H-band
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.
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The components of a sarcomere visible under an electron microscope (as depicted in the image)
- 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.
- 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.
- 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. Note that this is where the myosin heads interact with the actin proteins.
- 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.