Muscle Tissue PDF

Muscle tissue  Main function: contraction, responsible for body movements Elongated cells called muscle fibers Certain specific names are used in this type of cells: - Sarcolemma: plasma membrane - Sarcoplasm: cytoplasm - Sarcoplasmic reticulum: SER - Sarcosome: mitochondria...

Muscle tissue  Main function: contraction, responsible for body movements Elongated cells called muscle fibers Certain specific names are used in this type of cells: - Sarcolemma: plasma membrane - Sarcoplasm: cytoplasm - Sarcoplasmic reticulum: SER - Sarcosome: mitochondria Classification Types of muscle according to the presence of an arrangement of contractile filamentous proteins repeated regularly in the muscle cell: - Striated muscle: the arrangement of these filamentous proteins causes dark and light bands to be observed in the cell as striations. Two types of striated muscle: - Skeletal striated muscle - Cardiac striated muscle - Smooth muscle: the contractile proteins have a different arrangement, and no striations appear in the cells. Overview of Muscle Tissues - Concept | Anatomy and Physiology | JoVe  Skeletal striated muscle Present in muscles of the locomotor apparatus, responsible for the voluntary movements of the body. Made up of very large and multinucleated cells – muscle fibers – with flat nuclei that are always located in the periphery, next to the plasma membrane. These cells have a cylindrical shape. They are arranged in parallel with respect to each other, always leaving spaces between them where there will be connective tissue with rich vascularization. Around the muscle cells there are satellite cells, which have a single nucleus, surrounded by the same connective tissue that surrounds the fiber. These cells act as regenerative cells to repair the damage suffered by the muscle, since the skeletal muscle cells can not divide. Layers of connective tissue surrounding the muscle: Endomysium: surround muscle cell. Perimysium: surround muscle fascicle. Several fibers are grouped in parallel to form a fascicle. Epimysium: surround the entire muscle.  Myofibrils Cylindrical structures that extend along the entire length of the cell, in parallel and without branching. Formed by myofilaments, which are arranged in an orderly and repetitive model giving rise to dark and light bands. That organized structure that is repeated throughout the entire myofibril is called sarcomere. https://www.youtube.com/watch?v=SCznFaTwTPE  Sarcomere There are two types of myofilaments: thin (actin) or thick (myosin). In a sarcomere, 6 thin filaments surround 1 thick filament. Areas of the sarcomere: I-band - Only thin filaments. Clear band. A-band - Contains both thick and thin filaments. Dark zone. H-zone - Within the A-band there is an area where there are only thick filaments. M line - In the centre of the sarcomere. Z-disk - Joins thin filaments on both sides, so it is in the middle of the I-band. The space between one Z-disk and the next is called sarcomere, which will repeat throughout the entire myofibril. The Sarcomere - Concept | Anatomy and Physiology | JoVe  Myofilaments - Thin filaments. The main component of the thin filaments is F-actin, that is produced by polymerization of globular molecules into two filaments that form a helix. - Thick filaments. Formed mainly by myosin, which has two chains that form a helical tail and two globular heads. In addition, each actin molecule has a binding site for myosin, but this binding site is usually capped by filaments from another protein called tropomyosin. Another molecule that forms these fine filaments is troponin, which has three subunits: one that binds actin, another binds tropomyosin and the third binds calcium. In the central area of the thick filament there are only myosin tails, while at the ends, which is where it will get in touch with the actin, there are tails and heads. The sarcomeres are repeated to form the myofibrils, and the myofibrils extend from one end of the cell to the other. Most of the sarcoplasm of the cell is occupied by the myofibrils, and this is why the nuclei are displaced to the periphery. In between the myofibrils there are several structures related to muscle contraction: a large sarcoplasmic reticulum numerous mitochondria T-tubes  T tubes Tubular structures that extend from the plasma membrane and surround the myofibrils. In skeletal muscle they are specifically arranged in the plane of the union of A-bands and I- bands. Thus, each sarcomere has 2 sets of T- tubes. These tubes allow all myofibrils and all sarcomeres to be in contact with the plasma membrane even in the innermost areas of the cell. So, when a nervous impulse arrives at the cell, it is transmitted directly to all the myofibrils so that they all contract in unison. Sarcoplasmic reticulum: The sarcoplasmic reticulum is associated with the T tubes. It forms a mesh around each myofibril and in the junction zone of the A and I bands it ends in a dilated part called terminal cistern. Thus, each T tube is surrounded by two terminal cisterns and together form a triad. The nervous stimulus will also reach almost instantaneously the sarcoplasmic reticulum. Calcium is necessary for contraction and is stored within the reticulum. Upon receiving the stimulus, the reticulum will release calcium from its interior. To stop