transverse striations in their cytoplasm, but they present only one or two nuclei of oval and vesicular morphology, with one or two nucleoli and central location. In cross section these fibers are rounded and the spherical nucleus is located in the geometric center of the cytoplasm. 1 Figure 1. Cardiac muscle structure (adapted from Gartner y Hiat, 2013, Color Atlas and Text of Histology, 6th Ed.). The sarcolemma is surrounded, as we have said before, by a basal lamina and presents T tubules that are shorter and thicker than in skeletal muscle, penetrating the external lamina inside. The T tubules are associated with an underdeveloped cistern of the sarcoplasmic reticulum forming diads, which fulfil the same function as the triads in skeletal muscle but are located at the level of the Z line. Another difference with skeletal muscle is that in the cardiac Ca ++ must be actively transported from the extracellular fluid. In the sarcoplasm of the cardiac muscle cells there are many mitochondria (they occupy approximately half of the sarcoplasm volume) which indicates the great energy consumption that these cells need. They also contain abundant glycogen granules and small lipid vesicles since approximately 60% of their energy is obtained from triglycerides. There are two types of contractile cardiac muscle fibers, atrial and ventricular, and a third type of fibers specialized in conducting impulses, Purkinje fibers. • • Contractile fibers: they have all the characteristics described above. Atrial are smaller than ventricular, and muscle fibers of the right atrium and, to a lesser extent of left atrium, containing spherical granules, 0.3-0.4 µm in diameter and electron dense. These granules contain atrial natriuretic factor, a hormone that is released into the blood and acts on the convoluted renal tubules, reducing water and sodium absorption and, therefore, regulates blood volume by lowering blood pressure. Conductive fibers: in addition to contractile muscle cells, in the heart, there are specialized muscle fibers for conducting the nerve impulse. They are larger than contractile cells, the cytoplasm is wide and pale, in which they store abundant glycogen and few myofibrils. Under the epicardium, at the mouth of the vena cava, form the sinoatrial or Keith and Flack node. Under the endocardium, in the bottom of the interatrial septum, they are grouped to form the atrioventricular or Tawara node. Also, under the endocardium, the conductive fibers 2 form a bundle that runs by the heart wall and the interventricular septum called His bundle, which emits branches between the contractile muscle fibers. The conducting cells of the His bundle are called Purkinje fibers and are especially large in ungulates. The Purkinje fibers have little myofibrils, have not T tubules, the sarcoplasmic reticulum is sparse, but the gap junctions are highly developed between them and the adjacent contractile fibers, which transmit the depolarisation so that the contraction is coordinated producing the closure of the cavities (atria or ventricles) and, therefore, the pumping of blood. A B C Figure 2. Cardiomyocyte scheme (A); Electron microscope photograph (B) and light microscope photograph (C) of cardiac muscle fibers. Contraction of cardiac muscle fibers The arrangement of the myofilaments, as well as the organoids, which intervene in muscle contraction, is similar in cardiac and skeletal muscle fibers and, therefore, the mechanism of muscle contraction is very similar. However, its regulation is different. The heart muscle has a rhythmic contraction that is characteristic and differs in different regions of the heart. The impulse to initiate each contraction originates in the sinoatrial node, where numerous nerve endings of the autonomic nervous system arrive. Cardiac muscle fibers are innervated by the autonomic nervous system. Thus, the sympathetic system releases norepinephrine in the nerve endings, increasing depolarization and, therefore, the heart rate. The parasympathetic system releases acetylcholine as a neurotransmitter, decreasing depolarization and thus the heart rate. The nerve endings only reach some cardiac muscle fibers, which transmit the nerve impulse to neighbouring cells through the passage of Ca ++ ions through the communicating junctions, thus producing a rhythmic contraction of the cardiac muscles. II. SMOOTH MUSCLE Smooth muscle tissue is made up of slow and sustained involuntary contraction (except for the urinary bladder), mononuclear spindle cells. It is part of the viscera of the digestive, reproductive, urinary and respiratory systems, walls of blood vessels, gland ducts, dermis and ciliary bodies of the eye. It consists of fusiform muscle fibers, about 5-20 µm in diameter and from about 20 µm to 1 mm in length (e.g. a length of 1 mm can be seen on the fibers of the 3 myometrium of the uterus gravid). These muscle fibers have a central nucleus with a cylindrical morphology, finely granular chromatin, and one or two nucleoli. The nucleus is usually arranged like a helix, indicating a state of contraction, which usually occurs after death or secondary to fixation. In cross sections, the nuclei are spherical, centrally positioned and are not observed in all cells due to the larger size of the cytoplasm. The cytoplasm of smooth muscle fibers is eosinophilic and homogeneous, and there are no transverse striations in longitudinal sections. Smooth muscle cells have the capacity to divide, so they can regenerate and can also be originated by differentiation of pericytes. Between adjacent cells they present Gap type junctions. The structure of smooth muscle fibers is something different from that of striated muscle fibers, since the myofilaments are not arranged forming specific structures. However, they show a complex structuring to favour muscle contraction. Figure 3. Smooth muscle structure (adapted from de Gartner y Hiat, 2013, Color Atlas and Text of Histology, 6th Ed.). The sarcolemma or cytoplasmic membrane of these cells has numerous caveolae on both sides of the nuclei, which have the same function as the T tubules in skeletal and cardiac muscle, as well as pinocytic vesicles. There is also an external lamina surrounding the sarcolemma, as well as numerous communicating junctions with adjacent cells. As organelles, mitochondria, rough endoplasmic reticulum, and a small Golgi complex are usually arranged on either side of the nucleus. Intermediate filaments (desmin and vimentin) are also present in the sarcoplasm and help myofilaments in the process of contraction. Myofibrils have a very different arrangement than skeletal and cardiac muscle. The bundles of myofibrils are arranged parallel to each other, in a spiral fashion. The thin myofilaments contain actin, but not troponin, and in some types of smooth muscle neither tropomyosin. These filaments are very long, helical in arrangement, and are anchored in the sarcolemma by dense bodies, which act in a similar way to the Z line of skeletal muscle. Dense bodies can be revealed with the light microscope and iron hematoxylin stains, being very electron dense. The thick filaments are thicker than in skeletal muscle, but they are less abundant. Thus, the actin/myosin coefficient is the double that in skeletal muscle, 4 because each thick filament is surrounded by 15 thin filaments instead of 6 as in skeletal muscle. In addition, the thick filaments have heavy meromyosin throughout their length, so they can slide further over each other. This sliding is transmitted to dense bodies, wrinkling and shortening the cell, up to 10% of its length at rest, while skeletal muscle can only shorten up to 30% of its length at rest. Therefore, with equal surface area, smooth muscle can exert more force than skeletal muscle. The contraction of smooth muscle fibers does not follow the "all or none law" as in skeletal muscle, but a part of the cell can contract, as well as contract gradually and progressively. . B A Figure 4. Smooth muscle fiber scheme (A) and electron microscope image (B). 5