Why are the skeletal muscles polynuclear
The skeletal muscles consist of many multinucleated muscle cells (= muscle fibers) that contain contractile myofilaments (actin and myosin) in a regular arrangement. The consciously induced interaction of these proteins mainly serves to move the skeleton. The contraction is triggered by an action potential of a somatic nerve, which is transferred to the muscle via the so-called motor end plate. This process leads to an increase in the intracellular calcium concentration in the muscle cell, which in turn results in an interaction between actin and myosin (so-called electromechanical coupling).
Organization and structure
A skeletal muscle is a complex structure made up of various structures. The myofilaments actin and myosin, the smallest functional unit of which is called the “sarcomere”, are combined to form larger myofibrils that fill a large muscle cell (= muscle fiber). The muscle fibers are in turn combined into ever larger bundles by envelope structures. The muscle that is assembled in this way is still completely surrounded by the muscle fascia.
Organization of the skeletal muscle: from myofilament to muscle fascia
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The connective tissue sheaths of the muscle serve nerves and vessels as a guide structure to reach every single muscle cell through the endomysium!
Characteristics of a skeletal muscle fiber (= skeletal muscle cell)
Skeletal muscle cells have the same function as all muscle cells: They are supposed to enable contractions. To do this, they have the contractile myofilaments actin and myosin on the one hand and specialized cellular devices to control them on the other. The excitation that spreads across the cell membrane is brought into direct proximity to the intracellular myofilaments and cell organelles via deep indentations in the cell membrane (T tubules). This allows the excitation to be easily transferred to the sarcoplasmic reticulum, which serves as a calcium store and is expanded into terminal cisterns. For more information on the structure of muscle tissue and myofilaments, see: Basics of muscle tissue.
Skeletal muscle cells can only be regenerated by so-called satellite cells!
Skeletal muscles are controlled by motor nerve fibers from the spinal cord or cranial nerves. The action potential passed on via the nerves is transferred to the muscle at the so-called "motor end plate", a chemical synapse. The subsequent translation of this electrical signal into a mechanical contraction is called "electromechanical coupling". For details on the molecular interaction of myofilaments actin and myosin (filament slip theory) see: Fundamentals of muscle tissue.
The innervation of the muscle by an α-motor neuron is necessary to maintain its shape and function. If the connection between muscle and nerve is cut, for example by a cut ("denervation"), the muscle not only loses its strength in the long term (so-called paresis), but also fundamentally changes its structure: the sarcolemma forms a large number of acetylcholine receptors
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