Sarcomere
Sarcomere: A segment of a myofibril (muscle cell) from one Z disk to the next. This is the smallest functional unit of a muscle cell. A muscle shortens because all of the individual sarcomeres shorten, pulling Z disks closer to one another.
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The components of a sarcomere visible under an electron microscope (as depicted in the image)
- Z-disk (line): Refers to a dark line between adjacent I-bands under a microscope. This area is the border, or walls of the "sarcomere ," and is comprised of connectin protein that acts as an anchor for the actin filaments.
- M-line: This is not labeled below but is visible. It refers to a dark line through the middle of a sarcomere, bisecting the two halves between Z disks. The M-band is composed of myomesin protein and C-protein, as well as portions of myosin proteins and the cross-links between myosin.
- H-band: Refers to the central zone of a sarcomere that looks lighter under a microscope. This area of the sarcomere is a lighter portion of the A-band, where there is no overlap between the actin and myosin filaments.
- A-band: Refers to the part of the muscle cell that looks darker under a microscope. This area of the cell is darker due to the parallel arrangement and overlap of myosin and actin filaments.
- I-band: Refers to the part of the muscle cell that looks lighter under a microscope. This area of the cell is lighter due to the parallel arrangement of actin with no overlap with the thicker myosin filaments.