Brightfield light micrograph of human trachea smooth muscle cells. The trachea (wind pipe) runs from the larynx (voice box) to the lungs. The smooth muscle cells appear as columnar muscle fibres. The smooth muscle fibres at the back of the trachea are known as the trachealis muscle. The trachealis muscle relaxes with inhalation and contracts with exhalation. Smooth muscle is an involuntary non-striated muscle. Smooth muscle is made of many bundles of muscle fibres (cells). These muscle cells contain the contractile proteins actin and myosin, which slide over each other to contract the muscle. The muscle cells have typically elongated nuclei. The structure and function is basically the same in smooth muscle cells in different organs of the human body. Magnification: x100 when shortest axis printed at 25 millimetres.
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