Skeletmuskler PDF

### Skeletmuskler * **Muskelcelle** is often several centimeter long and 10-100 µm thick. But only in a few muscles does a muscle fiber reach from one end of the muscle to the other. * **The end of a striated muscle cell** connected to a tendon is swollen, whereas the end which wedges itself bet...

### Skeletmuskler * **Muskelcelle** is often several centimeter long and 10-100 µm thick. But only in a few muscles does a muscle fiber reach from one end of the muscle to the other. * **The end of a striated muscle cell** connected to a tendon is swollen, whereas the end which wedges itself between other fibers is pointed. * **A muscle cell** located in the middle of the muscle and which has no connection to a tendon is pointed at both ends. * **A large muscle cell** can be called a muscle fiber. Previously, the term *striated muscle fiber* was used instead of muscle cell, because of the large size and the many nuclei. Fig. 36b shows the 'ends' of a striated muscle cell. The middle section has been omitted and is marked with two dotted lines. This is necessary when wanting to draw the muscle cell at the specified thickness. If the length were to be drawn in the correct proportion to the thickness, the muscle cell would not fit on the paper. * **A striated muscle cell** is enclosed by a sarcolemma. That encloses sarcoplasm with myofibrils and nuclei. Mitochondria are found in large quantities. They function as the muscle cell's ‘power plants’ and provide energy for the contractions. Endoplasmic reticulum is also well-developed and is called sarcoplasmic reticulum. The nuclei (3) are located peripherally in the sarcoplasm right under the sarcolemma (1). Their number depends on the muscle cell's length. Long muscle cells have hundreds of nuclei. * **Fig. 48 Hi.Ku. shows** that the many nuclei (2) are located peripherally in striated muscle cells (1). This can be seen in cross sections of muscle cells, Fig. 49 Hi.Ku. (3). ### Myofibrils * **Myofibrils** extend through the entire muscle cell. They are either diffusely distributed or collected in bundles (4). Across a striated muscle cell, one sees ‘bands’ (1-3), which alternate regularly. The striation of muscle cells is caused by the fact that the transverse bands of the myofibrils are offset from one another and interrupt the light differently. * **The one transverse band** is anisotropic and is called the A-band (1). The other band is isotropic and is called the I-band (2). In resting conditions, the two bands are equally thick. This changes during contraction. In the middle of the I-band, a small, dark Z-line is found. Fig. 57. * **The sarcomere** is the structural and functional unit of skeletal muscle. It is bound by two Z-lines which approach one another during contraction, Fig. 57 and 58. ### Myofilaments and contraction * **Each muscle cell (C)** contains many bundles of myofibrils (D), which extend along the length of the cell. Each individual myofibril is longitudinally divided into uniform sarcomeres (S). Between the two lower, dotted lines from (D), a section of the myofibril is cut out and enlarged in (E). * **From each Z-line** emerge actin filaments. Between them, myosin filaments are located, which have the ability to draw the actin filaments towards them, so that the sarcomere shortens. * **The I-band** consists entirely of thin actin filaments. Actin filaments are connected to the Z-lines at one end, while their free ends wedge themselves between the thick myosin filaments in the A-band. The A-band thus comes to consist of both thick myosin filaments and thin actin filaments. * **Muscle contraction** is effected by the actin filaments sliding along the myosin filaments. * **Fig. 59 shows** that the thick myosin filaments (2) are not smooth as the drawings had shown previously. Their proteins have a number of fine ‘branches’ (4) which project from the surface. Each branch consists of a thin stem and a ‘head’ (3), which rests against the neighboring actin filament (1). During contraction, the ‘heads’ nod and pull the actin filaments towards one another. * **The heads bend** back to the resting position (B 3, dotted), and a new nodding movement (B 3, drawn) gives the actin filament (1) another ‘push’ towards the middle, Fig. 59. * **This leads to filament sliding** and to the myofibrils, muscle cells and muscle becoming shorter. * **A muscle contraction** is initiated by an impulse that spreads partly around the muscle fiber (13) and partly around the myofibrils (4). This occurs through channels (6) across the muscle fiber's long axis. The sections of the endoplasmic (sarcoplasmic) reticulum (5) that are located closest to a channel (6) form expansions, cisterns (2). * **When the impulse** passes through the channels (6), the cisterns (2) release calcium ions, which act on the filaments and initiate the ‘sliding’ and contraction. When the contraction is over, the calcium ions are pumped back to the cisterns.