the contraction, the reticulum re-sequesters calcium from the cytoplasm. A B C D Microscopic Anatomy of Skeletal Muscles - Concept | Anatomy and Physiology | JoVE Muscle contraction In resting state, the site of union of the myosin is covered in the thin filaments and for that reason the thin and thick filaments are separated. When a nervous impulse arrives, it is transmitted throughout the membrane and the T tubes and reaches the sarcoplasmic reticulum. The reticulum releases calcium into the cytoplasm. Calcium binds to troponin, changing its conformation. This will force the tropomyosin to move and will uncover the myosin binding site, so that actin and myosin may bind. Actin and Myosin in Muscle Contraction - Concept | Cell Biology | JoVE When the binding site is free, myosin spends the ATP it had in order to bind, releasing a phosphate and leaving ADP joined to myosin. Since myosin does not bind ADP, it releases it by changing its conformation. As the myosin molecule moves, it will displace the actin that was attached to it, causing a slippage of this filament. Once the ATP binding site has been released, a new molecule of ATP will join, causing the actin to be released again and the myosin to recover its conformation. But while there is still calcium in the cytoplasm, actin and myosin can be reattached starting the cycle again. This cycle will be repeated about 200 times for the contraction to complete.  Innervation Each muscle receives sensory and motor innervation and also some autonomous fibers. Motor fibers are those that cause contraction, while sensitive fibers collect sensitive information from the muscle. Autonomous fibers primarily innervate the vessels. A motor unit is formed by a neuron with the set of fibers it innervates. The axon of the neuron enters the conjunctive of the muscle, branches out and loses its myelin. The terminal of each branch dilates and covers the membrane of each individual muscle fiber. The area where the nerve ending meets the muscle is called the motor plate. Cardiac striated muscle It is another type of striated muscle, not voluntary and present only in the heart. It has an extensive network of capillaries that surround each muscle cell. This explains why these cells can get most of their energy from aerobic respiration. Cardiac muscle cells contract rhythmically and spontaneously thanks to a group of modified cardiac muscle cells that coordinate the contraction of this muscle. Cardiac muscle cells have a very limited capacity for regeneration. If they suffer significant damage, as in myocardial infarctions, this tissue can not be recovered and is replaced by scar tissue.  Cardiac muscle cells are also elongated and cylindrical, although they branch off at their ends, where they will bind other cardiac cells. They have a smaller size than those of the skeletal muscle and a single large oval nucleus in central position, although occasionally they may have two nuclei. They have cross striations Between the cells there is connective tissue with an extensive network of capillaries. At the ends, where the cells join each other, there is a binding structure called intercalated disc.  Intercalated discs These junctions between the cardiac cells have a stepped shape and in their transverse parts (vertical) there are a lot of adherent fascias and desmosomes that keep the two cells together. In the longitudinal (horizontal) part, there are abundant communicating unions (GAP junctions), that allow the interchange of ions and small molecules between the cells so that they are well coordinated. Cell structure. Comparison with skeletal striated muscle: The structure of sarcomeres is the same as in skeletal muscle and therefore the mechanism of contraction is also the same. The myofibrils have a more irregular shape and separate in the center of the cell to make room for the nucleus. The sarcoplasmic reticulum is not as extensive as in the skeletal muscle and does not form terminal cisterns. The T tubules, which have a greater diameter, are located at the height of the Z disks. In this case they form dyads. These cells contain many more mitochondria than the skeletal muscle cells due to the high energy consumption of the heart muscle.  Smooth muscle It is an involuntary muscle present in the walls of hollow viscera, blood vessels, large ducts of the glands, airways and in the dermis of the skin. - These cells do not have cross striations, so there are no myofibrils or a T-tube system. - They are short, spindle-shaped cells with an oval nucleus in the center. - They are surrounded by an external lamina and all cells are joined together forming bundles or layers. - They retain their mitotic capacity, so they can divide and regenerate damaged muscle. Contraction mechanism On the cytosolic side of its membrane, there are dense bodies as well as scattered throughout the cytoplasm. Thin filaments are inserted into these dense bodies and thick filaments will be placed between them. In this case, the thick filaments are arranged in an aligned manner, so that the heads of the myosin project over the entire length of the filament and not only at its ends, which allows a longer contraction. In addition, here are 15 thin filaments that surround a thick one instead of six. Elongated nucleus Spiral nucleus (cell relaxed) (contracted cell)