Based on the type of metabolism and the rate at which
tension develops, skeletal muscle fibers can be classified
into three types:
Slow oxidative
(SO)
fibers have small diameters,
contain many large
mitochondria,
and
appear
red
because
they contain large
amounts
of myoglobin.
They make ATP mainly by aerobic respiration, develop
tension relatively slowly, resist fatigue, and are capable
of prolonged, sustained contractions. Such muscle fibers
are found in large numbers in the muscles associated with
posture. These are also the most important muscle fiber
type for long-distance runners, who require endurance.
Fast oxidative-glycolytic
(FOG)
fibers are inter-
mediate in diameter. They have large amounts of
myoglobin and therefore appear red. They make ATP
through both aerobic respiration and glycolysis, so
they are moderately resistant to fatigue. Most of the
muscle fibers associated with large motor skills are of
this type. They contract and relax faster than SO fibers
but less quickly than the FG fiber type, providing an
intermediate level of contraction sustainability. Middle-
distance runners rely heavily on this type of muscle fiber.
Fast glycolytic (FG) fibers are large and white, with
considerable amounts of glycogen, but no myoglobin.
Because they make ATP mainly through glycolysis, they
fatigue rapidly. However, they contract and relax quickly,
providing a short surge of power. Many of these fibers are
found in areas where fine motor skills occur. Sprinters
need lots of this muscle fiber type, as these muscles allow
rapid reactions and short bursts of speed.
Most skeletal muscles have all three types of fibers. About
50 percent of the fiber composition is SO fibers. Depend-
ing on the muscle, the rest of the composition varies. For
example, arm and shoulder muscles have high proportions
of FG fibers, while leg muscles have large numbers of both
SO and FOG fibers. Regardless of the overall mixture of
muscle fibers within any given muscle, a single motor unit
consists of identical fiber types. Therefore, there are SO
motor units, FOG motor units, and FG motor units. These
motor units are recruited in specific orders, depending
on whether speed, force, or duration of contraction is re-
quired in the muscle action.
CONCEPT CHECK
1.
Which
skeletal muscle organelle stores and
releases calcium?
2.
What
does ATP do in the contraction, or cross-
bridge, cycle?
3.
Through
what steps is the action potential
stimulus translated into sliding filament contrac-
tions?
4.
Which
type of muscle fibers would dominate
the movements of your arm muscles in perform-
ing short but powerful contractions, as in throw-
ing a ball or swinging a tennis racket?
Smooth Muscle Tissue Is in the Walls
of Most Organs
LEARNING OBJECTIVES
1.
explain
the structure of smooth muscle.
2.
Differentiate
the classes of smooth muscle and
describe where they are found.
mooth muscles are found in the walls of all
hollow organs, such as blood vessels, the stom-
ach, and the intestines. (They are also found in
the skin, attached to hair follicles.) Because of
their unique structure, they are capable of being stretched
much more than skeletal or cardiac muscle and can main-
tain steady levels of contractions for long periods of time.
Smooth Muscle tissue looks Very
Different from Skeletal Muscle tissue
Smooth muscle cells are much smaller than cells of other
muscle types, and they are spindle-shaped. Smooth muscle
differs from skeletal muscle in a number of ways:
Each smooth muscle cell has a single nucleus and no
striations because the thin and thick filaments are not
arranged regularly.
Because smooth muscle cells are small and thin, there
are no transverse tubules.
Each
cell
contains
intermediate
filaments
and
dense bodies, which are equivalent to the Z-discs in
Smooth Muscle Tissue is in the Walls of Most organs 165
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