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Histology - 2 nd^ stage
Muscular tissues: is responsible for movement of the body and its parts for changes in
the size and shape of internal organs. This tissue is characterized by aggregates of specialized, elongated cells arranged in parallel array that have the primary role of contraction.
General Concepts
Specializations, unique terms are used for certain structures in muscle cells.
1. Individual muscle cells are called muscle fibers or myocytes 2. The cytoplasm of muscle fibers is called sarcoplasm. 3. The muscle fiber plasma membrane is called the sarcolemma. 4. The smooth endoplasmic reticulum is called the sarcoplasmic reticulum, while mitochondria called **sarcosomes.
- Myofilaments:** interaction is responsible for muscle cell contraction. It has two types' thin filaments (actin ) and Thick filaments ( myosin ) Certain forms of contractile cell function as single-cell contractile units: 1. Myoepithelial cells are an important component of certain secretory glands. 2. Pericytes are smooth muscle-like cells that surround blood vessels. 3. Myofibroblasts are cells that have a contractile role in addition to being able to secrete collagen. This type of cell found in normal tissues but becomes important in tissue damage during the process of healing and repair, leading to formation of a scar.
Classification of Muscle
➢ Functional classification : is based on the type of neural control divided in to two types: Voluntary Involuntary ➢ Structural classification: is based on the presence or absence of cross striations divided in to two types: Striated No striated (smooth) ➢ Combined functional and structural classification divided in to three types: Skeletal muscle Cardiac muscle
Histology - 2 nd^ stage
Smooth (visceral) muscle. Skeletal muscle tissues: contains bundles of very long, multinucleated cells with cross- striations. Their contraction is quick, forceful, and usually under voluntary control. Organization of a Skeletal Muscle
1. Epimysium is the sheath of dense connective tissue that surrounds a collection of fascicles that constitutes the muscle. The major vascular and nerve supply of the muscle penetrates the epimysium. 2. Perimysium is a thicker connective tissue layer that surrounds a group of fibers to form a bundle or fascicle. Fascicles are functional units of muscle fibers that tend to work together to perform a specific function. Larger blood vessels and nerves travel in the perimysium. 3. Endomysium is the delicate layer of reticular fibers that immediately surrounds individual muscle fibers .Only small-diameter blood vessels and the finest neuronal.
Myofibrils and Myofilaments (Actin and Myosin Filaments) A muscle fiber is filled with longitudinally arrayed structural subunits called myofibrils Myofibrils consist of an end-to end repetitive arrangement of sarcomeres. The A and I banding pattern in sarcomeres is due mainly to the regular arrangement of thick and thin myofilaments, composed of myosin and F-actin, respectively, organized within each myofibril in a symmetric pattern containing thousands of each filament type. Organization within Muscle Fibers
Histology - 2 nd^ stage
The conducting system for contractile in skeletal muscle (Sarcoplasmic Reticulum &Transverse Tubule System) In skeletal muscle fibers the smooth ER, or (sarcoplasmic reticulum), is specialized for Ca2+^ sequestration, and cause uniform contraction of all myofibrils, the sarcolemma is folded into a system of transverse or T tubules it is long fingerlike invaginations of the cell membrane penetrate deeply into the sarcoplasm and encircle every myofibril near the aligned A- and I-band boundaries of sarcomeres. Adjacent to each side of every T tubule are expanded terminal cisterns of the sarcoplasmic reticulum. The complex of a T tubule with two closely associated small cisterns of sarcoplasmic reticulum on each side is known as a triad. After depolarization of the sarcoplasmic reticulum membrane, calcium ions concentrated within these cisternae are released through Ca2+^ channels in the membrane into cytoplasm surrounding the thick and thin filaments. Ca2+ binds troponin and allows bridging between actin and myosin molecules. When the membrane depolarization ends, the sarcoplasmic reticulum pumps Ca2+ back into the cisternae, ending contractile activity.
Histology - 2 nd^ stage
Myofilament interaction is responsible for muscle cell contraction: Two
types of myofilaments are associated with cell contraction
1. Thin filaments are composed primarily of the protein actin. Each thin filament of fibrous actin ( F-actin ) is a polymer formed from globular actin molecules ( G-actin ). 2. Thick filaments are composed of the protein myosin consists of tail and head (contain binding site for actin and for ATP, ATPase). this myofilament consist of two types: A. Tropomyosin is protein of double helix of two polypeptides. B. Troponin complex consists of a three globular subunits. Troponin-C (TnC) It binds Ca2+ Troponin-T (TnT) , binds to tropomyosin Troponin-I (TnI) , thus inhibiting actin–myosin interaction
Smooth muscle tissues: is specialized for slow, steady contraction and is controlled by a variety of involuntary mechanisms (also called visceral muscle) are elongated, tapering, and nonstriated cells, form the walls of most hollow organs with the exception of the heart.
Histology - 2 nd^ stage
A Cardiac Muscle tissue: occurs only in the myocardium of the heart and, in the
roots of large vessels where they join the heart.
Structure of cardiac muscle fibers
- Intermediate in size between skeletal and smooth muscle
- Fibers are cylindrical, branch, and form interwoven bundles.
- Usually one nucleus per fiber located in the center
- Organelles are clustered at the poles of the nucleus.
- Myofilament organization into myofibrils is identical to skeletal muscle. Cross- striations of bands are presented, but not as prominent as in skeletal muscle.
- High vascularity and with large numbers of mitochondria reflect the high metabolic requirements of cardiac muscle fibers.
- Fibers are capable of hypertrophy but not hyperplasia. Intercalated discs Junctional complexes those are unique to cardiac muscle fibers. Consist of specialized cell junctions of the sarcolemma at the ends of the fibers. Contain three types of junctions zonula adherens of epithelia; serve to attach cardiac muscle fibers and anchor actin filaments of the terminal sarcomeres at the ends of the cell. Acts as a hemi-Z-line. Desmosomes. Bind ends of fibers together Gap junctions. Provide ionic coupling between fibers
