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Sarcomeres

Sarcomeres are repeating units of contractile proteins found in the myofibrils of skeletal and cardiac muscle cells. During a muscle contraction, the sarcomere shortens as the contractile proteins slide over one another. In your bicep brachii muscle, for example, approximately 100.000 sarcomeres are lined up from one end to the other.

The major building blocks of a sarcomere are:

  • Thick filament: Made of hundreds of intertwined proteins known as Myosin

  • Thin filament: Mostly composed of intertwined strands of a protein known as Actin

  • Z-disc: Defines the boundaries of a sarcomere and thus, one sarcomere extends from one Z-disc to the next. The Z-disc is composed of large proteins and serve as an anchoring site for contractile proteins. Characteristically, the Z-lines are pulled closer together during the muscle contraction.

  • M-line: This is the center of a sarcomere and contains proteins that function as anchoring sites for the Myosin molecules. The M-line also provides elasticity to the muscles due to the presence of elastic fibers.

A few special areas of a sarcomere can be identified:

  • A-band: Covers the length of the thick filament (Myosin) in a sarcomere. The amount of actin found in the A-band depends on how much the sarcomere is contracted. The size of the A-band remain constant during a muscle contraction.

  • I-band: The zone of a sarcomere that is not covered by Myosin molecules. The amount of actin molecules found in the I-band varies, depending on how much the sarcomere is contracted.

Molecular structure of sarcomere with its distinguishable areas. It consists of three, thin, vertical, purple lines, called M line, which are crossed by three horizontal, parallel purple tube structures, on which small purple spheres are spread, called myosins. In between the three tubes, the actin molecules are placed, shown as green structures, overlapping the purple tube rows, and extending a bit beyond that, where they are connected by thick, dark green, vertical line of proteins, called Z disc. Upon contraction, the purple spheres on the purple tubes are extending and connecting to aligned actin molecules, which results in Z discs brought closer together.

Figure 1: A sarcomere is shown relaxed (upper) and contracted (lower).

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References: OpenStax College, Biology. (OpenStax CNX. Mar 13, 2015)

Referred from:

Article
Skeletal Muscle Architecture