Download A Closer Look at Skeletal Muscles Fibers (Cells

A Closer Look at Skeletal Muscles Fibers (Cells) and Myofibrils
Each muscle fiber (cell) has tiny myofibrils running parallel in the cell. Myofibrils are the contractile elements of
muscle cells. Each myofibril is made up of chains of sarcomeres. When you look at a myofibril, you can see dark
and light bands. The dark bands are A-Bands. The light bands are I-Bands. These are the visible striations you
see in skeletal muscle. This banding pattern of the myofibril is from protein filaments called myofilaments. The
thick filaments extend the length of the A-Band. The thin filaments extend across the I-Band partway into the
A-Band.
In a relaxed muscle, inside the A-band there can be seen a light stripe called the H-Zone, which is lighter
because the thick and thin filaments don’t overlap there. The M-Line in the center of each H-Zone is where
the thick filaments connect. Down the center of the I-band there is a line called the Z-Line. This is the point of
attachment of the thin filaments. The region of the myofibril, between two Z-Lines is called a sarcomere. This is
the smallest contractile unit of a muscle cell.
mitochondria
sarcolemma
multi-nucleate
A-Band
one skeletal
muscle fiber
(or cell)
I-Band
Myofibril
A-Band
(Dark Band)
I-Band
(Light Band) H-Zone
Close up of a
myofibril
myofilaments: thin filament thick filament
(Actin)
©Sheri Amsel
www.exploringnature.org
(Myocin)
sarcomere
Z-Line M-Line
An Even Look Closer at a Myofibril
(Made up of Bundles of Myofilaments)
Illustrated Reveiw: Each muscle fiber (cell) has tiny myofibrils running parallel in the cell. Myofibrils are the
contractile elements of muscle cells. Each myofibril is made up of chains of sarcomeres. When you look at a
myofibril, you can see dark and light bands. The dark bands are A-Bands. The light bands are I-Bands. These are
the visible striations you see in skeletal muscle. This banding pattern of the myofibril is from protein filaments
called myofilaments. The thick filaments extend the length of the A-Band. The thin filaments extend across the
I-Band partway into the A-Band.
In a relaxed muscle, inside the A-band there can be seen a light stripe called the H-Zone, which is lighter
because the thick and thin filaments don’t overlap there. The M-Line in the center of each H-Zone is where
the thick filaments connect. Down the center of the I-band there is a line called the Z-Line. This is the point of
attachment of the thin filaments. The region of the myofibril, between two Z-Lines is called a sarcomere. This is
the smallest contractile unit of a muscle cell.
Close up of a Myofibril
Myofilaments:
thin filament thick filament
(Myocin)
(Actin)
sarcomere
I-Band
(Light
Band)
A-Band
(Dark Band)
Z-Line
M-Line
H-Zone
sarcomere
©Sheri Amsel
sarcomere
www.exploringnature.org