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Muscles BCH 443 First Semester 1427-28 Types of Muscles - Smooth muscle - Striated muscle (skeletal muscle) - Cardiac muscle. Smooth Muscle • Smooth muscle is involuntary (they cannot be controlled voluntarily). • It lines the gut, blood vessels, and reproductive tract, in the wall of the trachea, uterus, and bladder. • The cells are tapered on the ends. • Their cells have a variable length but are in the order of 0.1 mm. • The contraction of smooth muscle is controlled from the brain through the autonomic nervous system. Skeletal Muscles Functions of Skeletal Muscle • Movement • Posture • Stabilizes joints • Heat • Protects organs Striated muscles, are also called skeletal muscles • skeletal muscles are formed from a large number of muscle fibres, that range in length from 1 to 40 mm and in diameter from 0.01 to 0.1mm. • Each fibre forms a (muscle) cell and is distinguished by the presence of alternating dark and light bands. This is the origin of the description "striated," • Striated muscles are connected to the bones via tendons. • Such muscles are voluntary and form an essential part of the organ of support and motion. • The striated muscle fibre corresponds to an (unmyelinated) nerve fibre but is distinguished electrophysiologically from nerve by the presence of a periodic transverse tubular system (TTS), a complex structure that, in effect, continues the surface membrane into the interior of the muscle. • Propagation of the impulse over the surface membrane continues radially into the fibre via the TTS, and forms the trigger of myofibrillar contraction. • The presence of the TTS affects conduction of the muscle fibre so that it differs (although only slightly) from propagation on an (unmyelinated) nerve fibre. Cardiac Muscle • Cardiac muscle is found in the heart. The cells are short, branched, and striated. Intercalated disks are regions where cells join together. The blue arrows in the photograph point to branches. • Cardiac muscle is striated, but differs in other ways from skeletal muscle: • It is involuntary muscle. • When excited, it generates a much longer electric impulse than does skeletal muscle, lasting about 300ms. • Correspondingly, the mechanical contraction also lasts longer. • Cardiac muscle has a special property: The electric activity of one muscle cell spreads to all other surrounding muscle cells, owing to an elaborate system of intercellular junctions. Structure of Skeletal Muscles • Skeletal muscles consist of 100,000s of muscle cells (also known as "muscle fibers") that perform the functions of the specific muscle of which they are a part. • The components of skeletal muscle cells that are specific to muscle tissue are myofibrils. • Each muscle fibre ("muscle cell") is covered by a plasma membrane sheath which is called the sarcolemma. • Tunnel-like extensions from the sarcolemma pass through the muscle fibre from one side of it to the other in transverse sections through the diameter of the fibre. These tunnel-like extensions are known as transverse tubules ("T Tubules) Sarcoplasm • The cytoplasm present is muscle fibres (muscle cells) is called sarcoplasm • The sarcoplasm present in muscle fibres contains many mitochondria. • These mitochondria produce large amounts of "ATP’, required for contracting muscles, moving chromosomes during cell division, moving structures with cells, transporting substances across cell membranes, biosynthesis etc. • Myoglobin is present in the sarcoplasm of muscle fibres/cells. This reddish pigment results in the distinctive colour of skeletal muscle, and stores oxygen - until it is required by the mitochondria for the production of ATP. Sarcoplasmic reticulum • Sarcoplasmic reticulum is a network of membrane-enclosed tubules similar to smooth endoplasmic reticulum (SER). • Sarcoplasmic reticulum is present in muscle fibres/cells and extends throughout the sarcoplasm of the cell. • The function of the sarcoplasmic reticulum is to store calcium ions, which are necessary for muscle contraction. • The nuclei of muscle fibres ("muscle cells") are located at the edges of the diameter of the fibre, adjacent to the sarcolemma. A single muscle fibre may have many nuclei. Sarcomere: Contractile unit • Each myofibril consists of two types of protein filaments called "thick filaments", and "thin filaments". These two types of filament have different structures . • The thick filaments and the thin filaments within myofibrils overlap in a structured way, forming units called sarcomeres. • The extent to which the thick filaments and the thin filaments overlap determines the sizes of the H zone, the I band, and the A band of the sacromere formed by these filaments. Central less dense zone-H; Sarcomere repeated every 2.3 µm. Middle dark line- M line Light Band Dark Band • In the diagram below the Z discs are represented by the zig-zag lines that form the boundaries between adjacent sarcomeres • Thick Filaments Thick filaments (150 AO) are formed from a protein called myosin which has important properties of elasticity and contractibility. The shape of the myosin molecules has the appearance of two "hockey sticks" or "golf clubs" twisted together. These are the myosin tail, and the myosin heads, or "crossbridges" . Hydrolyzes ATP, binds Actin. An Enzyme with ATPase activity: ATP +H2O of energy for muscles ADP +pi+ H+ source • Thin Filaments (70 AO- Actin, Tropomyosin and Troponin): The main component of the thin filaments is actin. Actin molecules join together forming chains twisted into a helix configuration. These molecules are very important to the contraction mechanism of muscles because each actin molecule has a single "myosin-binding site" The other two protein molecules that form the thin filaments are called troponin and tropomyosin. The molecules of tropomyosin cover the myosin-binding sites on the actin molecules when the muscle fibres are relaxed. • Actin and myosin are the two principal muscle proteins, present in myofibrils. • arranged in a ring-like structure, usually with six (thin) actin strands surrounding a (thicker) myosin fibril. • run parallel and lengthwise. • The myosin fibril has numerous small protrusions called cross bridges. • The actin strand is actually intertwined with an even thinner, ribbon-like protein called tropomyosin, and a smaller molecule, called troponin, associates with tropomyosin in this structure. • Myosin and actin form the main contractile elements of muscles. This is because it is the binding of the thick filaments to the thin filaments - and in particular the positions of these points of attachment - that controls the state of contraction/relaxation of the muscle of which they are apart. • Tropomyosin: Two strands α-helical rod. – Located in the grooe between the two helical strands of actin. • Troponin: Complex of three polypeptide chains TpC, TpI, and TpT. The troponin is located in the thin filaments at intervals. HISTOLOGIC FEATURES OF CARDIAC MUSCLE. The cardiac muscle resembles skeletal muscle in having striated fibers. Cardiac muscle has intercalated discs. Note the cross striations, the central position of the nuclei, the intercalated discs, and the branched fibers The blue arrows in the photograph point to branches.