09.Muscle.pdf underlined.pdf

Muscle tissue It is formed by cells specialized in contraction that will be responsible for body movements. These cells are elongated and for that reason they are called muscle fibers. In addition, certain specific names are used in this type of cells: - Sarcolemma: plasma membrane - Sarcoplasm:...

Muscle tissue It is formed by cells specialized in contraction that will be responsible for body movements. These cells are elongated and for that reason they are called muscle fibers. In addition, certain specific names are used in this type of cells: - Sarcolemma: plasma membrane - Sarcoplasm: cytoplasm - Sarcoplasmic reticulum: SER - Sarcosoma: mitochondria Classification Depending on the presence of an arrangement of contractile filamentous proteins that is repeated regularly in the muscle cell, we will see two types of muscle: - Striated muscle: the arrangement of these filamentous proteins causes dark and light bands to be observed in the cell as striations. This muscle can be of two types: - Skeletal striated muscle - Cardiac striated muscle - Smooth muscle: the proteins have a different arrangement and no striations appear in the cells. Skeletal striated muscle It is the one that forms the muscles of the locomotor apparatus, responsible for the movements of the body. It is a muscle that we usually move voluntarily. It is made up of very large and multinucleated cells. The nuclei are flattened and are always located in the periphery, next to the plasma membrane. The cells have a cylindrical shape, but in the vicinity of the tendons they acquire a conical 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 we also find 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. The connective tissue that surrounds each muscle cell is called endomysium and is formed mainly by reticular fibers and an external lamina (basal lamina). Several fibers with their endomysium are grouped in parallel to form a fascicle, which is surrounded by another connective tissue called perimysium. This tissue is dense and rich in collagen fibers. The fascicles are grouped to form a muscle surrounded by the epimysium, an irregular dense connective tissue. All these connective t. elements are interconnected with each other and continue with the tendons and ligaments that connect the muscle to the bone. Thus, the contractile forces exerted by the muscle fibers are transmitted through the connective t. to generate movement. Dark and light bands called striations are observed throughout the cell. They appear due to the disposition of some filamentous proteins of the cytoskeleton that will be responsible for the contraction, the myofilaments. These myofilaments are grouped to form structures called myofibrils. Myofibrils: They are cylindrical structures that extend along the entire length of the cell, in parallel and without branching. They are 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, the thin filaments are anchored in a dense area called the Z-disk, and the thick filaments are arranged interspersed between them and surrounded by six thin filaments. The I-band is the zone where there are only thin filaments and corresponds to the clear band. The dark zone is the A-band that contains both thick and thin filaments. Within the A-band there is an area where there are only thick filaments that is called H-zone. The Z-disk will join 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. Actin 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. All actin molecules are arranged in the same orientation - filament polarity. Its positive end is the one that joins the Z disk that is negatively charged. 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. Myosin filaments Thick filaments are formed mainly by myosin, which has two chains that form a helical tail and two globular heads. The globular heads have a binding site for actin and another for ATP. The thick filament consists of 200300 molecules of myosin and all of them are joined in parallel and staggered at regular intervals orienting their heads towards the periphery. 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 we will find a large sarcoplasmic reticulum, numerous mitochondria and also structures called T-tubes. All this will be related to muscle contraction. T tubes: They are tubular structures that extend from the plasma membrane and surround the myofibrils, branching and fusing with each other. Furthermore, 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 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. 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. Muscle contraction When the thin filaments slide, the length of the sarcomere is effectively reduced and thus the myofibril and all the fiber are also shortened. The filaments maintain their structure and length, but they slide with respect to each other. This is what is known as the "Huxley sliding filament theory". In addition, this process will obey the "law of all or nothing", that is, a single fiber that receive a stimulus, contracts completely or do not contracts at all. When we make more or less force with a muscle, more or fewer fibers contract, but the fibers that contract, contract completely. Repose Contracted 3. Sarcomere Contraction - YouTube 4. Actin-Myosin Crosslinking - YouTube 5. Calcium and the Biochemistry of Muscle Contraction (Invertebrate) - YouTube 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 neuron innervates several muscle fibers, which will contract all at once upon receiving the same stimulus. The more accurate a muscle is, the smaller the number of fibers innervated by one neuron is. If it is an inaccurate muscle, the same neuron could innervate up to 1000 muscle fibers. 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, similarly to skeletal muscle, although the structure of their myofibrils is slightly different. Between the cells there is connective tissue with an extensive network of capillaries. At the extremes, 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. The structure of sarcomeres is the same as in skeletal muscle and therefore the mechanism of contraction is also the same. But the myofibrils have a more irregular shape and separate in the center of the cell to make room for the nucleus. In addition, the sarcoplasmic reticulum is not as extensive as in the skeletal muscle and does not form terminal cisterns. Its small terminations continue to come into contact with the T tubules, which have a greater diameter than in the skeletal muscle and are located at the height of the Z disks. In this case they do not form triads with the cisterns, but form dyads. Since the reticulum is smaller, there is less calcium stored, but the larger diameter of the T tubes allows more extracellular calcium enter the cytoplasm in the vicinity of the myofibrils. In addition, 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 (central part wider and narrow at the ends), with an oval nucleus in the center. They are also 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. https://www.youtube.com/watch?v=J6MaPhULIYQ