Biomedical Engineering Reference
In-Depth Information
2.2
µ
m
Filaments
100
1.8
µ
m
hin
hick
80
3.8
µ
m
60
40
20
40
60
80 100 120 140 160
Percentage sarcomere length
FIGURE A.8
Length-tension relationship in skeletal muscle fiber.
fiber is stretched to 175% of its optimal length, no myosin cross-bridges can bind
to thin filaments and no contraction occurs. At sarcomere length less than the
optimum, the force of the contraction also decreases. This is because thick filaments
crumple as they are compressed by the Z-discs, resulting in fewer myosin cross-
bridge contacts with thin filaments. In the intact body, resting muscle fiber length
is rarely less than 70% or more than 130% of optimum. This is because skeletal
muscles are firmly anchored to bones and other inelastic tissues.
Recruitment is the process of increasing the number of active motor units
(multiple motor unit summation). The various motor neurons to a whole muscle fire
asynchronously. While some motor units are active (contraction), others are inactive
(relaxed). This pattern of firing motor neurons prevents fatigue while maintaining
contraction by allowing a brief rest for the inactive units. The alternating motor units
relieve one another so that the contraction can be sustained for long periods. Recruit-
ment is also one factor responsible for producing smooth movements rather than a
series of jerky movements. As indicated before, the number of muscle fibers inner-
vated by one motor neuron varies greatly. Precise movements require tiny changes
in muscle contraction. Therefore, in such muscles, the motor units are small. In this
way when a motor unit is recruited or turned off, slight but controlled changes occur
in muscle contraction. On the other hand, large motor units are employed where
maintaining a constant position or posture is important and precision is not.
A.2
ROLE OF STRUCTURAL COMPONENTS
OF MUSCLE FIBERS
As thin filaments start to slide past thick filaments, they pull on the Z-discs, which
in turn pull on the rest of the muscle fibers and their connective tissue wrappings.
Some of these structural components are elastic; that is, they stretch slightly before
they start to relay the force or tension generated by the sliding filaments.
