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The Sliding-Filament Theory of Muscle Action
The "Sliding-Filament
Theory of Muscle Action"
explains how the movement of thick- and thin-filaments
relative
to each other leads to the
contraction
and relaxation
of
whole
muscles - hence ultimately to the movement
of the limbs or tissues attached to those muscles:
As explained on the previous page (Structures
of Muscle Filaments), there are two physical
units that are important for the action of muscles.
They are thick
filaments and thin
filaments.
Muscle tissue can be described in terms of units called
sacromeres.
These units are defined in terms of groups of overlapping
filaments (the thin and thick filaments previously
described). Sacromeres are arrangements
of thick and thin filaments.
The length of a sacromere and the zones (H
zone, I
band and A
band) within each sacromere, are determined
by the positions of the thick and thin filaments
relative to each other. This is illustrated in
the three diagrams below - showing the relative
length and configuration of two sacromeres of
relaxed muscle (top), partially
contracted muscle (centre) and fully
contracted muscle (lower diagram).
Note that although the length of the sarcomere changes,
the lengths of the filaments remain the same - the length
of the sarcomere is a consequence of the extent to which
these filaments overlap.
What happens
?
During Muscle Contraction:
The myosin
heads on the thick filaments "hook"
onto, and so pull, the thin filaments towards
the centre (labelled "M-line") of
each sacromere. The appearance of this action
is shown above as the transistion from "relaxed"
to "fully contracted" muscle. As the
thin filaments slide over the thick filaments,
the I
bands and H
zones becomes narrower and narrower until
they disappear when the muscle reaches its fully
contracted state.
During Muscle Relaxation:
When the myosin
heads on the thick filaments relax they release
their hold on the thin filaments, thereby allowing
them to slide back to their "relaxed"
positions in which the I
bands and H
zones appear again.
How and why does this occur
?
This leads to questions about what causes the
myosin heads to lock onto the thin filaments and pull them,
and what causes them to relax and release their
hold on the thin filaments.
These processes happen as a result of instructions sent
via the nervous system to activate and deactive these tissues.
The
muscular
and nervous systems are connected to each other by neuromuscular
junctions. The anatomical structures that form
neuromuscular junctions, together with the actions they
perform, are described
on the next page. First make
a note of the conditions that must exist in order for the
processes
described sliding-filament theory to take place ...
Necessary Conditions:
This sliding
filament mechanism can only occur when there are sufficient
calcium ions (Ca2+) and sufficient ATP is
also available.
... continue on the the next page: Anatomy
of Neuromuscular Junctions.
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