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Muscle
Muscle tissue composed of cells that optimize the universal cell property of contractility. Actin microfilaments and associated proteins generate the forces necessary for the muscle contraction, which drives movement within organ systems, of blood, and of the body as a whole. muscle cells are of mesodermal origin and differentiate by a gradual process of cell lengthening with abundant synthesis of the myofibrillar proteins actin and myosin.
Composed of elongated muscle cells and intercellular substance in the form of loose areolar connective tissue. The sarcoplasm of muscle cells are myofibrils. Myofibrils are fine thread like structures, which are responsible for muscle contraction and may appear striated or not.
Three main types of muscle tissue ● Skeletal muscle contains bundles of very long, multi-nucleated cells with cross-striations. Their contraction is quick, forceful, and usually under voluntary control. Responsible for the movement of the skeleton. Arrangement of the contractile proteins give rise to the appearance of cross-striations.
● Cardiac muscle also has cross-striations and is com- posed of elongated, often branched cells bound to one another at structures called intercalated discs, which are unique to cardiac muscle. Contraction is involuntary, vigorous, and rhythmic. Provided with cross-straitions
● Smooth muscle consists of collections of fusiform cells, which lack striations and have slow, involuntary contractions. Forms the muscle components such as blood vessels, the gastrointestinal tract, the uterus and the urinary bladder. Involuntary (under inherent autonomic and hormonal control)
● Cytoplasm is called Sarcoplasm ● Cell membrane or plasmalemma is called Sarcolemma ● Sarcoplasm contains numerous myofibrils that contains
two types of contractile protein filaments:
A. Actin (thin filament)
● Composition:
○ Made of globular actin (G-actin) monomers polymerized into filamentous actin (F-actin).
○ Bound by regulatory proteins:
■ Tropomyosin: Blocks myosin-binding sites on actin at rest.
■ Troponin complex (TnT, TnI, TnC): Binds Ca²⁺ to initiate contraction.
● Diameter: ~5–7 nm.
● Function:
○ Provides binding sites for myosin heads during contraction.
○ Slides past myosin filaments (sliding filament mechanism).
B. Myosin (thick filaments)
● Composition:
● Composed of myosin II (motor protein) with:
○ Tail regions (bundled into filaments).
○ Head regions (bind actin + hydrolyze ATP).
● Diameter: ~15 nm.
● Function:
● Uses ATP to "walk" along actin filaments (power stroke).
● Generates force for muscle contraction.
Types of muscles
- Smooth muscles
- Cardiac muscle
- Skeletal muscle
- Smooth muscle
Muscles are spindle-shaped cells with one central nucleus. Grouped into sheets often running perpendicular to each other. Plays a major role in Peristalsis. Smooth muscle cells have no striations (no sarcomeres) and contractions are slow, sustained and resistant to fatigue. Does not always require a nervous signal and can be stimulated by stretching or hormones. Sarcoplasm is homogenous with fine threadlike structures called myofibrils (sacrostyle), arranged parallel to the long axis of the muscle cell. Smooth muscle in cross section appears like a mosaic of rounded or irregularly polygonal structures of varying sizes
Functions of smooth muscle
● Contraction of the bladder to force urine out ● Move food through the intestines (peristalsis) ● Peristaltic movement to move feces down the digestive system ● Contraction of smooth muscle in the trachea and bronchi which decreases the size of the air passageway ● Constriction and dilation of blood vessels ● Constriction, accommodation and dilation of pupil
Cardiac muscle
Has characteristics of both skeletal and smooth muscle and functions to provide the contractile activity of the heart. The muscle is very fatigue resistant and activation of cardiac muscle is involuntary (like smooth muscle) and made up of elongated muscle fibers that branches. Cardiac muscle has a single spherical centrally located nucleus (Branches have no nucleus). Myofibrils appears striated with distinct actin & myosin with an intercalated disk of Eberth: serves as junction between cardiac cells, with intercalated disk (cell junction between cardiac cells). The numerous Loose Areolar CT since the cells are branching, Cardiac muscle fibres are shorter than the skeletal muscle fibres and show branching patterns. One or two nuclei placed in the centre presence of darkly staining transverse lines across the fibres -> intercalated discs -> Specialised cell junctions between the ends of adjacent muscle fibres
Functions of the cardiac muscle tissue
● Cardiac muscle tissue plays the most important role in the contraction of the atria and ventricles of the heart. ● It causes the rhythmical beating of the heart, circulating the blood and its contents throughout the body as a consequence. ● Distribution ○ Muscle layer of the heart (myocardium) ● Walls of the aorta, vena cava and pulmonary vessels
Skeletal muscles
Are voluntary muscles that move bones. They are called striated because they look striped and are attached to tendons, which are attached to bones. It comprises most of the musculature of the body and are related or attached to the skeletal system. They’re grouped together into elongated bundles called Fasciculi , which are surrounded by loose collage out tissue called Perimysium. The whole muscle mass is invested in a dense collagenous sheath called the epimysium. Each individual fiber is surrounded with a delicate supporting tissue called endomysium.
In a Longitudinal section the section is elongated and doesn’t branch. Cells are multinucleated, nuclei located in the periphery just beneath the Sarcolemma(thin and transparent and has glycoprotein external coating with a delicate network of reticular fibers.)
Myofibrils are grouped into parallel bundles longitudinally arranged along the muscle fibers. Arrangement results in the appearance of the alternating light and dark bands in the muscle fiber. Regions seen in sarcomere (fundamental structure sub unit of all striated muscle): A. Anisotropic band (A band) - dark staining, appear bright, length remains constant in all phases of contraction, represents the entire length of thick filaments
B. Isotropic band (I band) - light staining, appear dark, most prominent in stretched muscles, shorter at resting stage, region exclusively of thin filaments insert at Z line
C. Telophragma (Z line) - dark traverse line bisecting I band
D. Intermediate disk of hensen (H band) - paler zone, traversing the center of the A band
E. Mesophragma (M band) - narrow dark line in the center of the H band, thickest point of the thick filaments
● maintain body temperature ● store nutrients
Distribution ● Widely distributed because this type of muscle is attached to the entire skeletal system of the body (external urethra and external anal sphincter) ● Although the tongue & upper part of the esophagus are unattached to the skeletal system, it is classified as striated voluntarily. Its lower part is involuntary in nature.
