Structures of Muscle Filaments
This page is a continuation of the page about
the structure of a muscle
cell - which is a continuation of the more
general page about the structure
Each muscle cell
(also known as a "muscle fibre")
contains many specialised components described
page about the structure of a muscle cell.
components within muscle cells include myofibrils,
which consist of two types of protein
filaments called "thick
filaments", and "thin
These two types of filament have different structures that
enable then to work together.
Their structures are shown below:
Above: Diagrams of Muscle Filaments
Thick filaments are formed from a protein called
which has important properties of elasticity and
The shape of the myosin molecules has the apperance
of two "hockey sticks" or "golf
clubs" twisted together. This is illustrated
in the diagram above - indicating the two parts
of the myosin molecule referred
to in Advanced
Textbooks about Muscles
These are the myosin tail, and the myosin
heads, or "crossbridges"
The main component of the thin filaments is a
protein called actin.
Actin molecules join together forming chains twisted
into a helix configuration. These molecules are
very important to the contraction mechanism of
muscles because each actin molecule has a single
(not illustrated above).
The other two protein molecules that form the
thin filaments are called troponin
The molecules of tropomyosin cover the myosin-binding
sites on the actin molecules when the muscle fibres are relaxed.
Myosin and actin form the
main contractile elements of muscles.
This is because it is the binding of the thick filaments to
the thin filaments - and in particular the
positions of these points of attachment - that controls the
state of contraction/relaxation of the muscle of which they
Recall (from the previous
page) that the thick filaments
and the thin filaments together
form units called sacromeres.
The diagram of a sacromere is repeated below:
Above: Diagram of a Sacromere
As shown above, the extent to which
the thick filaments and the thin
filaments overlap with each other determines
the sizes of
zone, the I
band, and the A
band of the
sacromere formed by these filaments.
For information about how the muscle filaments cause
muscles to contract, read about the Sliding-Filament