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:
Muscle tissue can be described in terms of units called sarcomeres. 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 ...
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.