Sarcomere

A sarcomere is the fundamental contractile unit of striated muscle fibers. It is the segment of a myofibril between two adjacent Z lines (Z discs). Each sarcomere contains overlapping thick filaments (myosin) and thin filaments (actin); muscle contraction occurs when these filaments slide past each other, shortening the sarcomere.

• Structural landmarks: <u>Z lines</u> define the borders of the sarcomere (actin filaments are anchored here). The <u>M line</u> runs down the center (where myosin filaments anchor). The <u>I band</u> is the light band with only thin filaments, and the <u>A band</u> is the dark band that spans the length of the thick filaments.
• During contraction: the <u>H zone</u> (center region with only thick filaments) and the I band both shrink in width, while the A band remains constant in length. This is because thick and thin filaments slide over each other (the filaments themselves do not shorten). Remember: "Z lines come closer, I and H get smaller, A stays the same.
• Many sarcomeres end-to-end form a myofibril. The aligned arrangement of sarcomeres in a muscle fiber is what gives skeletal and cardiac muscle their striated (banded) appearance under the microscope.

• A common exam question is which bands change size during muscle contraction - correct answer: the I band and H zone shorten, while the A band does not change (because the length of the myosin filaments doesn't change).
• If asked for the "functional unit of skeletal muscle contraction," the answer is the <u>sarcomere</u>. Also, be prepared to identify a sarcomere on a diagram (Z line to Z line).
• Visualization: Questions might refer to an electron micrograph of muscle. Knowing terms like Z line, A band, I band, etc., and how they relate to the sarcomere will help interpret such images and answer related questions.