The muscles are the active part of the apparatus of locomotion. Their task is to move the body. The muscles comprise 40% of the body's weight. The body has 300 individual muscles (musculi) in different shapes and sizes.
The basis of the muscles are muscle cells, capable of contracting longitudinally in response to nervous stimulation. The muscle cells contain small contractile strands of protein (so-called myofibrils). They contract when stimulated, returning to their original state as soon as the stimulation ceases. Muscle fibers and muscle cells are distinguished according to their construction:
· Smooth muscles · Striped muscles · Cardiac muscle (myocardium)
The smooth muscles consist of smooth muscle fibers. Their myofibrils run through the muscle fibers longitudinally and are of equal length. Consequently, the muscle fibers look smooth and not striated. These cells are 40-500µm long and 4-20µm thick. The smooth muscles carry out the involuntary movements of the inner organs, e.g. the (peristaltic) bowel movement. The nerves are supplied exclusively by the vegetative (autonomous) nervous system.
The striped muscles are composed of different kinds of tissue: most of them striped muscle fibers. They make up all the skeletal muscles, facilitating our voluntary movements and reflexes. These voluntary movements are driven by stimuli within the cerebral cortex (cortex cerebri) and reach the muscles via nerve paths in the spinal cord (medulla spinalis) and myokinetic nerves. In contrast, involuntary movements stem from myokinetic centers in the brain stem (truncus cerebri) and take place automatically through a voluntary impulse within the cerebral cortex.
The muscle fibers of the striped muscles are multinuclear, cylindrical cells, which can be up to 12 cm (4,68 in) long and 100µm thick. Their striations are due to myofibrils within the cells. The cells and their nuclei are directly under the surface, with their longitudinal axes running in the same direction as the fibers.
The Human 3D software contains both diagrams of human body muscles as well as diagrams of the human body muscular system.
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