Download MUSCLES Muscles Fibres Types Muscle Tension

MUSCLES
Muscles Fibres Types
There are three types of muscles in the human body - skeletal, smooth
and cardiac, which differ in their structure, innervation, and function. This lab
deals almost exclusively with the skeletal muscle system, which also has three
different types categorized by their maximal shortening velocity and the major
pathway used to form ATP. There are fast and slow fibers, and glycolytic or
oxidative fibers.
Skeletal muscle has myosin isozymes which differ in their maximal rates
of ATP splitting, which affects the rate of cross-bridge cycling, and hence the
rate of contraction. Fast fibers have isozymes with high ATP-ase activity, while
slow fiber isozymes have low activity.
Oxidative fibers are supplied with numerous small blood vessels, lots of
myoglobin, many mitochondria, and thus a high capacity for oxidative
phosphorylation (the regeneration of ATP from various energy stores). The
myoglobin affects the diffusion rate for oxygen, and also gives the muscle its
characteristic red appearance. Because of all of this cellular capability for ATP
production, oxidative fibers fatigue far slower than glycolytic fibers. Glycolytic
fibers have low mitochondria levels, but large stores of glycogen and glycolytic
enzymes. Because they are typically not as rich in myoglobin, they are white in
appearance. Glycolytic fibers are also somewhat larger in diameter, and can
produce more force because of this greater cross-sectional area.
Below are listed the various muscle types with some of their
properties. A similar and more complete table can be found in Vander.
Fiber Type
ATP-ase Metabolism Fatigue
Color
Slow-oxidative Low
Oxidative
Slow
Red
Fast-oxidative
Hi
Oxidative
Intermediate Pink
Fast-glycolytic Hi
Glycolytic
Fast
White
Muscle Tension
The action-potential frequency critically affects the force produced because The
amount of tension (or force) developed by a muscle is dependent upon the
number of fibers and the tension developed by each fiber. The number of fibers
used depends on the motor unit organization of the muscle and the number of
active motor units. The tension in the individual fibers is determined by fiber
length, fiber diameter, fatigue, and the stimulation frequency.
The fiber length changes as the muscle is stretched out, or relaxed. Because
there must be some overlap between the thick and thin filaments, the muscle
can not be pulled too far apart. Likewise, the muscle cannot achieve maximum
force generation if the thin filament themselves overlap, because the maximum
number of cross-bridge sites is not presented. The optimum length is that
which simply positions the thick and thin filaments for greatest non-interfering
overlap.
Fiber diameter is influenced by fiber type (fast or slow), with glycolytic fibers
have larger cross-sectional areas and greater force production. Fatigue is also
influenced by fiber type, (see above), and duration of activity.
multiple stimuli, if timed close enough together (in other words, with a relative
high frequency) will add up in summation. With one single stimulus, the muscle
produces one twitch. If two stimuli are given, the muscle twitches twice. If the
second stimuli is timed so that it just overlaps the first, the two will add to
produce greater tension. With multiple stimuli of high frequency (little time
between each) the muscle can achieve tetanus, or a sustained contraction.
Tetanus tension can be 3-5 times that of a single twitch. The frequency
required for maximal tetanus varies with fiber type because of the different
contraction times.