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Proteins
of the myofilaments. Thick filament assembly
In
the middle portion of the thick filament the molecules are
assembled in a tail-to-tail fashion. Along the rest of the
filament they are arranged head to tail. The tail parts
of the molecules formthe core of the filament: the head
portions project out from the filament. The cross bridges
are actually the globular head regions of myosin molecules
extending outward from the filament, and the smooth pseudo-H
zone is the region of tail-to-tail aggregation in which
there are only tails and no heads.
The
precise three-dimensional arrangement of the cross bridges
projecting from the thick filament cannot be seen easily
in electron micrographs but can be determined from X-ray
diffraction study of living muscle. The three bridges project
120 degrees from the opposite sides of the filament every
14.3 nanometres along the length of the filament. Each successive
set of bridges is located in a position rotated 40 degrees
farther around the filament. The pattern of nine bridges
(three sets of three bridges) repeats itself every 42.9
nanometres along the thick filament. Some variation may
exist from species to species and muscle to muscle.
