Muscular tissues In the first embryonic life the muscular tissues arise from mesoderm, The function of movement in multicellular organisms is usually assumed by specialized cells called muscle fibers which contract upon appropriate stimulation. This property is also manifested by other structures such as cilia and flagella. These various motile system have in common the ability to transform chemical in to mechanical energy through the enzymatic splitting of ATP and each possesses precisely arranged filamentous proteins. Proteins have recently been implicated in motility in wide variety of cell systems. In muscle cells filaments are oriented parallel to the direction of the movement and because of their arrangement constitute the actual contractile machinery of the cells in the vertebrate body there are 3 type of muscle based on the appearance and location of cells :smooth, skeletal,and cardiac all 3 types are composed of systematic cells or fibers with long axis arranged in the direction of movement: Skeletal muscle General features : Skeletal muscle consist of long bundles of more or less parallel cells called muscles fibers in longitudinal section cylindrical these cells are marked by transverse striations and have multinucleated are located just beneath the cell membrane or sarcolemma(peripheral position) the fibers contain smaller parallel units the myofibril's which are also transversely striated and are composed of myofilaments that are visibly only in electron microscope the myofilaments are responsible for the striations because of their arrangements within the myofibril Skeletal muscle are attached to the bony structures (voluntary) by tendons which are continues with connective tissues covering over the entire muscle the epimysium this outer most connective tissues extends in to the muscle and surrounds bundles of muscle fibers forming the perimysium which divided in to delicate sheathes of reticular fibers around each muscle fiber called the endo mysium. Blood vessels and nerves follow these sheaths in to the interior of the muscle and rich capillary net work closely invests each muscle fiber. Skeletal muscle fiber contain a cytoplasm or sarcoplasm which occupies the limited space between the myofibrils The sarcoplasm contain the sarcoplasmic reticulum and Golgi apparatus, glycogen lipid droplets,both lipid and glycogen provide metabolic fuel for the contractile machinery.
The banding pattern of the skeletal muscle fiber reflect the ultra structural organization of each myofibrils and knowledge of this pattern is fundamental to an under standing the mechanisms of contraction. The 2 largest bands are named according to their appearance in polarized light. the anisotropic or A bands are bright when viewed with a polarizing microscope, they alternate with dark isotropic I bands the pattern is repeated along the length of the myofibrils,each A band has central zone called H band, and each I band is bisected by distinct Z line.the M line marks the center of H band,the segment between 2 successive Z line is called sarcomere this structural units is repeated along the length of the myofibrils, Biochemical analysis has revealed that the myofibrils consist of number of proteins 2 of these is actin and myosin. Myofibrils is composed also of 2 major types of filaments one of type being thicker than the other,the thick filaments composed of myosin on the other hand thin filaments composed of actin. Red and white muscle : In fresh skeletal muscles one can distinguished red and white fibers the red color is due to abundance of muscle pigment (myoglobin) and less glycogen, and increase in mitochondria and a richer vascularity in the smaller red muscle fibers the larger, pale white fibers have fewer mitochondria less pigment and poorer blood supply functionally red and white muscles are also different the red muscles contract slowly than the white and there for called slow muscles. ( A)
(B) (C) skeletal muscle
Cardiac muscle General features: Cardiac muscle forming the bulk of the walls and septa of the heart as well as the origins of great vessels contrast to pump blood through cardiovascular system similar to skeletal muscle,the cardiac muscle fiber has distinct cross striation however the myofibrils are more delicate than skeletal muscle fibers making the striation less prominent,there is a greater amount of sarcoplasm surrounding the nuclei and the myofibrils than in skeletal muscle, the cardiac muscle fiber has thin sarcolemma and oval prominent nucleus is in the center of the fiber as in smooth muscle fiber, cardiac muscle fiber differ from other muscle type in that is fiber is short and branch to form a complicated network. under LM dark irregular lines intercalated disks may be seen passing across the cardiac fiber at un even intervals, electron microscope reveals these disks to be adhesive connecting junctions between separate cells thus a cardiac fibers is composed of several mononucleated cells joined by intercalated disks, the walls of the heart are highly vascular between the individual cardiac fibers is a rich plexus of blood capillaries these vessel supply oxygen and nutrients to the cardiac cells to sustain vigorous continuous heart beat the high metabolic needs of the heart are also reflected in an abundance of mitochondria and as well developed sarcoplasmic reticulum in cardiac cells. Purkinje fibers : Purkinje fibers are modified cardiac muscle fibers specialized for the condition of electrical impulses similar to those nerve they are located beneath the endocardium as a bundle of His, Purkinje fibers are thicker larger fibers than cardiac and contain glycogen they have a large a mount of sarcoplasm surrounding the central nucleus
( A) Smooth muscle: (B) cardiac muscle Smooth muscle also called involuntary or visceral muscle is structurally the simplest of muscle type it is called smooth because it has no visible cross striations involuntary because it is under conscious control and visceral because it is be predominant found in organs the individual fibers are elongated spindle shaped cell, a cigar shaped nucleus lies near the center of each fibers the cytoplasm in the muscle called sarcpolasm appears rather homogenous even thought it is filled with very fine contractile elements, the myofilaments. In electron microscope the cytoplasm of smooth muscle is dominated by longitudinal thin microfilaments with few thick myofilaments these filaments however are not arranged as in striated muscle and there for
there are no cross striation,smooth muscle have also sarcoplasmic reticulum and a well developed Golgi complex (A) (B)smooth muscle