Page 71 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Slow-twitch fibers are the least fatigable and
Motility and Muscle Types
are therefore equipped for sustained perform-
Active motility (ability to move) is due to ance. They have high densities of capillaries
either the interaction of energy-consuming and mitochondria and high concentrations of
motor proteins (fueled by ATPase) such as my- fat droplets (high-energy substrate reserves)
osin, kinesin and dynein with other proteins and the red pigment myoglobin (short-term O 2
such as actin or the polymerization and storage). They are also rich in oxidative
depolymerization of actin and tubulin. Cell di- enzymes (! p. 72). Fast-twitch fibers are
vision (cytokinesis), cell migration (! p. 30), mainly responsible for brief and rapid contrac-
Nerve and Muscle, Physical Work (! p. 366), and ciliary motility (! p. 110) are pends on the muscle type. Motor units of the S
tions. They are quickly fatigued (FF ! FR) and
intracellular vesicular transport and cytosis
are rich in glycogen (FF ! FR) but contain little
(! p. 12f.), sperm motility (! p. 306f.), axonal
transport (! p. 42), electromotility of hair cells
myoglobin (FF"FR).
The fiber type distribution of a muscle de-
examples of cell and organelle motility.
type predominate in “red” muscles such as the
The muscles consist of cells (fibers) that
soleus muscle, which helps to maintain the
contract when stimulated. Skeletal muscle is
responsible for locomotion, positional change,
body in an upright position, whereas the F type
predominates in “white” muscles such as the
and the convection of respiratory gases. Car-
gastrocnemius muscle, which is involved in
diac muscle (! p. 190 ff.) is responsible for
converted to the other type. If, for example, the
(! p. 70) serves as the motor of internal organs
and blood vessels. The different muscle types
prolonged activation of fast-twitch fibers leads
2 pumping the blood, and smooth muscle running activity. Each fiber type can also be
con-
2+
to a chronic increase in the cytosolic Ca
are distinguished by several functional charac-
teristics (! A). centration, fast-twitch fibers will be converted
to slow-twitch fibers and vice versa.
Graded muscle activity is possible because a
Motor Unit of Skeletal Muscle
variable number of motor units can be re-
Unlike some types of smooth muscle (single- cruited as needed. The more motor units a
unit type; ! p. 70) and cardiac muscle fibers, muscle has, the more finely graded its contrac-
which pass electric stimuli to each other tions. Contractions are much finer in the exter-
through gap junctions or nexus (! A; p. 16f.), nal eye muscles, for example, which have
skeletal muscle fibers are not stimulated by around 2000 motor units, than in the lumbri-
adjacent muscle fibers, but by motor neurons. cal muscles, which have only around 100
In fact, muscle paralysis occurs if the nerve is motor units. The larger the number of motor
severed. units recruited, the stronger the contraction.
One motor neuron together with all muscle The number and type of motor units recruited
fibers innervated by it is called a motor unit depends on the type of movement involved
(MU). Muscle fibers belonging to a single (fine or coarse movement, intermittent or per-
motor unit can be distributed over large por- sistent contraction, reflex activity, voluntary or
2
tions (1 cm ) of the muscle cross-sectional involuntary movement, etc.). In addition, the
area. To supply its muscle fibers, a motor neu- strength of each motor unit can be increased
ron splits into collaterals with terminal by increasing the frequency of neuronal im-
branches (! p. 42). A given motor neuron may pulses, as in the tetanization of skeletal muscle
supply only 25 muscle fibers (mimetic muscle) (! p. 67 A).
or well over 1000 (temporal muscle).
Two types of skeletal muscle fibers can be
distinguished: S – slow-twitch fibers (type 1)
and F – fast-twitch fibers (type 2), including
two subtypes, FR (2 A) and FF (2 B). Since each
motor unit contains only one type of fiber, this
58 classification also applies to the motor unit.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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