The Myofibril

Each muscle fiber (muscle cell) is made up of rod like myofibrils that run parallel to the length of the muscle fiber. They contain the contractile elements of the muscle.

The Myofibril

Myofibrils The myofibrils number in the hundreds to the thousands in a single muscle fiber.

Myofibrils The myofibrils number in the hundreds to the thousands in a single muscle fiber. They make up 80% of the cell volume.

Myofibrils The myofibrils number in the hundreds to the thousands in a single muscle fiber. They make up 80% of the cell volume. The myofibrils are made up of smaller rod like structures called myofilaments.

Myofibrils Examination of a muscle fiber will show a series of dark and light bands which create the striations seen in skeletal muscle

Myofibrils These striations are produced by the myofilaments which make up the myofibrils.

Myofibrils The dark bands are called A bands and the light bands, the I bands.

Myofibrils These striations are created by the myofilaments actin and myosin. These are the contractile elements of the muscle.

Myofibrils These striations are created by the myofilaments actin and myosin. Actin filaments are called the thin fibers and make up the I band.

Myofibrils These striations are created by the myofilaments actin and myosin. Actin filaments are called the thin fibers and make up the I band. Myosin filaments are called the thick fibers and make up the A band.

Myofibrils

The dark A band has a lighter zone in it called the H zone. Each H zone is bisected by a dark line called the M line.

Myofibrils The I bands are lighter and have a midline dark band called the Z band. The Z band is used to define the functional unit called the sarcomere.

The Sarcomere Because muscle fibers are very long, they are divided up into small contractile units called the Sarcomere. It averages 2 micrometers and is the distance between two successive Z discs

I band A band Sarcomere H zone Thin (actin) filament Thick (myosin) filament Z disc M line

Actin The thin filaments contain actin extend across the I band and part way into the A band. The Z disc anchors the thin filaments.

Actin and Myosin Myosin makes up the thick filament. The M line is formed by myosin molecules that are linked together.

Copyright © 2010 Pearson Education, Inc. Figure 9.2c Microscopic anatomy of a skeletal muscle fiber. I band A band Sarcomere H zone Thin (actin) filament Thick (myosin) filament Z disc M line (c) Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next.

Copyright © 2010 Pearson Education, Inc. I band thin filaments only Actin filament Myosin filament H zone thick filaments only M line thick filaments linked by accessory proteins Outer edge of A band thick and thin filaments overlap (e) Cross-sectional view of a sarcomere cut through in different locations.

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