At
high magnification small bridgelike structures can be seen
on the thick filaments extending toward the thin filaments
in the overlap region. They are called cross bridges and
are believed to be responsible for the movement and force
developed during contraction (for the relation of cross
bridges to the molecular architecture of thick filaments,
see below). In the middle of the A band, where only thick
filaments are present, is a region called the H zone; it
looks somewhat lighter than the overlap region of the A
band. Also in the A band is a narrow, lightly stained region
that contains bare thick filaments without cross bridges
and is called the pseudo-H zone. In the centre of the A
band is a narrow, darkly stained region called the M band,
in which occur fine bridges between the thick filaments.
These bridges differ from the cross bridges between the
thick and thin filaments and are in fact composed of an
entirely different protein.
If
cross sections of the myofi
at different levels of the
sarcomere are examined in the electron microscope, the filaments
can be seen end-on, and the three-dimensional nature of
the lattice of filaments can be appreciated. The I band
contains only thin filaments, with a diameter of six to
eight nanometres. In the A band, in the overlap region,
the thin filaments appear with thick ones (diameter of 12
nanometres) in an extremely regular pattern or lattice.
In vertebratesthe thick filaments are arranged in a hexagonal
lattice and the thin ones are located at the centre of the
equilateral triangles formed by the thick filaments. Sections
through the H zone contain only thick filaments arranged
in the same hexagonal pattern they form in the overlap region.
In the M band the hexagonal array of thick filaments can
be seen with M bridges running between them.
