Myofilament Arrangement and the Sarcomere

Muscular System, Ch 9, Histology and Physiology

“I” bands

where only thin filaments found are light colored.

“A” bands

where thick filaments found are dark colored.

Zone of Overlap

The “A” band region where we find thick and thin filaments and where tension is generated during contraction.

True or False: In the middle of the “A” band where only thick filament exist is the “H” zone.

True: H is in the A band. (mnemonic: Ha!)

“M” line

is a dark line in the middle of the “A” band made up of structural proteins. (mnemonic: M for middle)

“Z” disc

is found in the middle of the “I” band, also composed of structural proteins.

Sarcomere

The functional contractile unit of the myofibril; extends from one “Z” disc to the next.

True or False: During a muscle contraction, both the I band and H zone narrow, while the A band remains unchanged.

True

True or False: Myosin heads attach to actin to shorten the thin filaments toward the M line bringing the Z discs closer together, shortening the sarcomere.

False: Actin pulls the thin filaments toward the M line, bringing Z discs closer together, thus shortening the sarcomere

How does the sarcomere arrangement affect the whole muscle fiber?

Sarcomeres are arranged end to end within each myofibril and when simultaneously contracted, contract the whole muscle fiber.