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Sliding
of filaments
The
discovery that during contraction the filaments do not shorten
but that the two sets—thick and thin—merely move relative
to each other is crucial for our current understanding of
musclephysiology. During contraction the thin filaments
move deeper into the A band, and the overlap of the thick
and thin filaments increases. If a longitudinal section
of the sarcomere is considered, the thin filaments on the
left side of the A band would move to the right into the
A band and the filaments on the right of the A band would
move to the left into the A band. Directionality of the
motion partly results from the structural polarity of both
the thick filaments, since in the two halves of the filament
the myosin molecules are oriented in opposite directions,
and the actin filaments, in which the actin molecules are
oriented with respect to the Z bands.
