| The plasma membrane of the
skeletal muscle fiber is called a sarcolemma.
The muscle fiber contains long cylindrical structures, the myofibrils.
The myofibrils almost entirely fill the cell and push the nuclei
to the outer edges of the cell under the sarcolemma. The many
myofibrils each have light and dark bands and are aligned with one
another so that the light and dark bands are next to one another.
This gives the cell its
| The light
bands are called I bands and
the dark bands are called A bands.
In the middle of the I bands there is a line called the Z
line (or disc). In the middle of the A bands (or dark bands)
there is a light zone called the H zone. In the middle of the H zone there is another line, the M
line. The precise arrangement of these features is due to a
chain of functional units in the myofibrils,
| The sarcomere consists of a
number of individual protein elements. Some of these proteins are
thread-like proteins called myofilaments.
There are two major types of myofilaments:
| Thick myofilaments are made up of
proteins molecules called
The myosin molecules are shaped like golf clubs with
long shafts. Myosin forms the thick myofilaments by
forming bundles in which the heads of the “golf
clubs” stick out at either end of the filament and the
shafts form a “bare” zone in the middle of the
heads of the
thick myofilaments form attachments with the other type
of myofilaments, the thin actin myofilaments. These
attachments are called cross
bridges. The heads are also the places on the thick
myofilaments that use the energy in the ATP
molecule to power the muscle contraction.
| The thin myofilaments are composed
of the protein
The thin myofilaments have the binding
sites to which the heads of the thick myofilaments